Re: [PATCH] knfsd: fix the fallback implementation of the get_name export operation

[Trond Myklebust <trondmy@hammerspace.com>]

On Sat, 2023-12-30 at 08:23 +0200, Amir Goldstein wrote:
> On Sat, Dec 30, 2023 at 1:50 AM Trond Myklebust
> <trondmy@hammerspace.com> wrote:
> > 
> > On Fri, 2023-12-29 at 18:29 -0500, Chuck Lever wrote:
> > > On Fri, Dec 29, 2023 at 07:44:20PM +0200, Amir Goldstein wrote:
> > > > On Fri, Dec 29, 2023 at 4:35 PM Chuck Lever
> > > > <chuck.lever@oracle.com> wrote:
> > > > > 
> > > > > On Fri, Dec 29, 2023 at 07:46:54AM +0200, Amir Goldstein
> > > > > wrote:
> > > > > > [CC: fsdevel, viro]
> > > > > 
> > > > > Thanks for picking this up, Amir, and for copying
> > > > > viro/fsdevel. I
> > > > > was planning to repost this next week when more folks are
> > > > > back,
> > > > > but
> > > > > this works too.
> > > > > 
> > > > > Trond, if you'd like, I can handle review changes if you
> > > > > don't
> > > > > have
> > > > > time to follow up.
> > > > > 
> > > > > 
> > > > > > On Thu, Dec 28, 2023 at 10:22 PM <trondmy@kernel.org>
> > > > > > wrote:
> > > > > > > 
> > > > > > > From: Trond Myklebust <trond.myklebust@hammerspace.com>
> > > > > > > 
> > > > > > > The fallback implementation for the get_name export
> > > > > > > operation
> > > > > > > uses
> > > > > > > readdir() to try to match the inode number to a filename.
> > > > > > > That filename
> > > > > > > is then used together with lookup_one() to produce a
> > > > > > > dentry.
> > > > > > > A problem arises when we match the '.' or '..' entries,
> > > > > > > since
> > > > > > > that
> > > > > > > causes lookup_one() to fail. This has sometimes been seen
> > > > > > > to
> > > > > > > occur for
> > > > > > > filesystems that violate POSIX requirements around
> > > > > > > uniqueness
> > > > > > > of inode
> > > > > > > numbers, something that is common for snapshot
> > > > > > > directories.
> > > > > > 
> > > > > > Ouch. Nasty.
> > > > > > 
> > > > > > Looks to me like the root cause is "filesystems that
> > > > > > violate
> > > > > > POSIX
> > > > > > requirements around uniqueness of inode numbers".
> > > > > > This violation can cause any of the parent's children to
> > > > > > wrongly match
> > > > > > get_name() not only '.' and '..' and fail the d_inode
> > > > > > sanity
> > > > > > check after
> > > > > > lookup_one().
> > > > > > 
> > > > > > I understand why this would be common with parent of
> > > > > > snapshot
> > > > > > dir,
> > > > > > but the only fs that support snapshots that I know of
> > > > > > (btrfs,
> > > > > > bcachefs)
> > > > > > do implement ->get_name(), so which filesystem did you
> > > > > > encounter
> > > > > > this behavior with? can it be fixed by implementing a
> > > > > > snapshot
> > > > > > aware ->get_name()?
> > > > > > 
> > > > > > > This patch just ensures that we skip '.' and '..' rather
> > > > > > > than
> > > > > > > allowing a
> > > > > > > match.
> > > > > > 
> > > > > > I agree that skipping '.' and '..' makes sense, but...
> > > > > 
> > > > > Does skipping '.' and '..' make sense for file systems that
> > > > > do
> > > > 
> > > > It makes sense because if the child's name in its parent would
> > > > have been "." or ".." it would have been its own parent or its
> > > > own
> > > > grandparent (ELOOP situation).
> > > > IOW, we can safely skip "." and "..", regardless of anything
> > > > else.
> > > 
> > > This new comment:
> > > 
> > > +     /* Ignore the '.' and '..' entries */
> > > 
> > > then seems inadequate to explain why dot and dot-dot are now
> > > never
> > > matched. Perhaps the function's documenting comment could expand
> > > on
> > > this a little. I'll give it some thought.
> > 
> > The point of this code is to attempt to create a valid path that
> > connects the inode found by the filehandle to the export point. The
> > readdir() must determine a valid name for a dentry that is a
> > component
> > of that path, which is why '.' and '..' can never be acceptable.
> > 
> > This is why I think we should keep the 'Fixes:' line. The commit it
> > points to explains quite concisely why this patch is needed.
> > 
> 
> By all means, mention this commit, just not with a fixed tag please.
> IIUC, commit 21d8a15ac333 did not introduce a regression that this
> patch fixes. Right?
> So why insist on abusing Fixes: tag instead of a mention?

I don't see it as being that straightforward.

Prior to commit 21d8a15ac333, the call to lookup_one_len() could return
a dentry (albeit one with an invalid name) depending on whether or not
the filesystem lookup succeeds. Note that knfsd does support a lookup
of "." and "..", as do several other NFS servers.

With commit 21d8a15ac333 applied, however, lookup_one_len()
automatically returns an EACCES error.

So while I agree that there are good reasons for introducing commit
21d8a15ac333, it does change the behaviour in this code path.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com