Re: [PATCH 4.9 183/271] signal: Allow cifs and drbd to receive their terminating signals

[Sasha Levin]

On Wed, Jan 29, 2020 at 12:36:43PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Jan 29, 2020 at 12:10:47PM +0100, Thomas Voegtle wrote:
> > On Tue, 28 Jan 2020, Greg Kroah-Hartman wrote:
> >
> > > From: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> > >
> > > [ Upstream commit 33da8e7c814f77310250bb54a9db36a44c5de784 ]
> > >
> > > My recent to change to only use force_sig for a synchronous events
> > > wound up breaking signal reception cifs and drbd.  I had overlooked
> > > the fact that by default kthreads start out with all signals set to
> > > SIG_IGN.  So a change I thought was safe turned out to have made it
> > > impossible for those kernel thread to catch their signals.
> > >
> > > Reverting the work on force_sig is a bad idea because what the code
> > > was doing was very much a misuse of force_sig.  As the way force_sig
> > > ultimately allowed the signal to happen was to change the signal
> > > handler to SIG_DFL.  Which after the first signal will allow userspace
> > > to send signals to these kernel threads.  At least for
> > > wake_ack_receiver in drbd that does not appear actively wrong.
> > >
> > > So correct this problem by adding allow_kernel_signal that will allow
> > > signals whose siginfo reports they were sent by the kernel through,
> > > but will not allow userspace generated signals, and update cifs and
> > > drbd to call allow_kernel_signal in an appropriate place so that their
> > > thread can receive this signal.
> > >
> > > Fixing things this way ensures that userspace won't be able to send
> > > signals and cause problems, that it is clear which signals the
> > > threads are expecting to receive, and it guarantees that nothing
> > > else in the system will be affected.
> > >
> > > This change was partly inspired by similar cifs and drbd patches that
> > > added allow_signal.
> > >
> > > Reported-by: ronnie sahlberg <ronniesahlberg@xxxxxxxxx>
> > > Reported-by: Christoph Böhmwalder <christoph.boehmwalder@xxxxxxxxxx>
> > > Tested-by: Christoph Böhmwalder <christoph.boehmwalder@xxxxxxxxxx>
> > > Cc: Steve French <smfrench@xxxxxxxxx>
> > > Cc: Philipp Reisner <philipp.reisner@xxxxxxxxxx>
> > > Cc: David Laight <David.Laight@xxxxxxxxxx>
> > > Fixes: 247bc9470b1e ("cifs: fix rmmod regression in cifs.ko caused by force_sig changes")
> > > Fixes: 72abe3bcf091 ("signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of force_sig")
> >
> > These two commits come with that release, but...
> >
> > > Fixes: fee109901f39 ("signal/drbd: Use send_sig not force_sig")
> > > Fixes: 3cf5d076fb4d ("signal: Remove task parameter from force_sig")
> >
> > ...these two commits not and were never added to 4.9.y.
> >
> > Are these both really not needed?
>
> I don't think so, do you feel otherwise?

Both of those commits read as a cleanup to me. I've actually slightly
modified to patch to not need those commits (they were less than trivial
to backport as is).

--
Thanks,
Sasha