On Wed, 29 Jan 2020, Eric W. Biederman wrote: > Sasha Levin 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 >>>>> >>>>> [ 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 >>>>> Reported-by: Christoph Böhmwalder >>>>> Tested-by: Christoph Böhmwalder >>>>> Cc: Steve French >>>>> Cc: Philipp Reisner >>>>> Cc: David Laight >>>>> 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