reduce vfs_path_lookup() to do_path_lookup()

New lookup flag: LOOKUP_ROOT.  nd->root is set (and held) by caller,
path_init() starts walking from that place and all pathname resolution
machinery never drops nd->root if that flag is set.  That turns
vfs_path_lookup() into a special case of do_path_lookup() *and*
gets us down to 3 callers of link_path_walk(), making it finally
feasible to rip the handling of trailing symlink out of link_path_walk().
That will not only simply the living hell out of it, but make life
much simpler for unionfs merge.  Trailing symlink handling will
become iterative, which is a good thing for stack footprint in
a lot of situations as well.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 43 insertions, 52 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 0bebd13e5cb7..8ee7785d5642 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -401,9 +401,11 @@ static int nameidata_drop_rcu(struct nameidata *nd)
 {
         struct fs_struct *fs = current->fs;
         struct dentry *dentry = nd->path.dentry;
+        int want_root = 0;
 
         BUG_ON(!(nd->flags & LOOKUP_RCU));
-        if (nd->root.mnt) {
+        if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
+                want_root = 1;
                 spin_lock(&fs->lock);
                 if (nd->root.mnt != fs->root.mnt ||
                                 nd->root.dentry != fs->root.dentry)
@@ -414,7 +416,7 @@ static int nameidata_drop_rcu(struct nameidata *nd)
                 goto err;
         BUG_ON(nd->inode != dentry->d_inode);
         spin_unlock(&dentry->d_lock);
-        if (nd->root.mnt) {
+        if (want_root) {
                 path_get(&nd->root);
                 spin_unlock(&fs->lock);
         }
@@ -427,7 +429,7 @@ static int nameidata_drop_rcu(struct nameidata *nd)
 err:
         spin_unlock(&dentry->d_lock);
 err_root:
-        if (nd->root.mnt)
+        if (want_root)
                 spin_unlock(&fs->lock);
         return -ECHILD;
 }
@@ -454,9 +456,11 @@ static int nameidata_dentry_drop_rcu(struct nameidata *nd, struct dentry *dentry
 {
         struct fs_struct *fs = current->fs;
         struct dentry *parent = nd->path.dentry;
+        int want_root = 0;
 
         BUG_ON(!(nd->flags & LOOKUP_RCU));
-        if (nd->root.mnt) {
+        if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
+                want_root = 1;
                 spin_lock(&fs->lock);
                 if (nd->root.mnt != fs->root.mnt ||
                                 nd->root.dentry != fs->root.dentry)
@@ -476,7 +480,7 @@ static int nameidata_dentry_drop_rcu(struct nameidata *nd, struct dentry *dentry
         parent->d_count++;
         spin_unlock(&dentry->d_lock);
         spin_unlock(&parent->d_lock);
-        if (nd->root.mnt) {
+        if (want_root) {
                 path_get(&nd->root);
                 spin_unlock(&fs->lock);
         }
@@ -490,7 +494,7 @@ err:
         spin_unlock(&dentry->d_lock);
         spin_unlock(&parent->d_lock);
 err_root:
-        if (nd->root.mnt)
+        if (want_root)
                 spin_unlock(&fs->lock);
         return -ECHILD;
 }
@@ -501,7 +505,8 @@ static inline int nameidata_dentry_drop_rcu_maybe(struct nameidata *nd, struct d
         if (nd->flags & LOOKUP_RCU) {
                 if (unlikely(nameidata_dentry_drop_rcu(nd, dentry))) {
                         nd->flags &= ~LOOKUP_RCU;
-                        nd->root.mnt = NULL;
+                        if (!(nd->flags & LOOKUP_ROOT))
+                                nd->root.mnt = NULL;
                         rcu_read_unlock();
                         br_read_unlock(vfsmount_lock);
                         return -ECHILD;
@@ -525,7 +530,8 @@ static int nameidata_drop_rcu_last(struct nameidata *nd)
 
         BUG_ON(!(nd->flags & LOOKUP_RCU));
         nd->flags &= ~LOOKUP_RCU;
-        nd->root.mnt = NULL;
+        if (!(nd->flags & LOOKUP_ROOT))
+                nd->root.mnt = NULL;
         spin_lock(&dentry->d_lock);
         if (!__d_rcu_to_refcount(dentry, nd->seq))
                 goto err_unlock;
@@ -1053,7 +1059,8 @@ static int follow_dotdot_rcu(struct nameidata *nd)
 
 failed:
         nd->flags &= ~LOOKUP_RCU;
-        nd->root.mnt = NULL;
+        if (!(nd->flags & LOOKUP_ROOT))
+                nd->root.mnt = NULL;
         rcu_read_unlock();
         br_read_unlock(vfsmount_lock);
         return -ECHILD;
@@ -1310,7 +1317,8 @@ static void terminate_walk(struct nameidata *nd)
                 path_put(&nd->path);
         } else {
                 nd->flags &= ~LOOKUP_RCU;
-                nd->root.mnt = NULL;
+                if (!(nd->flags & LOOKUP_ROOT))
+                        nd->root.mnt = NULL;
                 rcu_read_unlock();
                 br_read_unlock(vfsmount_lock);
         }
@@ -1477,6 +1485,25 @@ static int path_init(int dfd, const char *name, unsigned int flags,
         nd->last_type = LAST_ROOT; /* if there are only slashes... */
         nd->flags = flags | LOOKUP_JUMPED;
         nd->depth = 0;
+        if (flags & LOOKUP_ROOT) {
+                struct inode *inode = nd->root.dentry->d_inode;
+                if (!inode->i_op->lookup)
+                        return -ENOTDIR;
+                retval = inode_permission(inode, MAY_EXEC);
+                if (retval)
+                        return retval;
+                nd->path = nd->root;
+                nd->inode = inode;
+                if (flags & LOOKUP_RCU) {
+                        br_read_lock(vfsmount_lock);
+                        rcu_read_lock();
+                        nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
+                } else {
+                        path_get(&nd->path);
+                }
+                return 0;
+        }
+
         nd->root.mnt = NULL;
 
         if (*name=='/') {
@@ -1587,7 +1614,7 @@ static int path_lookupat(int dfd, const char *name,
         if (base)
                 fput(base);
 
-        if (nd->root.mnt) {
+        if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
                 path_put(&nd->root);
                 nd->root.mnt = NULL;
         }
@@ -1638,46 +1665,10 @@ int vfs_path_lookup(struct dentry *dentry, struct vfsmount *mnt,
                     const char *name, unsigned int flags,
                     struct nameidata *nd)
 {
-        int result;
-
-        /* same as do_path_lookup */
-        nd->last_type = LAST_ROOT;
-        nd->flags = flags | LOOKUP_JUMPED;
-        nd->depth = 0;
-
-        nd->path.dentry = dentry;
-        nd->path.mnt = mnt;
-        path_get(&nd->path);
-        nd->root = nd->path;
-        path_get(&nd->root);
-        nd->inode = nd->path.dentry->d_inode;
-
-        current->total_link_count = 0;
-
-        result = link_path_walk(name, nd);
-        if (!result)
-                result = handle_reval_path(nd);
-        if (result == -ESTALE) {
-                /* nd->path had been dropped */
-                current->total_link_count = 0;
-                nd->path.dentry = dentry;
-                nd->path.mnt = mnt;
-                nd->inode = dentry->d_inode;
-                path_get(&nd->path);
-                nd->flags = flags | LOOKUP_JUMPED | LOOKUP_REVAL;
-
-                result = link_path_walk(name, nd);
-                if (!result)
-                        result = handle_reval_path(nd);
-        }
-        if (unlikely(!result && !audit_dummy_context() && nd->path.dentry &&
-                                nd->inode))
-                audit_inode(name, nd->path.dentry);
-
-        path_put(&nd->root);
-        nd->root.mnt = NULL;
-
-        return result;
+        nd->root.dentry = dentry;
+        nd->root.mnt = mnt;
+        /* the first argument of do_path_lookup() is ignored with LOOKUP_ROOT */
+        return do_path_lookup(AT_FDCWD, name, flags | LOOKUP_ROOT, nd);
 }
 
 static struct dentry *__lookup_hash(struct qstr *name,
@@ -2320,7 +2311,7 @@ static struct file *path_openat(int dfd, const char *pathname,
                 path_put(&link);
         }
 out:
-        if (nd.root.mnt)
+        if (nd.root.mnt && !(nd.flags & LOOKUP_ROOT))
                 path_put(&nd.root);
         if (base)
                 fput(base);