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

[Eric W. Biederman]

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