[git pull] breakage in LOOKUP_MOUNTPOINT handling

From: Al Viro <viro@zeniv.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: [git pull] breakage in LOOKUP_MOUNTPOINT handling
Date: Thu, 8 Apr 2021 02:07:33 +0000	[thread overview]
Message-ID: <YG5lZcubiudwsz4I@zeniv-ca.linux.org.uk> (raw)

	Brown paperbag time: dumb braino in the series that went into
5.7 broke the "don't step into ->d_weak_revalidate() when umount(2)
looks the victim up" behaviour.  Spotted only now - saw
	if (!err && unlikely(nd->flags & LOOKUP_MOUNTPOINT)) {
		err = handle_lookup_down(nd);
		nd->flags &= ~LOOKUP_JUMPED; // no d_weak_revalidate(), please...
	}
went "why do we clear that flag here - nothing below that point is going
to check it anyway" / "wait a minute, what is it doing *after* complete_walk()
(which is where we check that flag and call ->d_weak_revalidate())" / "how could
that possibly _not_ break?", followed by reproducing the breakage and verifying
that the obvious fix of that braino does, indeed, fix it.

FWIW, reproducer is (assuming that $DIR exists and is exported r/w to localhost)
mkdir $DIR/a
mkdir /tmp/foo
mount --bind /tmp/foo /tmp/foo
mkdir /tmp/foo/a
mkdir /tmp/foo/b
mount -t nfs4 localhost:$DIR/a /tmp/foo/a
mount -t nfs4 localhost:$DIR /tmp/foo/b
rmdir /tmp/foo/b/a
umount /tmp/foo/b
umount /tmp/foo/a
umount -l /tmp/foo	# will get everything under /tmp/foo, no matter what

Correct behaviour is successful umount; broken kernels (5.7-rc1 and later) get
umount.nfs4: /tmp/foo/a: Stale file handle
Note that bind mount is there to be able to recover - on broken kernels we'd
get stuck with impossible-to-umount filesystem if not for that.

	FWIW, that braino had been posted for review back then, at least
twice.  Unfortunately, the call of complete_walk() was outside of diff
context, so the bogosity hadn't been immediately obvious from the patch
alone ;-/

The following changes since commit 7d01ef7585c07afaf487759a48486228cd065726:

  Make sure nd->path.mnt and nd->path.dentry are always valid pointers (2021-04-06 12:33:07 -0400)

are available in the git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git fixes

for you to fetch changes up to 4f0ed93fb92d3528c73c80317509df3f800a222b:

  LOOKUP_MOUNTPOINT: we are cleaning "jumped" flag too late (2021-04-06 20:33:00 -0400)

----------------------------------------------------------------
Al Viro (1):
      LOOKUP_MOUNTPOINT: we are cleaning "jumped" flag too late

 fs/namei.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)