Re: NFS/d_splice_alias breakage

From: Al Viro
Date: Fri Jun 03 2016 - 00:27:02 EST

On Thu, Jun 02, 2016 at 11:43:59PM -0400, Oleg Drokin wrote:

> > Which of the call sites had that been and how does one reproduce that fun?
> > If you feel that posting a reproducer in the open is a bad idea, just send
> > it off-list...
> This is fs/nfs/dir.c::nfs_lookup() right after no_entry label.

Bloody hell... Was that sucker hashed on the entry into nfs_lookup()?
If we get that with call as a method, we are really fucked.


Ho-hum... One of the goto no_open; in nfs_atomic_open()? That would
mean a stale negative dentry in dcache that has escaped revalidation...
Wait a minute, didn't nfs ->d_revalidate() skip everything in some
cases, leaving it to ->atomic_open()?

Looks like the right thing to do would be to do d_drop() at no_open:,
just before we call nfs_lookup(). If not earlier, actually... How
about the following?

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));