linux-kernel - Re: [PATCH] signal: Allow cifs and drbd to receive their terminating signals

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Date:   Mon, 19 Aug 2019 10:37:59 +0200
From:   Christoph Böhmwalder 
        <christoph.boehmwalder@...bit.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Philipp Reisner <philipp.reisner@...bit.com>,
        David Laight <David.Laight@...lab.com>,
        Jens Axboe <axboe@...nel.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Steve French <smfrench@...il.com>,
        ronnie sahlberg <ronniesahlberg@...il.com>,
        Jeff Layton <jlayton@...marydata.com>,
        linux-cifs <linux-cifs@...r.kernel.org>
Subject: Re: [PATCH] signal: Allow cifs and drbd to receive their terminating
 signals

On Fri, Aug 16, 2019 at 05:19:38PM -0500, Eric W. Biederman wrote:
> 
> 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@...il.com>
> Reported-by: Christoph Böhmwalder <christoph.boehmwalder@...bit.com>
> Cc: Steve French <smfrench@...il.com>
> Cc: Philipp Reisner <philipp.reisner@...bit.com>
> Cc: David Laight <David.Laight@...LAB.COM>
> 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")
> Fixes: fee109901f39 ("signal/drbd: Use send_sig not force_sig")
> Fixes: 3cf5d076fb4d ("signal: Remove task parameter from force_sig")
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  drivers/block/drbd/drbd_main.c |  2 ++
>  fs/cifs/connect.c              |  2 +-
>  include/linux/signal.h         | 15 ++++++++++++++-
>  kernel/signal.c                |  5 +++++
>  4 files changed, 22 insertions(+), 2 deletions(-)
> 

Just tested this patch, and I can confirm that it makes DRBD work as
intended again.

Tested-by: Christoph Böhmwalder <christoph.boehmwalder@...bit.com>

--
Christoph Böhmwalder
LINBIT | Keeping the Digital World Running
DRBD HA —  Disaster Recovery — Software defined Storage