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

[Thomas Voegtle]

On Wed, 29 Jan 2020, Eric W. Biederman wrote:

> Sasha Levin <sashal@xxxxxxxxxx> writes:
>
> > 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 BÃhmwalder <christoph.boehmwalder@xxxxxxxxxx>
> > > > > Tested-by: Christoph BÃhmwalder <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).
>
> All of these changes were cleanup.  Which is why I didn't tag any of
> them for stable.
>
> Not to say that there weren't real problems using force_sig instead
> of send_sig.  force_sig does nothing to ensure the task it is sending
> signals to won't, and hasn't gone away.  Which is why it is a bad
> idea to use force_sig on anything but current.  As I recall drbd used
> force_sig on a kernel_thread which didn't go away.
>
> When fixing the force_sig vs send_sig confusion I didn't realize that
> some places were using force_sig because they had not enabled receiving
> the signals they depended on.  Which is where allow_kernel_signal comes
> from.  But while using force_sig allow_kernel_signal is not necessary.
>
> Eric


Thanks for clarification.


      Thomas