added a lot of printk output to ease writing of emulator

[linux-2.4.21-pre4.git] / fs / namei.c

/*
 *  linux/fs/namei.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 */

/*
 * Some corrections by tytso.
 */

/* [Feb 1997 T. Schoebel-Theuer] Complete rewrite of the pathname
 * lookup logic.
 */
/* [Feb-Apr 2000, AV] Rewrite to the new namespace architecture.
 */

#include <linux/init.h>
#include <linux/slab.h>
#include <linux/fs.h>
#include <linux/quotaops.h>
#include <linux/pagemap.h>
#include <linux/dnotify.h>
#include <linux/smp_lock.h>
#include <linux/personality.h>

#include <asm/namei.h>
#include <asm/uaccess.h>

#define ACC_MODE(x) ("\000\004\002\006"[(x)&O_ACCMODE])

/* [Feb-1997 T. Schoebel-Theuer]
 * Fundamental changes in the pathname lookup mechanisms (namei)
 * were necessary because of omirr.  The reason is that omirr needs
 * to know the _real_ pathname, not the user-supplied one, in case
 * of symlinks (and also when transname replacements occur).
 *
 * The new code replaces the old recursive symlink resolution with
 * an iterative one (in case of non-nested symlink chains).  It does
 * this with calls to <fs>_follow_link().
 * As a side effect, dir_namei(), _namei() and follow_link() are now 
 * replaced with a single function lookup_dentry() that can handle all 
 * the special cases of the former code.
 *
 * With the new dcache, the pathname is stored at each inode, at least as
 * long as the refcount of the inode is positive.  As a side effect, the
 * size of the dcache depends on the inode cache and thus is dynamic.
 *
 * [29-Apr-1998 C. Scott Ananian] Updated above description of symlink
 * resolution to correspond with current state of the code.
 *
 * Note that the symlink resolution is not *completely* iterative.
 * There is still a significant amount of tail- and mid- recursion in
 * the algorithm.  Also, note that <fs>_readlink() is not used in
 * lookup_dentry(): lookup_dentry() on the result of <fs>_readlink()
 * may return different results than <fs>_follow_link().  Many virtual
 * filesystems (including /proc) exhibit this behavior.
 */

/* [24-Feb-97 T. Schoebel-Theuer] Side effects caused by new implementation:
 * New symlink semantics: when open() is called with flags O_CREAT | O_EXCL
 * and the name already exists in form of a symlink, try to create the new
 * name indicated by the symlink. The old code always complained that the
 * name already exists, due to not following the symlink even if its target
 * is nonexistent.  The new semantics affects also mknod() and link() when
 * the name is a symlink pointing to a non-existant name.
 *
 * I don't know which semantics is the right one, since I have no access
 * to standards. But I found by trial that HP-UX 9.0 has the full "new"
 * semantics implemented, while SunOS 4.1.1 and Solaris (SunOS 5.4) have the
 * "old" one. Personally, I think the new semantics is much more logical.
 * Note that "ln old new" where "new" is a symlink pointing to a non-existing
 * file does succeed in both HP-UX and SunOs, but not in Solaris
 * and in the old Linux semantics.
 */

/* [16-Dec-97 Kevin Buhr] For security reasons, we change some symlink
 * semantics.  See the comments in "open_namei" and "do_link" below.
 *
 * [10-Sep-98 Alan Modra] Another symlink change.
 */

/* [Feb-Apr 2000 AV] Complete rewrite. Rules for symlinks:
 *      inside the path - always follow.
 *      in the last component in creation/removal/renaming - never follow.
 *      if LOOKUP_FOLLOW passed - follow.
 *      if the pathname has trailing slashes - follow.
 *      otherwise - don't follow.
 * (applied in that order).
 *
 * [Jun 2000 AV] Inconsistent behaviour of open() in case if flags==O_CREAT
 * restored for 2.4. This is the last surviving part of old 4.2BSD bug.
 * During the 2.4 we need to fix the userland stuff depending on it -
 * hopefully we will be able to get rid of that wart in 2.5. So far only
 * XEmacs seems to be relying on it...
 */

/* In order to reduce some races, while at the same time doing additional
 * checking and hopefully speeding things up, we copy filenames to the
 * kernel data space before using them..
 *
 * POSIX.1 2.4: an empty pathname is invalid (ENOENT).
 * PATH_MAX includes the nul terminator --RR.
 */
static inline int do_getname(const char *filename, char *page)
{
        int retval;
        unsigned long len = PATH_MAX;

        if ((unsigned long) filename >= TASK_SIZE) {
                if (!segment_eq(get_fs(), KERNEL_DS))
                        return -EFAULT;
        } else if (TASK_SIZE - (unsigned long) filename < PATH_MAX)
                len = TASK_SIZE - (unsigned long) filename;

        retval = strncpy_from_user((char *)page, filename, len);
        if (retval > 0) {
                if (retval < len)
                        return 0;
                return -ENAMETOOLONG;
        } else if (!retval)
                retval = -ENOENT;
        return retval;
}

char * getname(const char * filename)
{
        char *tmp, *result;

        result = ERR_PTR(-ENOMEM);
        tmp = __getname();
        if (tmp)  {
                int retval = do_getname(filename, tmp);

                result = tmp;
                if (retval < 0) {
                        putname(tmp);
                        result = ERR_PTR(retval);
                }
        }
        return result;
}

/*
 *      vfs_permission()
 *
 * is used to check for read/write/execute permissions on a file.
 * We use "fsuid" for this, letting us set arbitrary permissions
 * for filesystem access without changing the "normal" uids which
 * are used for other things..
 */
int vfs_permission(struct inode * inode, int mask)
{
        umode_t                 mode = inode->i_mode;

        if (mask & MAY_WRITE) {
                /*
                 * Nobody gets write access to a read-only fs.
                 */
                if (IS_RDONLY(inode) &&
                    (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))
                        return -EROFS;

                /*
                 * Nobody gets write access to an immutable file.
                 */
                if (IS_IMMUTABLE(inode))
                        return -EACCES;
        }

        if (current->fsuid == inode->i_uid)
                mode >>= 6;
        else if (in_group_p(inode->i_gid))
                mode >>= 3;

        /*
         * If the DACs are ok we don't need any capability check.
         */
        if (((mode & mask & (MAY_READ|MAY_WRITE|MAY_EXEC)) == mask))
                return 0;

        /*
         * Read/write DACs are always overridable.
         * Executable DACs are overridable if at least one exec bit is set.
         */
        if ((mask & (MAY_READ|MAY_WRITE)) || (inode->i_mode & S_IXUGO))
                if (capable(CAP_DAC_OVERRIDE))
                        return 0;

        /*
         * Searching includes executable on directories, else just read.
         */
        if (mask == MAY_READ || (S_ISDIR(inode->i_mode) && !(mask & MAY_WRITE)))
                if (capable(CAP_DAC_READ_SEARCH))
                        return 0;

        return -EACCES;
}

int permission(struct inode * inode,int mask)
{
        if (inode->i_op && inode->i_op->permission) {
                int retval;
                lock_kernel();
                retval = inode->i_op->permission(inode, mask);
                unlock_kernel();
                return retval;
        }
        return vfs_permission(inode, mask);
}

/*
 * get_write_access() gets write permission for a file.
 * put_write_access() releases this write permission.
 * This is used for regular files.
 * We cannot support write (and maybe mmap read-write shared) accesses and
 * MAP_DENYWRITE mmappings simultaneously. The i_writecount field of an inode
 * can have the following values:
 * 0: no writers, no VM_DENYWRITE mappings
 * < 0: (-i_writecount) vm_area_structs with VM_DENYWRITE set exist
 * > 0: (i_writecount) users are writing to the file.
 *
 * Normally we operate on that counter with atomic_{inc,dec} and it's safe
 * except for the cases where we don't hold i_writecount yet. Then we need to
 * use {get,deny}_write_access() - these functions check the sign and refuse
 * to do the change if sign is wrong. Exclusion between them is provided by
 * spinlock (arbitration_lock) and I'll rip the second arsehole to the first
 * who will try to move it in struct inode - just leave it here.
 */
static spinlock_t arbitration_lock = SPIN_LOCK_UNLOCKED;
int get_write_access(struct inode * inode)
{
        spin_lock(&arbitration_lock);
        if (atomic_read(&inode->i_writecount) < 0) {
                spin_unlock(&arbitration_lock);
                return -ETXTBSY;
        }
        atomic_inc(&inode->i_writecount);
        spin_unlock(&arbitration_lock);
        return 0;
}
int deny_write_access(struct file * file)
{
        spin_lock(&arbitration_lock);
        if (atomic_read(&file->f_dentry->d_inode->i_writecount) > 0) {
                spin_unlock(&arbitration_lock);
                return -ETXTBSY;
        }
        atomic_dec(&file->f_dentry->d_inode->i_writecount);
        spin_unlock(&arbitration_lock);
        return 0;
}

void path_release(struct nameidata *nd)
{
        dput(nd->dentry);
        mntput(nd->mnt);
}

/*
 * Internal lookup() using the new generic dcache.
 * SMP-safe
 */
static struct dentry * cached_lookup(struct dentry * parent, struct qstr * name, int flags)
{
        struct dentry * dentry = d_lookup(parent, name);

        if (dentry && dentry->d_op && dentry->d_op->d_revalidate) {
                if (!dentry->d_op->d_revalidate(dentry, flags) && !d_invalidate(dentry)) {
                        dput(dentry);
                        dentry = NULL;
                }
        }
        return dentry;
}

/*
 * This is called when everything else fails, and we actually have
 * to go to the low-level filesystem to find out what we should do..
 *
 * We get the directory semaphore, and after getting that we also
 * make sure that nobody added the entry to the dcache in the meantime..
 * SMP-safe
 */
static struct dentry * real_lookup(struct dentry * parent, struct qstr * name, int flags)
{
        struct dentry * result;
        struct inode *dir = parent->d_inode;

        down(&dir->i_sem);
        /*
         * First re-do the cached lookup just in case it was created
         * while we waited for the directory semaphore..
         *
         * FIXME! This could use version numbering or similar to
         * avoid unnecessary cache lookups.
         */
        result = d_lookup(parent, name);
        if (!result) {
                struct dentry * dentry = d_alloc(parent, name);
                result = ERR_PTR(-ENOMEM);
                if (dentry) {
                        lock_kernel();
                        result = dir->i_op->lookup(dir, dentry);
                        unlock_kernel();
                        if (result)
                                dput(dentry);
                        else
                                result = dentry;
                }
                up(&dir->i_sem);
                return result;
        }

        /*
         * Uhhuh! Nasty case: the cache was re-populated while
         * we waited on the semaphore. Need to revalidate.
         */
        up(&dir->i_sem);
        if (result->d_op && result->d_op->d_revalidate) {
                if (!result->d_op->d_revalidate(result, flags) && !d_invalidate(result)) {
                        dput(result);
                        result = ERR_PTR(-ENOENT);
                }
        }
        return result;
}

/*
 * This limits recursive symlink follows to 8, while
 * limiting consecutive symlinks to 40.
 *
 * Without that kind of total limit, nasty chains of consecutive
 * symlinks can cause almost arbitrarily long lookups. 
 */
static inline int do_follow_link(struct dentry *dentry, struct nameidata *nd)
{
        int err;
        if (current->link_count >= 5)
                goto loop;
        if (current->total_link_count >= 40)
                goto loop;
        if (current->need_resched) {
                current->state = TASK_RUNNING;
                schedule();
        }
        current->link_count++;
        current->total_link_count++;
        UPDATE_ATIME(dentry->d_inode);
        err = dentry->d_inode->i_op->follow_link(dentry, nd);
        current->link_count--;
        return err;
loop:
        path_release(nd);
        return -ELOOP;
}

static inline int __follow_up(struct vfsmount **mnt, struct dentry **base)
{
        struct vfsmount *parent;
        struct dentry *dentry;
        spin_lock(&dcache_lock);
        parent=(*mnt)->mnt_parent;
        if (parent == *mnt) {
                spin_unlock(&dcache_lock);
                return 0;
        }
        mntget(parent);
        dentry=dget((*mnt)->mnt_mountpoint);
        spin_unlock(&dcache_lock);
        dput(*base);
        *base = dentry;
        mntput(*mnt);
        *mnt = parent;
        return 1;
}

int follow_up(struct vfsmount **mnt, struct dentry **dentry)
{
        return __follow_up(mnt, dentry);
}

static inline int __follow_down(struct vfsmount **mnt, struct dentry **dentry)
{
        struct vfsmount *mounted;

        spin_lock(&dcache_lock);
        mounted = lookup_mnt(*mnt, *dentry);
        if (mounted) {
                *mnt = mntget(mounted);
                spin_unlock(&dcache_lock);
                dput(*dentry);
                mntput(mounted->mnt_parent);
                *dentry = dget(mounted->mnt_root);
                return 1;
        }
        spin_unlock(&dcache_lock);
        return 0;
}

int follow_down(struct vfsmount **mnt, struct dentry **dentry)
{
        return __follow_down(mnt,dentry);
}
 
static inline void follow_dotdot(struct nameidata *nd)
{
        while(1) {
                struct vfsmount *parent;
                struct dentry *dentry;
                read_lock(&current->fs->lock);
                if (nd->dentry == current->fs->root &&
                    nd->mnt == current->fs->rootmnt)  {
                        read_unlock(&current->fs->lock);
                        break;
                }
                read_unlock(&current->fs->lock);
                spin_lock(&dcache_lock);
                if (nd->dentry != nd->mnt->mnt_root) {
                        dentry = dget(nd->dentry->d_parent);
                        spin_unlock(&dcache_lock);
                        dput(nd->dentry);
                        nd->dentry = dentry;
                        break;
                }
                parent=nd->mnt->mnt_parent;
                if (parent == nd->mnt) {
                        spin_unlock(&dcache_lock);
                        break;
                }
                mntget(parent);
                dentry=dget(nd->mnt->mnt_mountpoint);
                spin_unlock(&dcache_lock);
                dput(nd->dentry);
                nd->dentry = dentry;
                mntput(nd->mnt);
                nd->mnt = parent;
        }
        while (d_mountpoint(nd->dentry) && __follow_down(&nd->mnt, &nd->dentry))
                ;
}

/*
 * Name resolution.
 *
 * This is the basic name resolution function, turning a pathname
 * into the final dentry.
 *
 * We expect 'base' to be positive and a directory.
 */
int link_path_walk(const char * name, struct nameidata *nd)
{
        struct dentry *dentry;
        struct inode *inode;
        int err;
        unsigned int lookup_flags = nd->flags;

        while (*name=='/')
                name++;
        if (!*name)
                goto return_reval;

        inode = nd->dentry->d_inode;
        if (current->link_count)
                lookup_flags = LOOKUP_FOLLOW;

        /* At this point we know we have a real path component. */
        for(;;) {
                unsigned long hash;
                struct qstr this;
                unsigned int c;

                err = permission(inode, MAY_EXEC);
                dentry = ERR_PTR(err);
                if (err)
                        break;

                this.name = name;
                c = *(const unsigned char *)name;

                hash = init_name_hash();
                do {
                        name++;
                        hash = partial_name_hash(c, hash);
                        c = *(const unsigned char *)name;
                } while (c && (c != '/'));
                this.len = name - (const char *) this.name;
                this.hash = end_name_hash(hash);

                /* remove trailing slashes? */
                if (!c)
                        goto last_component;
                while (*++name == '/');
                if (!*name)
                        goto last_with_slashes;

                /*
                 * "." and ".." are special - ".." especially so because it has
                 * to be able to know about the current root directory and
                 * parent relationships.
                 */
                if (this.name[0] == '.') switch (this.len) {
                        default:
                                break;
                        case 2: 
                                if (this.name[1] != '.')
                                        break;
                                follow_dotdot(nd);
                                inode = nd->dentry->d_inode;
                                /* fallthrough */
                        case 1:
                                continue;
                }
                /*
                 * See if the low-level filesystem might want
                 * to use its own hash..
                 */
                if (nd->dentry->d_op && nd->dentry->d_op->d_hash) {
                        err = nd->dentry->d_op->d_hash(nd->dentry, &this);
                        if (err < 0)
                                break;
                }
                /* This does the actual lookups.. */
                dentry = cached_lookup(nd->dentry, &this, LOOKUP_CONTINUE);
                if (!dentry) {
                        dentry = real_lookup(nd->dentry, &this, LOOKUP_CONTINUE);
                        err = PTR_ERR(dentry);
                        if (IS_ERR(dentry))
                                break;
                }
                /* Check mountpoints.. */
                while (d_mountpoint(dentry) && __follow_down(&nd->mnt, &dentry))
                        ;

                err = -ENOENT;
                inode = dentry->d_inode;
                if (!inode)
                        goto out_dput;
                err = -ENOTDIR; 
                if (!inode->i_op)
                        goto out_dput;

                if (inode->i_op->follow_link) {
                        err = do_follow_link(dentry, nd);
                        dput(dentry);
                        if (err)
                                goto return_err;
                        err = -ENOENT;
                        inode = nd->dentry->d_inode;
                        if (!inode)
                                break;
                        err = -ENOTDIR; 
                        if (!inode->i_op)
                                break;
                } else {
                        dput(nd->dentry);
                        nd->dentry = dentry;
                }
                err = -ENOTDIR; 
                if (!inode->i_op->lookup)
                        break;
                continue;
                /* here ends the main loop */

last_with_slashes:
                lookup_flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
last_component:
                if (lookup_flags & LOOKUP_PARENT)
                        goto lookup_parent;
                if (this.name[0] == '.') switch (this.len) {
                        default:
                                break;
                        case 2: 
                                if (this.name[1] != '.')
                                        break;
                                follow_dotdot(nd);
                                inode = nd->dentry->d_inode;
                                /* fallthrough */
                        case 1:
                                goto return_reval;
                }
                if (nd->dentry->d_op && nd->dentry->d_op->d_hash) {
                        err = nd->dentry->d_op->d_hash(nd->dentry, &this);
                        if (err < 0)
                                break;
                }
                dentry = cached_lookup(nd->dentry, &this, 0);
                if (!dentry) {
                        dentry = real_lookup(nd->dentry, &this, 0);
                        err = PTR_ERR(dentry);
                        if (IS_ERR(dentry))
                                break;
                }
                while (d_mountpoint(dentry) && __follow_down(&nd->mnt, &dentry))
                        ;
                inode = dentry->d_inode;
                if ((lookup_flags & LOOKUP_FOLLOW)
                    && inode && inode->i_op && inode->i_op->follow_link) {
                        err = do_follow_link(dentry, nd);
                        dput(dentry);
                        if (err)
                                goto return_err;
                        inode = nd->dentry->d_inode;
                } else {
                        dput(nd->dentry);
                        nd->dentry = dentry;
                }
                err = -ENOENT;
                if (!inode)
                        goto no_inode;
                if (lookup_flags & LOOKUP_DIRECTORY) {
                        err = -ENOTDIR; 
                        if (!inode->i_op || !inode->i_op->lookup)
                                break;
                }
                goto return_base;
no_inode:
                err = -ENOENT;
                if (lookup_flags & (LOOKUP_POSITIVE|LOOKUP_DIRECTORY))
                        break;
                goto return_base;
lookup_parent:
                nd->last = this;
                nd->last_type = LAST_NORM;
                if (this.name[0] != '.')
                        goto return_base;
                if (this.len == 1)
                        nd->last_type = LAST_DOT;
                else if (this.len == 2 && this.name[1] == '.')
                        nd->last_type = LAST_DOTDOT;
return_reval:
                /*
                 * We bypassed the ordinary revalidation routines.
                 * Check the cached dentry for staleness.
                 */
                dentry = nd->dentry;
                if (dentry && dentry->d_op && dentry->d_op->d_revalidate) {
                        err = -ESTALE;
                        if (!dentry->d_op->d_revalidate(dentry, 0)) {
                                d_invalidate(dentry);
                                break;
                        }
                }
return_base:
                return 0;
out_dput:
                dput(dentry);
                break;
        }
        path_release(nd);
return_err:
        return err;
}

int path_walk(const char * name, struct nameidata *nd)
{
        current->total_link_count = 0;
        return link_path_walk(name, nd);
}

/* SMP-safe */
/* returns 1 if everything is done */
static int __emul_lookup_dentry(const char *name, struct nameidata *nd)
{
        if (path_walk(name, nd))
                return 0;               /* something went wrong... */

        if (!nd->dentry->d_inode || S_ISDIR(nd->dentry->d_inode->i_mode)) {
                struct nameidata nd_root;
                /*
                 * NAME was not found in alternate root or it's a directory.  Try to find
                 * it in the normal root:
                 */
                nd_root.last_type = LAST_ROOT;
                nd_root.flags = nd->flags;
                read_lock(&current->fs->lock);
                nd_root.mnt = mntget(current->fs->rootmnt);
                nd_root.dentry = dget(current->fs->root);
                read_unlock(&current->fs->lock);
                if (path_walk(name, &nd_root))
                        return 1;
                if (nd_root.dentry->d_inode) {
                        path_release(nd);
                        nd->dentry = nd_root.dentry;
                        nd->mnt = nd_root.mnt;
                        nd->last = nd_root.last;
                        return 1;
                }
                path_release(&nd_root);
        }
        return 1;
}

void set_fs_altroot(void)
{
        char *emul = __emul_prefix();
        struct nameidata nd;
        struct vfsmount *mnt = NULL, *oldmnt;
        struct dentry *dentry = NULL, *olddentry;
        if (emul) {
                read_lock(&current->fs->lock);
                nd.mnt = mntget(current->fs->rootmnt);
                nd.dentry = dget(current->fs->root);
                read_unlock(&current->fs->lock);
                nd.flags = LOOKUP_FOLLOW|LOOKUP_DIRECTORY|LOOKUP_POSITIVE;
                if (path_walk(emul,&nd) == 0) {
                        mnt = nd.mnt;
                        dentry = nd.dentry;
                }
        }
        write_lock(&current->fs->lock);
        oldmnt = current->fs->altrootmnt;
        olddentry = current->fs->altroot;
        current->fs->altrootmnt = mnt;
        current->fs->altroot = dentry;
        write_unlock(&current->fs->lock);
        if (olddentry) {
                dput(olddentry);
                mntput(oldmnt);
        }
}

/* SMP-safe */
static inline int
walk_init_root(const char *name, struct nameidata *nd)
{
        read_lock(&current->fs->lock);
        if (current->fs->altroot && !(nd->flags & LOOKUP_NOALT)) {
                nd->mnt = mntget(current->fs->altrootmnt);
                nd->dentry = dget(current->fs->altroot);
                read_unlock(&current->fs->lock);
                if (__emul_lookup_dentry(name,nd))
                        return 0;
                read_lock(&current->fs->lock);
        }
        nd->mnt = mntget(current->fs->rootmnt);
        nd->dentry = dget(current->fs->root);
        read_unlock(&current->fs->lock);
        return 1;
}

/* SMP-safe */
int path_lookup(const char *path, unsigned flags, struct nameidata *nd)
{
        int error = 0;
        if (path_init(path, flags, nd))
                error = path_walk(path, nd);
        return error;
}


/* SMP-safe */
int path_init(const char *name, unsigned int flags, struct nameidata *nd)
{
        nd->last_type = LAST_ROOT; /* if there are only slashes... */
        nd->flags = flags;
        if (*name=='/')
                return walk_init_root(name,nd);
        read_lock(&current->fs->lock);
        nd->mnt = mntget(current->fs->pwdmnt);
        nd->dentry = dget(current->fs->pwd);
        read_unlock(&current->fs->lock);
        return 1;
}

/*
 * Restricted form of lookup. Doesn't follow links, single-component only,
 * needs parent already locked. Doesn't follow mounts.
 * SMP-safe.
 */
struct dentry * lookup_hash(struct qstr *name, struct dentry * base)
{
        struct dentry * dentry;
        struct inode *inode;
        int err;

        inode = base->d_inode;
        err = permission(inode, MAY_EXEC);
        dentry = ERR_PTR(err);
        if (err)
                goto out;

        /*
         * See if the low-level filesystem might want
         * to use its own hash..
         */
        if (base->d_op && base->d_op->d_hash) {
                err = base->d_op->d_hash(base, name);
                dentry = ERR_PTR(err);
                if (err < 0)
                        goto out;
        }

        dentry = cached_lookup(base, name, 0);
        if (!dentry) {
                struct dentry *new = d_alloc(base, name);
                dentry = ERR_PTR(-ENOMEM);
                if (!new)
                        goto out;
                lock_kernel();
                dentry = inode->i_op->lookup(inode, new);
                unlock_kernel();
                if (!dentry)
                        dentry = new;
                else
                        dput(new);
        }
out:
        return dentry;
}

/* SMP-safe */
struct dentry * lookup_one_len(const char * name, struct dentry * base, int len)
{
        unsigned long hash;
        struct qstr this;
        unsigned int c;

        this.name = name;
        this.len = len;
        if (!len)
                goto access;

        hash = init_name_hash();
        while (len--) {
                c = *(const unsigned char *)name++;
                if (c == '/' || c == '\0')
                        goto access;
                hash = partial_name_hash(c, hash);
        }
        this.hash = end_name_hash(hash);

        return lookup_hash(&this, base);
access:
        return ERR_PTR(-EACCES);
}

/*
 *      namei()
 *
 * is used by most simple commands to get the inode of a specified name.
 * Open, link etc use their own routines, but this is enough for things
 * like 'chmod' etc.
 *
 * namei exists in two versions: namei/lnamei. The only difference is
 * that namei follows links, while lnamei does not.
 * SMP-safe
 */
int __user_walk(const char *name, unsigned flags, struct nameidata *nd)
{
        char *tmp;
        int err;

        tmp = getname(name);
        err = PTR_ERR(tmp);
        if (!IS_ERR(tmp)) {
                err = 0;
                err = path_lookup(tmp, flags, nd);
                putname(tmp);
        }
        return err;
}

/*
 * It's inline, so penalty for filesystems that don't use sticky bit is
 * minimal.
 */
static inline int check_sticky(struct inode *dir, struct inode *inode)
{
        if (!(dir->i_mode & S_ISVTX))
                return 0;
        if (inode->i_uid == current->fsuid)
                return 0;
        if (dir->i_uid == current->fsuid)
                return 0;
        return !capable(CAP_FOWNER);
}

/*
 *      Check whether we can remove a link victim from directory dir, check
 *  whether the type of victim is right.
 *  1. We can't do it if dir is read-only (done in permission())
 *  2. We should have write and exec permissions on dir
 *  3. We can't remove anything from append-only dir
 *  4. We can't do anything with immutable dir (done in permission())
 *  5. If the sticky bit on dir is set we should either
 *      a. be owner of dir, or
 *      b. be owner of victim, or
 *      c. have CAP_FOWNER capability
 *  6. If the victim is append-only or immutable we can't do antyhing with
 *     links pointing to it.
 *  7. If we were asked to remove a directory and victim isn't one - ENOTDIR.
 *  8. If we were asked to remove a non-directory and victim isn't one - EISDIR.
 *  9. We can't remove a root or mountpoint.
 */
static inline int may_delete(struct inode *dir,struct dentry *victim, int isdir)
{
        int error;
        if (!victim->d_inode || victim->d_parent->d_inode != dir)
                return -ENOENT;
        error = permission(dir,MAY_WRITE | MAY_EXEC);
        if (error)
                return error;
        if (IS_APPEND(dir))
                return -EPERM;
        if (check_sticky(dir, victim->d_inode)||IS_APPEND(victim->d_inode)||
            IS_IMMUTABLE(victim->d_inode))
                return -EPERM;
        if (isdir) {
                if (!S_ISDIR(victim->d_inode->i_mode))
                        return -ENOTDIR;
                if (IS_ROOT(victim))
                        return -EBUSY;
        } else if (S_ISDIR(victim->d_inode->i_mode))
                return -EISDIR;
        if (IS_DEADDIR(dir))
                return -ENOENT;
        return 0;
}

/*      Check whether we can create an object with dentry child in directory
 *  dir.
 *  1. We can't do it if child already exists (open has special treatment for
 *     this case, but since we are inlined it's OK)
 *  2. We can't do it if dir is read-only (done in permission())
 *  3. We should have write and exec permissions on dir
 *  4. We can't do it if dir is immutable (done in permission())
 */
static inline int may_create(struct inode *dir, struct dentry *child) {
        if (child->d_inode)
                return -EEXIST;
        if (IS_DEADDIR(dir))
                return -ENOENT;
        return permission(dir,MAY_WRITE | MAY_EXEC);
}

/* 
 * Special case: O_CREAT|O_EXCL implies O_NOFOLLOW for security
 * reasons.
 *
 * O_DIRECTORY translates into forcing a directory lookup.
 */
static inline int lookup_flags(unsigned int f)
{
        unsigned long retval = LOOKUP_FOLLOW;

        if (f & O_NOFOLLOW)
                retval &= ~LOOKUP_FOLLOW;
        
        if ((f & (O_CREAT|O_EXCL)) == (O_CREAT|O_EXCL))
                retval &= ~LOOKUP_FOLLOW;
        
        if (f & O_DIRECTORY)
                retval |= LOOKUP_DIRECTORY;

        return retval;
}

int vfs_create(struct inode *dir, struct dentry *dentry, int mode)
{
        int error;

        mode &= S_IALLUGO;
        mode |= S_IFREG;

        down(&dir->i_zombie);
        error = may_create(dir, dentry);
        if (error)
                goto exit_lock;

        error = -EACCES;        /* shouldn't it be ENOSYS? */
        if (!dir->i_op || !dir->i_op->create)
                goto exit_lock;

        DQUOT_INIT(dir);
        lock_kernel();
        error = dir->i_op->create(dir, dentry, mode);
        unlock_kernel();
exit_lock:
        up(&dir->i_zombie);
        if (!error)
                inode_dir_notify(dir, DN_CREATE);
        return error;
}

/*
 *      open_namei()
 *
 * namei for open - this is in fact almost the whole open-routine.
 *
 * Note that the low bits of "flag" aren't the same as in the open
 * system call - they are 00 - no permissions needed
 *                        01 - read permission needed
 *                        10 - write permission needed
 *                        11 - read/write permissions needed
 * which is a lot more logical, and also allows the "no perm" needed
 * for symlinks (where the permissions are checked later).
 * SMP-safe
 */
int open_namei(const char * pathname, int flag, int mode, struct nameidata *nd)
{
        int acc_mode, error = 0;
        struct inode *inode;
        struct dentry *dentry;
        struct dentry *dir;
        int count = 0;

        acc_mode = ACC_MODE(flag);

        /*
         * The simplest case - just a plain lookup.
         */
        if (!(flag & O_CREAT)) {
                error = path_lookup(pathname, lookup_flags(flag), nd);
                if (error)
                        return error;
                dentry = nd->dentry;
                goto ok;
        }

        /*
         * Create - we need to know the parent.
         */
        error = path_lookup(pathname, LOOKUP_PARENT, nd);
        if (error)
                return error;

        /*
         * We have the parent and last component. First of all, check
         * that we are not asked to creat(2) an obvious directory - that
         * will not do.
         */
        error = -EISDIR;
        if (nd->last_type != LAST_NORM || nd->last.name[nd->last.len])
                goto exit;

        dir = nd->dentry;
        down(&dir->d_inode->i_sem);
        dentry = lookup_hash(&nd->last, nd->dentry);

do_last:
        error = PTR_ERR(dentry);
        if (IS_ERR(dentry)) {
                up(&dir->d_inode->i_sem);
                goto exit;
        }

        /* Negative dentry, just create the file */
        if (!dentry->d_inode) {
                error = vfs_create(dir->d_inode, dentry,
                                   mode & ~current->fs->umask);
                up(&dir->d_inode->i_sem);
                dput(nd->dentry);
                nd->dentry = dentry;
                if (error)
                        goto exit;
                /* Don't check for write permission, don't truncate */
                acc_mode = 0;
                flag &= ~O_TRUNC;
                goto ok;
        }

        /*
         * It already exists.
         */
        up(&dir->d_inode->i_sem);

        error = -EEXIST;
        if (flag & O_EXCL)
                goto exit_dput;

        if (d_mountpoint(dentry)) {
                error = -ELOOP;
                if (flag & O_NOFOLLOW)
                        goto exit_dput;
                while (__follow_down(&nd->mnt,&dentry) && d_mountpoint(dentry));
        }
        error = -ENOENT;
        if (!dentry->d_inode)
                goto exit_dput;
        if (dentry->d_inode->i_op && dentry->d_inode->i_op->follow_link)
                goto do_link;

        dput(nd->dentry);
        nd->dentry = dentry;
        error = -EISDIR;
        if (dentry->d_inode && S_ISDIR(dentry->d_inode->i_mode))
                goto exit;
ok:
        error = -ENOENT;
        inode = dentry->d_inode;
        if (!inode)
                goto exit;

        error = -ELOOP;
        if (S_ISLNK(inode->i_mode))
                goto exit;
        
        error = -EISDIR;
        if (S_ISDIR(inode->i_mode) && (flag & FMODE_WRITE))
                goto exit;

        error = permission(inode,acc_mode);
        if (error)
                goto exit;

        /*
         * FIFO's, sockets and device files are special: they don't
         * actually live on the filesystem itself, and as such you
         * can write to them even if the filesystem is read-only.
         */
        if (S_ISFIFO(inode->i_mode) || S_ISSOCK(inode->i_mode)) {
                flag &= ~O_TRUNC;
        } else if (S_ISBLK(inode->i_mode) || S_ISCHR(inode->i_mode)) {
                error = -EACCES;
                if (nd->mnt->mnt_flags & MNT_NODEV)
                        goto exit;

                flag &= ~O_TRUNC;
        } else {
                error = -EROFS;
                if (IS_RDONLY(inode) && (flag & 2))
                        goto exit;
        }
        /*
         * An append-only file must be opened in append mode for writing.
         */
        error = -EPERM;
        if (IS_APPEND(inode)) {
                if  ((flag & FMODE_WRITE) && !(flag & O_APPEND))
                        goto exit;
                if (flag & O_TRUNC)
                        goto exit;
        }

        /*
         * Ensure there are no outstanding leases on the file.
         */
        error = get_lease(inode, flag);
        if (error)
                goto exit;

        if (flag & O_TRUNC) {
                error = get_write_access(inode);
                if (error)
                        goto exit;

                /*
                 * Refuse to truncate files with mandatory locks held on them.
                 */
                error = locks_verify_locked(inode);
                if (!error) {
                        DQUOT_INIT(inode);
                        
                        error = do_truncate(dentry, 0);
                }
                put_write_access(inode);
                if (error)
                        goto exit;
        } else
                if (flag & FMODE_WRITE)
                        DQUOT_INIT(inode);

        return 0;

exit_dput:
        dput(dentry);
exit:
        path_release(nd);
        return error;

do_link:
        error = -ELOOP;
        if (flag & O_NOFOLLOW)
                goto exit_dput;
        /*
         * This is subtle. Instead of calling do_follow_link() we do the
         * thing by hands. The reason is that this way we have zero link_count
         * and path_walk() (called from ->follow_link) honoring LOOKUP_PARENT.
         * After that we have the parent and last component, i.e.
         * we are in the same situation as after the first path_walk().
         * Well, almost - if the last component is normal we get its copy
         * stored in nd->last.name and we will have to putname() it when we
         * are done. Procfs-like symlinks just set LAST_BIND.
         */
        UPDATE_ATIME(dentry->d_inode);
        error = dentry->d_inode->i_op->follow_link(dentry, nd);
        dput(dentry);
        if (error)
                return error;
        if (nd->last_type == LAST_BIND) {
                dentry = nd->dentry;
                goto ok;
        }
        error = -EISDIR;
        if (nd->last_type != LAST_NORM)
                goto exit;
        if (nd->last.name[nd->last.len]) {
                putname(nd->last.name);
                goto exit;
        }
        error = -ELOOP;
        if (count++==32) {
                putname(nd->last.name);
                goto exit;
        }
        dir = nd->dentry;
        down(&dir->d_inode->i_sem);
        dentry = lookup_hash(&nd->last, nd->dentry);
        putname(nd->last.name);
        goto do_last;
}

/* SMP-safe */
static struct dentry *lookup_create(struct nameidata *nd, int is_dir)
{
        struct dentry *dentry;

        down(&nd->dentry->d_inode->i_sem);
        dentry = ERR_PTR(-EEXIST);
        if (nd->last_type != LAST_NORM)
                goto fail;
        dentry = lookup_hash(&nd->last, nd->dentry);
        if (IS_ERR(dentry))
                goto fail;
        if (!is_dir && nd->last.name[nd->last.len] && !dentry->d_inode)
                goto enoent;
        return dentry;
enoent:
        dput(dentry);
        dentry = ERR_PTR(-ENOENT);
fail:
        return dentry;
}

int vfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
{
        int error = -EPERM;

        down(&dir->i_zombie);
        if ((S_ISCHR(mode) || S_ISBLK(mode)) && !capable(CAP_MKNOD))
                goto exit_lock;

        error = may_create(dir, dentry);
        if (error)
                goto exit_lock;

        error = -EPERM;
        if (!dir->i_op || !dir->i_op->mknod)
                goto exit_lock;

        DQUOT_INIT(dir);
        lock_kernel();
        error = dir->i_op->mknod(dir, dentry, mode, dev);
        unlock_kernel();
exit_lock:
        up(&dir->i_zombie);
        if (!error)
                inode_dir_notify(dir, DN_CREATE);
        return error;
}

asmlinkage long sys_mknod(const char * filename, int mode, dev_t dev)
{
        int error = 0;
        char * tmp;
        struct dentry * dentry;
        struct nameidata nd;

        if (S_ISDIR(mode))
                return -EPERM;
        tmp = getname(filename);
        if (IS_ERR(tmp))
                return PTR_ERR(tmp);

        error = path_lookup(tmp, LOOKUP_PARENT, &nd);
        if (error)
                goto out;
        dentry = lookup_create(&nd, 0);
        error = PTR_ERR(dentry);

        mode &= ~current->fs->umask;
        if (!IS_ERR(dentry)) {
                switch (mode & S_IFMT) {
                case 0: case S_IFREG:
                        error = vfs_create(nd.dentry->d_inode,dentry,mode);
                        break;
                case S_IFCHR: case S_IFBLK: case S_IFIFO: case S_IFSOCK:
                        error = vfs_mknod(nd.dentry->d_inode,dentry,mode,dev);
                        break;
                case S_IFDIR:
                        error = -EPERM;
                        break;
                default:
                        error = -EINVAL;
                }
                dput(dentry);
        }
        up(&nd.dentry->d_inode->i_sem);
        path_release(&nd);
out:
        putname(tmp);

        return error;
}

int vfs_mkdir(struct inode *dir, struct dentry *dentry, int mode)
{
        int error;

        down(&dir->i_zombie);
        error = may_create(dir, dentry);
        if (error)
                goto exit_lock;

        error = -EPERM;
        if (!dir->i_op || !dir->i_op->mkdir)
                goto exit_lock;

        DQUOT_INIT(dir);
        mode &= (S_IRWXUGO|S_ISVTX);
        lock_kernel();
        error = dir->i_op->mkdir(dir, dentry, mode);
        unlock_kernel();

exit_lock:
        up(&dir->i_zombie);
        if (!error)
                inode_dir_notify(dir, DN_CREATE);
        return error;
}

asmlinkage long sys_mkdir(const char * pathname, int mode)
{
        int error = 0;
        char * tmp;

        tmp = getname(pathname);
        error = PTR_ERR(tmp);
        if (!IS_ERR(tmp)) {
                struct dentry *dentry;
                struct nameidata nd;

                error = path_lookup(tmp, LOOKUP_PARENT, &nd);
                if (error)
                        goto out;
                dentry = lookup_create(&nd, 1);
                error = PTR_ERR(dentry);
                if (!IS_ERR(dentry)) {
                        error = vfs_mkdir(nd.dentry->d_inode, dentry,
                                          mode & ~current->fs->umask);
                        dput(dentry);
                }
                up(&nd.dentry->d_inode->i_sem);
                path_release(&nd);
out:
                putname(tmp);
        }

        return error;
}

/*
 * We try to drop the dentry early: we should have
 * a usage count of 2 if we're the only user of this
 * dentry, and if that is true (possibly after pruning
 * the dcache), then we drop the dentry now.
 *
 * A low-level filesystem can, if it choses, legally
 * do a
 *
 *      if (!d_unhashed(dentry))
 *              return -EBUSY;
 *
 * if it cannot handle the case of removing a directory
 * that is still in use by something else..
 */
static void d_unhash(struct dentry *dentry)
{
        dget(dentry);
        spin_lock(&dcache_lock);
        switch (atomic_read(&dentry->d_count)) {
        default:
                spin_unlock(&dcache_lock);
                shrink_dcache_parent(dentry);
                spin_lock(&dcache_lock);
                if (atomic_read(&dentry->d_count) != 2)
                        break;
        case 2:
                list_del_init(&dentry->d_hash);
        }
        spin_unlock(&dcache_lock);
}

int vfs_rmdir(struct inode *dir, struct dentry *dentry)
{
        int error;

        error = may_delete(dir, dentry, 1);
        if (error)
                return error;

        if (!dir->i_op || !dir->i_op->rmdir)
                return -EPERM;

        DQUOT_INIT(dir);

        double_down(&dir->i_zombie, &dentry->d_inode->i_zombie);
        d_unhash(dentry);
        if (d_mountpoint(dentry))
                error = -EBUSY;
        else {
                lock_kernel();
                error = dir->i_op->rmdir(dir, dentry);
                unlock_kernel();
                if (!error)
                        dentry->d_inode->i_flags |= S_DEAD;
        }
        double_up(&dir->i_zombie, &dentry->d_inode->i_zombie);
        if (!error) {
                inode_dir_notify(dir, DN_DELETE);
                d_delete(dentry);
        }
        dput(dentry);

        return error;
}

asmlinkage long sys_rmdir(const char * pathname)
{
        int error = 0;
        char * name;
        struct dentry *dentry;
        struct nameidata nd;

        name = getname(pathname);
        if(IS_ERR(name))
                return PTR_ERR(name);

        error = path_lookup(name, LOOKUP_PARENT, &nd);
        if (error)
                goto exit;

        switch(nd.last_type) {
                case LAST_DOTDOT:
                        error = -ENOTEMPTY;
                        goto exit1;
                case LAST_DOT:
                        error = -EINVAL;
                        goto exit1;
                case LAST_ROOT:
                        error = -EBUSY;
                        goto exit1;
        }
        down(&nd.dentry->d_inode->i_sem);
        dentry = lookup_hash(&nd.last, nd.dentry);
        error = PTR_ERR(dentry);
        if (!IS_ERR(dentry)) {
                error = vfs_rmdir(nd.dentry->d_inode, dentry);
                dput(dentry);
        }
        up(&nd.dentry->d_inode->i_sem);
exit1:
        path_release(&nd);
exit:
        putname(name);
        return error;
}

int vfs_unlink(struct inode *dir, struct dentry *dentry)
{
        int error;

        down(&dir->i_zombie);
        error = may_delete(dir, dentry, 0);
        if (!error) {
                error = -EPERM;
                if (dir->i_op && dir->i_op->unlink) {
                        DQUOT_INIT(dir);
                        if (d_mountpoint(dentry))
                                error = -EBUSY;
                        else {
                                lock_kernel();
                                error = dir->i_op->unlink(dir, dentry);
                                unlock_kernel();
                                if (!error)
                                        d_delete(dentry);
                        }
                }
        }
        up(&dir->i_zombie);
        if (!error)
                inode_dir_notify(dir, DN_DELETE);
        return error;
}

asmlinkage long sys_unlink(const char * pathname)
{
        int error = 0;
        char * name;
        struct dentry *dentry;
        struct nameidata nd;

        name = getname(pathname);
        if(IS_ERR(name))
                return PTR_ERR(name);

        error = path_lookup(name, LOOKUP_PARENT, &nd);
        if (error)
                goto exit;
        error = -EISDIR;
        if (nd.last_type != LAST_NORM)
                goto exit1;
        down(&nd.dentry->d_inode->i_sem);
        dentry = lookup_hash(&nd.last, nd.dentry);
        error = PTR_ERR(dentry);
        if (!IS_ERR(dentry)) {
                /* Why not before? Because we want correct error value */
                if (nd.last.name[nd.last.len])
                        goto slashes;
                error = vfs_unlink(nd.dentry->d_inode, dentry);
        exit2:
                dput(dentry);
        }
        up(&nd.dentry->d_inode->i_sem);
exit1:
        path_release(&nd);
exit:
        putname(name);

        return error;

slashes:
        error = !dentry->d_inode ? -ENOENT :
                S_ISDIR(dentry->d_inode->i_mode) ? -EISDIR : -ENOTDIR;
        goto exit2;
}

int vfs_symlink(struct inode *dir, struct dentry *dentry, const char *oldname)
{
        int error;

        down(&dir->i_zombie);
        error = may_create(dir, dentry);
        if (error)
                goto exit_lock;

        error = -EPERM;
        if (!dir->i_op || !dir->i_op->symlink)
                goto exit_lock;

        DQUOT_INIT(dir);
        lock_kernel();
        error = dir->i_op->symlink(dir, dentry, oldname);
        unlock_kernel();

exit_lock:
        up(&dir->i_zombie);
        if (!error)
                inode_dir_notify(dir, DN_CREATE);
        return error;
}

asmlinkage long sys_symlink(const char * oldname, const char * newname)
{
        int error = 0;
        char * from;
        char * to;

        from = getname(oldname);
        if(IS_ERR(from))
                return PTR_ERR(from);
        to = getname(newname);
        error = PTR_ERR(to);
        if (!IS_ERR(to)) {
                struct dentry *dentry;
                struct nameidata nd;

                error = path_lookup(to, LOOKUP_PARENT, &nd);
                if (error)
                        goto out;
                dentry = lookup_create(&nd, 0);
                error = PTR_ERR(dentry);
                if (!IS_ERR(dentry)) {
                        error = vfs_symlink(nd.dentry->d_inode, dentry, from);
                        dput(dentry);
                }
                up(&nd.dentry->d_inode->i_sem);
                path_release(&nd);
out:
                putname(to);
        }
        putname(from);
        return error;
}

int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry)
{
        struct inode *inode;
        int error;

        down(&dir->i_zombie);
        error = -ENOENT;
        inode = old_dentry->d_inode;
        if (!inode)
                goto exit_lock;

        error = may_create(dir, new_dentry);
        if (error)
                goto exit_lock;

        error = -EXDEV;
        if (dir->i_dev != inode->i_dev)
                goto exit_lock;

        /*
         * A link to an append-only or immutable file cannot be created.
         */
        error = -EPERM;
        if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
                goto exit_lock;
        if (!dir->i_op || !dir->i_op->link)
                goto exit_lock;

        DQUOT_INIT(dir);
        lock_kernel();
        error = dir->i_op->link(old_dentry, dir, new_dentry);
        unlock_kernel();

exit_lock:
        up(&dir->i_zombie);
        if (!error)
                inode_dir_notify(dir, DN_CREATE);
        return error;
}

/*
 * Hardlinks are often used in delicate situations.  We avoid
 * security-related surprises by not following symlinks on the
 * newname.  --KAB
 *
 * We don't follow them on the oldname either to be compatible
 * with linux 2.0, and to avoid hard-linking to directories
 * and other special files.  --ADM
 */
asmlinkage long sys_link(const char * oldname, const char * newname)
{
        int error;
        char * to;

        to = getname(newname);
        error = PTR_ERR(to);
        if (!IS_ERR(to)) {
                struct dentry *new_dentry;
                struct nameidata nd, old_nd;

                error = __user_walk(oldname, LOOKUP_POSITIVE, &old_nd);
                if (error)
                        goto exit;
                error = path_lookup(to, LOOKUP_PARENT, &nd);
                if (error)
                        goto out;
                error = -EXDEV;
                if (old_nd.mnt != nd.mnt)
                        goto out_release;
                new_dentry = lookup_create(&nd, 0);
                error = PTR_ERR(new_dentry);
                if (!IS_ERR(new_dentry)) {
                        error = vfs_link(old_nd.dentry, nd.dentry->d_inode, new_dentry);
                        dput(new_dentry);
                }
                up(&nd.dentry->d_inode->i_sem);
out_release:
                path_release(&nd);
out:
                path_release(&old_nd);
exit:
                putname(to);
        }
        return error;
}

/*
 * The worst of all namespace operations - renaming directory. "Perverted"
 * doesn't even start to describe it. Somebody in UCB had a heck of a trip...
 * Problems:
 *      a) we can get into loop creation. Check is done in is_subdir().
 *      b) race potential - two innocent renames can create a loop together.
 *         That's where 4.4 screws up. Current fix: serialization on
 *         sb->s_vfs_rename_sem. We might be more accurate, but that's another
 *         story.
 *      c) we have to lock _three_ objects - parents and victim (if it exists).
 *         And that - after we got ->i_sem on parents (until then we don't know
 *         whether the target exists at all, let alone whether it is a directory
 *         or not). Solution: ->i_zombie. Taken only after ->i_sem. Always taken
 *         on link creation/removal of any kind. And taken (without ->i_sem) on
 *         directory that will be removed (both in rmdir() and here).
 *      d) some filesystems don't support opened-but-unlinked directories,
 *         either because of layout or because they are not ready to deal with
 *         all cases correctly. The latter will be fixed (taking this sort of
 *         stuff into VFS), but the former is not going away. Solution: the same
 *         trick as in rmdir().
 *      e) conversion from fhandle to dentry may come in the wrong moment - when
 *         we are removing the target. Solution: we will have to grab ->i_zombie
 *         in the fhandle_to_dentry code. [FIXME - current nfsfh.c relies on
 *         ->i_sem on parents, which works but leads to some truely excessive
 *         locking].
 */
int vfs_rename_dir(struct inode *old_dir, struct dentry *old_dentry,
               struct inode *new_dir, struct dentry *new_dentry)
{
        int error;
        struct inode *target;

        if (old_dentry->d_inode == new_dentry->d_inode)
                return 0;

        error = may_delete(old_dir, old_dentry, 1);
        if (error)
                return error;

        if (new_dir->i_dev != old_dir->i_dev)
                return -EXDEV;

        if (!new_dentry->d_inode)
                error = may_create(new_dir, new_dentry);
        else
                error = may_delete(new_dir, new_dentry, 1);
        if (error)
                return error;

        if (!old_dir->i_op || !old_dir->i_op->rename)
                return -EPERM;

        /*
         * If we are going to change the parent - check write permissions,
         * we'll need to flip '..'.
         */
        if (new_dir != old_dir) {
                error = permission(old_dentry->d_inode, MAY_WRITE);
        }
        if (error)
                return error;

        DQUOT_INIT(old_dir);
        DQUOT_INIT(new_dir);
        down(&old_dir->i_sb->s_vfs_rename_sem);
        error = -EINVAL;
        if (is_subdir(new_dentry, old_dentry))
                goto out_unlock;
        /* Don't eat your daddy, dear... */
        /* This also avoids locking issues */
        if (old_dentry->d_parent == new_dentry)
                goto out_unlock;
        target = new_dentry->d_inode;
        if (target) { /* Hastur! Hastur! Hastur! */
                triple_down(&old_dir->i_zombie,
                            &new_dir->i_zombie,
                            &target->i_zombie);
                d_unhash(new_dentry);
        } else
                double_down(&old_dir->i_zombie,
                            &new_dir->i_zombie);
        if (d_mountpoint(old_dentry)||d_mountpoint(new_dentry))
                error = -EBUSY;
        else 
                error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry);
        if (target) {
                if (!error)
                        target->i_flags |= S_DEAD;
                triple_up(&old_dir->i_zombie,
                          &new_dir->i_zombie,
                          &target->i_zombie);
                if (d_unhashed(new_dentry))
                        d_rehash(new_dentry);
                dput(new_dentry);
        } else
                double_up(&old_dir->i_zombie,
                          &new_dir->i_zombie);
                
        if (!error)
                d_move(old_dentry,new_dentry);
out_unlock:
        up(&old_dir->i_sb->s_vfs_rename_sem);
        return error;
}

int vfs_rename_other(struct inode *old_dir, struct dentry *old_dentry,
               struct inode *new_dir, struct dentry *new_dentry)
{
        int error;

        if (old_dentry->d_inode == new_dentry->d_inode)
                return 0;

        error = may_delete(old_dir, old_dentry, 0);
        if (error)
                return error;

        if (new_dir->i_dev != old_dir->i_dev)
                return -EXDEV;

        if (!new_dentry->d_inode)
                error = may_create(new_dir, new_dentry);
        else
                error = may_delete(new_dir, new_dentry, 0);
        if (error)
                return error;

        if (!old_dir->i_op || !old_dir->i_op->rename)
                return -EPERM;

        DQUOT_INIT(old_dir);
        DQUOT_INIT(new_dir);
        double_down(&old_dir->i_zombie, &new_dir->i_zombie);
        if (d_mountpoint(old_dentry)||d_mountpoint(new_dentry))
                error = -EBUSY;
        else
                error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry);
        double_up(&old_dir->i_zombie, &new_dir->i_zombie);
        if (error)
                return error;
        /* The following d_move() should become unconditional */
        if (!(old_dir->i_sb->s_type->fs_flags & FS_ODD_RENAME)) {
                d_move(old_dentry, new_dentry);
        }
        return 0;
}

int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
               struct inode *new_dir, struct dentry *new_dentry)
{
        int error;
        if (S_ISDIR(old_dentry->d_inode->i_mode))
                error = vfs_rename_dir(old_dir,old_dentry,new_dir,new_dentry);
        else
                error = vfs_rename_other(old_dir,old_dentry,new_dir,new_dentry);
        if (!error) {
                if (old_dir == new_dir)
                        inode_dir_notify(old_dir, DN_RENAME);
                else {
                        inode_dir_notify(old_dir, DN_DELETE);
                        inode_dir_notify(new_dir, DN_CREATE);
                }
        }
        return error;
}

static inline int do_rename(const char * oldname, const char * newname)
{
        int error = 0;
        struct dentry * old_dir, * new_dir;
        struct dentry * old_dentry, *new_dentry;
        struct nameidata oldnd, newnd;

        error = path_lookup(oldname, LOOKUP_PARENT, &oldnd);
        if (error)
                goto exit;

        error = path_lookup(newname, LOOKUP_PARENT, &newnd);
        if (error)
                goto exit1;

        error = -EXDEV;
        if (oldnd.mnt != newnd.mnt)
                goto exit2;

        old_dir = oldnd.dentry;
        error = -EBUSY;
        if (oldnd.last_type != LAST_NORM)
                goto exit2;

        new_dir = newnd.dentry;
        if (newnd.last_type != LAST_NORM)
                goto exit2;

        double_lock(new_dir, old_dir);

        old_dentry = lookup_hash(&oldnd.last, old_dir);
        error = PTR_ERR(old_dentry);
        if (IS_ERR(old_dentry))
                goto exit3;
        /* source must exist */
        error = -ENOENT;
        if (!old_dentry->d_inode)
                goto exit4;
        /* unless the source is a directory trailing slashes give -ENOTDIR */
        if (!S_ISDIR(old_dentry->d_inode->i_mode)) {
                error = -ENOTDIR;
                if (oldnd.last.name[oldnd.last.len])
                        goto exit4;
                if (newnd.last.name[newnd.last.len])
                        goto exit4;
        }
        new_dentry = lookup_hash(&newnd.last, new_dir);
        error = PTR_ERR(new_dentry);
        if (IS_ERR(new_dentry))
                goto exit4;

        lock_kernel();
        error = vfs_rename(old_dir->d_inode, old_dentry,
                                   new_dir->d_inode, new_dentry);
        unlock_kernel();

        dput(new_dentry);
exit4:
        dput(old_dentry);
exit3:
        double_up(&new_dir->d_inode->i_sem, &old_dir->d_inode->i_sem);
exit2:
        path_release(&newnd);
exit1:
        path_release(&oldnd);
exit:
        return error;
}

asmlinkage long sys_rename(const char * oldname, const char * newname)
{
        int error;
        char * from;
        char * to;

        from = getname(oldname);
        if(IS_ERR(from))
                return PTR_ERR(from);
        to = getname(newname);
        error = PTR_ERR(to);
        if (!IS_ERR(to)) {
                error = do_rename(from,to);
                putname(to);
        }
        putname(from);
        return error;
}

int vfs_readlink(struct dentry *dentry, char *buffer, int buflen, const char *link)
{
        int len;

        len = PTR_ERR(link);
        if (IS_ERR(link))
                goto out;

        len = strlen(link);
        if (len > (unsigned) buflen)
                len = buflen;
        if (copy_to_user(buffer, link, len))
                len = -EFAULT;
out:
        return len;
}

static inline int
__vfs_follow_link(struct nameidata *nd, const char *link)
{
        int res = 0;
        char *name;
        if (IS_ERR(link))
                goto fail;

        if (*link == '/') {
                path_release(nd);
                if (!walk_init_root(link, nd))
                        /* weird __emul_prefix() stuff did it */
                        goto out;
        }
        res = link_path_walk(link, nd);
out:
        if (current->link_count || res || nd->last_type!=LAST_NORM)
                return res;
        /*
         * If it is an iterative symlinks resolution in open_namei() we
         * have to copy the last component. And all that crap because of
         * bloody create() on broken symlinks. Furrfu...
         */
        name = __getname();
        if (!name) {
                path_release(nd);
                return -ENOMEM;
        }
        strcpy(name, nd->last.name);
        nd->last.name = name;
        return 0;
fail:
        path_release(nd);
        return PTR_ERR(link);
}

int vfs_follow_link(struct nameidata *nd, const char *link)
{
        return __vfs_follow_link(nd, link);
}

/* get the link contents into pagecache */
static char *page_getlink(struct dentry * dentry, struct page **ppage)
{
        struct page * page;
        struct address_space *mapping = dentry->d_inode->i_mapping;
        page = read_cache_page(mapping, 0, (filler_t *)mapping->a_ops->readpage,
                                NULL);
        if (IS_ERR(page))
                goto sync_fail;
        wait_on_page(page);
        if (!Page_Uptodate(page))
                goto async_fail;
        *ppage = page;
        return kmap(page);

async_fail:
        page_cache_release(page);
        return ERR_PTR(-EIO);

sync_fail:
        return (char*)page;
}

int page_readlink(struct dentry *dentry, char *buffer, int buflen)
{
        struct page *page = NULL;
        char *s = page_getlink(dentry, &page);
        int res = vfs_readlink(dentry,buffer,buflen,s);
        if (page) {
                kunmap(page);
                page_cache_release(page);
        }
        return res;
}

int page_follow_link(struct dentry *dentry, struct nameidata *nd)
{
        struct page *page = NULL;
        char *s = page_getlink(dentry, &page);
        int res = __vfs_follow_link(nd, s);
        if (page) {
                kunmap(page);
                page_cache_release(page);
        }
        return res;
}

struct inode_operations page_symlink_inode_operations = {
        readlink:       page_readlink,
        follow_link:    page_follow_link,
};