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(-)
The pull request you sent on Thu, 8 Apr 2021 02:07:33 +0000:
> git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git fixes
has been merged into torvalds/linux.git:
https://git.kernel.org/torvalds/c/035d80695fae55ed3e788cd8a62525657a43b924
Thank you!
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