Re: Apparent mount behaviour change in 6.15

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, May 26, 2025 at 06:47:25AM +0200, Christian Brauner wrote:
> On Sat, May 24, 2025 at 12:22:13AM +0100, Al Viro wrote:
> > On Fri, May 23, 2025 at 10:37:35PM +0100, Al Viro wrote:
> > > On Fri, May 23, 2025 at 10:29:58PM +0100, Al Viro wrote:
> > > 
> > > > This is bogus, IMO.  I'm perfectly fine with propagate_one() returning 0
> > > > on anon_ns(m->mnt); that would refuse to propagate into *any* anon ns,
> > > > but won't screw the propagation between the mounts that are in normal, non-anon
> > > > namespaces.
> > > 
> > > IOW, I mean this variant - the only difference from what you've posted is
> > > the location of is_anon_ns() test; you do it in IS_MNT_NEW(), this variant
> > > has it in propagate_one().  Does the variant below fix regression?
> > 
> > AFAICS, it does suffice to revert the behaviour change on the reproducer
> > upthread.
> > 
> > I've replaced the top of viro/vfs.git#fixes with that; commit message there
> > is tentative - if nothing else, that's a patch from Christian with slight
> > modifications from me.  It also needs reported-by, etc.
> > 
> > Said that, could somebody (original reporter) confirm that the variant
> > in git.kernel.org:/pub/scm/linux/kernel/git/viro/vfs.git #fixes (head at
> > 63e90fcc1807) is OK with them?
> > 
> > And yes, it will need a proper commit message.  Christian?
> 
> Yes, that looks good to me, thank you!

OK, I went with the following for commit message:

-----
Don't propagate mounts into detached trees

All versions up to 6.14 did not propagate mount events into detached
tree.  Shortly after 6.14 a merge of vfs-6.15-rc1.mount.namespace
(130e696aa68b) has changed that.

Unfortunately, that has caused userland regressions (reported in
https://lore.kernel.org/all/CAOYeF9WQhFDe+BGW=Dp5fK8oRy5AgZ6zokVyTj1Wp4EUiYgt4w@mail.gmail.com/)

Straight revert wouldn't be an option - in particular, the variant in 6.14
had a bug that got fixed in d1ddc6f1d9f0 ("fix IS_MNT_PROPAGATING uses")
and we don't want to bring the bug back.

This is a modification of manual revert posted by Christian, with changes
needed to avoid reintroducing the breakage in scenario described in
d1ddc6f1d9f0.

Cc: stable@vger.kernel.org
Reported-by: Allison Karlitskaya <lis@redhat.com>
Tested-by: Allison Karlitskaya <lis@redhat.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
-----

It's in viro/vfs.git #fixes; if everyone's OK with the commit message, I'm
sending a pull request tomorrow.