Re: NFS/d_splice_alias breakage

From: Al Viro
Date: Fri Jun 03 2016 - 01:57:06 EST

On Fri, Jun 03, 2016 at 12:58:10AM -0400, Oleg Drokin wrote:

> This one cures the insta-crash I was having, and I see no other ill-effects so far.

OK... I can take it through vfs.git, but I think it'd be better off in
NFS tree. Is everyone OK with something like the following?

make nfs_atomic_open() call d_drop() on all ->open_context() errors.

In "NFSv4: Move dentry instantiation into the NFSv4-specific atomic open code"
unconditional d_drop() after the ->open_context() had been removed. It had
been correct for success cases (there ->open_context() itself had been doing
dcache manipulations), but not for error ones. Only one of those (ENOENT)
got a compensatory d_drop() added in that commit, but in fact it should've
been done for all errors. As it is, the case of O_CREAT non-exclusive open
on a hashed negative dentry racing with e.g. symlink creation from another
client ended up with ->open_context() getting an error and proceeding to
call nfs_lookup(). On a hashed dentry, which would've instantly triggered
BUG_ON() in d_materialise_unique() (or, these days, its equivalent in

Cc: stable@xxxxxxxxxxxxxxx # v3.10+
Tested-by: Oleg Drokin <green@xxxxxxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index aaf7bd0..6e3a6f4 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1536,9 +1536,9 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry,
err = PTR_ERR(inode);
trace_nfs_atomic_open_exit(dir, ctx, open_flags, err);
+ d_drop(dentry);
switch (err) {
case -ENOENT:
- d_drop(dentry);
d_add(dentry, NULL);
nfs_set_verifier(dentry, nfs_save_change_attribute(dir));