Re: [PATCH] VFS: Add back check for !inode in walk_component()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, May 07, 2015 at 07:13:35PM +0100, Al Viro wrote:
> On Thu, May 07, 2015 at 01:39:35PM -0400, Steven Rostedt wrote:
> > I had them printed in my previous traces. The flags were 0x200088, and
> > they were 0 just before the call.
> 
> Not dentry->d_flags, nd->flags.  Most interesting part is bit 6 in those
> (LOOKUP_RCU, 0x40).
> 
> As for creation...  I think I see what might be going on:
> 
> A: finds a negative dentry, picks NULL ->d_inode from it and whatever
> ->d_seq it had.
> B: d_instantiate(): sets ->d_inode non-NULL, ->d_flags accordingly and
> bumps ->d_seq.
> A: fetches ->d_flags, sees non-negative, assumes ->d_inode is non-NULL.
> 
> In reality, the last assumption should've been "->d_inode is non-NULL or
> we have a stale ->d_seq and will end up discarding that fscker anyway".
> 
> Hmm...  Smells like we ought to
[snip]

Actually, could you try the following on top of -rc2?

diff --git a/fs/namei.c b/fs/namei.c
index 4a8d998b..421e597 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1415,6 +1415,7 @@ static int lookup_fast(struct nameidata *nd,
 	 */
 	if (nd->flags & LOOKUP_RCU) {
 		unsigned seq;
+		bool negative;
 		dentry = __d_lookup_rcu(parent, &nd->last, &seq);
 		if (!dentry)
 			goto unlazy;
@@ -1424,8 +1425,11 @@ static int lookup_fast(struct nameidata *nd,
 		 * the dentry name information from lookup.
 		 */
 		*inode = dentry->d_inode;
+		negative = d_is_negative(dentry);
 		if (read_seqcount_retry(&dentry->d_seq, seq))
 			return -ECHILD;
+		if (negative)
+			return -ENOENT;
 
 		/*
 		 * This sequence count validates that the parent had no
@@ -1472,9 +1476,13 @@ unlazy:
 		goto need_lookup;
 	}
 
-	path->mnt = mnt;
-	path->dentry = dentry;
-	err = follow_managed(path, nd->flags);
+	if (unlikely(d_is_negative(dentry))) {
+		err = -ENOENT;
+	} else {
+		path->mnt = mnt;
+		path->dentry = dentry;
+		err = follow_managed(path, nd->flags);
+	}
 	if (unlikely(err < 0)) {
 		path_put_conditional(path, nd);
 		return err;
@@ -1583,10 +1591,10 @@ static inline int walk_component(struct nameidata *nd, struct path *path,
 			goto out_err;
 
 		inode = path->dentry->d_inode;
+		err = -ENOENT;
+		if (d_is_negative(path->dentry))
+			goto out_path_put;
 	}
-	err = -ENOENT;
-	if (d_is_negative(path->dentry))
-		goto out_path_put;
 
 	if (should_follow_link(path->dentry, follow)) {
 		if (nd->flags & LOOKUP_RCU) {
@@ -3036,14 +3044,13 @@ retry_lookup:
 
 	BUG_ON(nd->flags & LOOKUP_RCU);
 	inode = path->dentry->d_inode;
-finish_lookup:
-	/* we _can_ be in RCU mode here */
 	error = -ENOENT;
 	if (d_is_negative(path->dentry)) {
 		path_to_nameidata(path, nd);
 		goto out;
 	}
-
+finish_lookup:
+	/* we _can_ be in RCU mode here */
 	if (should_follow_link(path->dentry, !symlink_ok)) {
 		if (nd->flags & LOOKUP_RCU) {
 			if (unlikely(nd->path.mnt != path->mnt ||