[PATCH] autofs4: check for directory re-create in lookup

This problem was identified and fixed some time ago by Jeff Moyer but it fell
through the cracks somehow.

It is possible that a user space application could remove and re-create a
directory during a request.  To avoid returning a failure from lookup
incorrectly when our current dentry is unhashed we need to check if another
positive, hashed dentry matching this one exists and if so return it instead
of a fail.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 4 deletions

diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index 47adf270106..b4631046867 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -655,14 +655,29 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
 
         /*
          * If this dentry is unhashed, then we shouldn't honour this
-         * lookup even if the dentry is positive.  Returning ENOENT here
-         * doesn't do the right thing for all system calls, but it should
-         * be OK for the operations we permit from an autofs.
+         * lookup.  Returning ENOENT here doesn't do the right thing
+         * for all system calls, but it should be OK for the operations
+         * we permit from an autofs.
          */
         if (dentry->d_inode && d_unhashed(dentry)) {
+                /*
+                 * A user space application can (and has done in the past)
+                 * remove and re-create this directory during the callback.
+                 * This can leave us with an unhashed dentry, but a
+                 * successful mount!  So we need to perform another
+                 * cached lookup in case the dentry now exists.
+                 */
+                struct dentry *parent = dentry->d_parent;
+                struct dentry *new = d_lookup(parent, &dentry->d_name);
+                if (new != NULL)
+                        dentry = new;
+                else
+                        dentry = ERR_PTR(-ENOENT);
+
                 if (unhashed)
                         dput(unhashed);
-                return ERR_PTR(-ENOENT);
+
+                return dentry;
         }
 
         if (unhashed)