Re: NFS/d_splice_alias breakage

[Oleg Drokin <green@linuxhacker.ru>]

It looks like this patch was totally forgotten?
I don't see it in neither vfs nor nfs trees and yet it fixes a very easy to cause
crash in nfs code. And I think it's unrelated to the other parallel case too.

On Jun 3, 2016, at 1:56 AM, Al Viro wrote:

> 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
> d_splice_alias()).
> 
> Cc: stable@vger.kernel.org # v3.10+
> Tested-by: Oleg Drokin <green@linuxhacker.ru>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
> 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);
> 		put_nfs_open_context(ctx);
> +		d_drop(dentry);
> 		switch (err) {
> 		case -ENOENT:
> -			d_drop(dentry);
> 			d_add(dentry, NULL);
> 			nfs_set_verifier(dentry, nfs_save_change_attribute(dir));
> 			break;