Re: [PATCH] xfs: remove racy hasattr check from attr get

[Brian Foster <bfoster@redhat.com>]

On Sun, Jan 08, 2017 at 07:30:08AM -0800, Christoph Hellwig wrote:
> On Fri, Jan 06, 2017 at 02:37:12PM -0500, Brian Foster wrote:
> > xfs_attr_get() has an unlocked attribute fork check to optimize away a
> > lock cycle in cases where the fork does not exist or is otherwise empty.
> > This check is not safe, however, because an attribute fork short form to
> > extent format conversion includes a transient state that causes the
> > xfs_inode_hasattr() check to fail. Specifically,
> > xfs_attr_shortform_to_leaf() creates an empty extent format attribute
> > fork and then adds the existing shortform attributes to it.
> > 
> > This means that lookup of an existing xattr can spuriously return
> > -ENOATTR when racing against a setxattr that causes the associated
> > format conversion. This was originally reproduced by an untar on a
> > particularly configured glusterfs volume, but can also be reproduced on
> > demand with properly crafted xattr requests.
> > 
> > The format conversion occurs under the exclusive ilock. xfs_attr_get()
> > already has the proper locking and checks further down in the function
> > to handle this situation correctly. Drop the unlocked check to avoid the
> > spurious failure and rely on the existing logic.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> > 
> > This survives a local xfstests run and the original glusterfs
> > reproducer. I also have an xfstests reproducer that I will post shortly.
> > 
> > Brian
> > 
> >  fs/xfs/libxfs/xfs_attr.c | 3 ---
> >  1 file changed, 3 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_attr.c b/fs/xfs/libxfs/xfs_attr.c
> > index af1ecb1..8fcbc52 100644
> > --- a/fs/xfs/libxfs/xfs_attr.c
> > +++ b/fs/xfs/libxfs/xfs_attr.c
> > @@ -131,9 +131,6 @@ xfs_attr_get(
> >  	if (XFS_FORCED_SHUTDOWN(ip->i_mount))
> >  		return -EIO;
> >  
> > -	if (!xfs_inode_hasattr(ip))
> > -		return -ENOATTR;
> > -
> 
> What about the similar pre-lock check in xfs_attr_remove?

Yep, will fix. Thanks for catching that..

Brian

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