From: ebiederm@xmission.com (Eric W. Biederman)
To: Oleg Nesterov <oleg@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
jolsa@redhat.com, Namhyung Kim <namhyung@kernel.org>,
luca abeni <luca.abeni@santannapisa.it>,
syzkaller <syzkaller@googlegroups.com>
Subject: Re: [PATCH 1/2] signal: Always notice exiting tasks
Date: Mon, 11 Feb 2019 18:42:39 -0600 [thread overview]
Message-ID: <87zhr1g7ls.fsf@xmission.com> (raw)
In-Reply-To: <20190211141340.GA21430@redhat.com> (Oleg Nesterov's message of "Mon, 11 Feb 2019 15:13:40 +0100")
Oleg Nesterov <oleg@redhat.com> writes:
> sorry again for delay...
>
> On 02/07, Eric W. Biederman wrote:
>>
>> --- a/kernel/signal.c
>> +++ b/kernel/signal.c
>> @@ -2393,6 +2393,11 @@ bool get_signal(struct ksignal *ksig)
>> goto relock;
>> }
>>
>> + /* Has this task already been marked for death? */
>> + ksig->info.si_signo = signr = SIGKILL;
>> + if (signal_group_exit(signal))
>> + goto fatal;
>> +
>> for (;;) {
>> struct k_sigaction *ka;
>>
>> @@ -2488,6 +2493,7 @@ bool get_signal(struct ksignal *ksig)
>> continue;
>> }
>>
>> + fatal:
>> spin_unlock_irq(&sighand->siglock);
>
> Eric, but this is wrong. At least this is the serious user-visible
> change.
>
> Afaics, with this patch the tracee will never stop in PTRACE_EVENT_EXIT in case
> of group_exit/exec, because schedule() in TASK_TRACED state won't block due to
> __fatal_signal_pending().
>
> Yes, yes, as I said many times the semantics of PTRACE_EVENT_EXIT was never really
> defined, it depends on /dev/random, but still I don't think we should break it even
> more.
Well it changes PTRACE_EVENT_EXIT I grant that. It looks like that
changes makes PTRACE_EVENT_EXIT is less than useful.
The only way to perfectly preserve the previous semantics is probably to
do something like my JOBCTL_TASK_EXIT proposal.
That said I don't think even adding a JOBCTL_TASK_EXIT is enough to have
a reliable stop of ptrace_event_exit after a process has exited. As any
other pending signal can cause problems there as well.
I have received a report that strace -f in some cases is not noticing
children before they die and it looks like a stop in PTRACE_EVENT_EXIT
would fix that strace behavior.
Sigh.
Here I was trying for the simple minimal change and I hit this landmine.
Which leaves me with the question of what should be semantics of signal
handling after exit.
I think from dim memory of previous conversations the desired semantics
look like:
a) Ignore all signal state except for SIGKILL.
b) Letting SIGKILL wake up the process should be sufficient.
I will see if I can reproduce the strace failure and see if I can cook
up something minimal that addresses just that. If you have suggestions
I would love to hear them.
As this was a minimal fix for SIGKILL being broken I have already sent
the fix to Linus. So we are looking at an incremental fix at this point.
Eric
next prev parent reply other threads:[~2019-02-12 0:42 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-01 16:48 perf_event_open+clone = unkillable process Dmitry Vyukov
2019-02-01 17:06 ` Dmitry Vyukov
2019-02-02 18:30 ` Jiri Olsa
2019-02-03 15:21 ` Jiri Olsa
2019-02-04 9:27 ` Thomas Gleixner
2019-02-04 9:38 ` Dmitry Vyukov
2019-02-04 17:38 ` Thomas Gleixner
2019-02-05 3:00 ` Eric W. Biederman
2019-02-05 4:27 ` Eric W. Biederman
2019-02-05 6:07 ` Eric W. Biederman
2019-02-05 15:26 ` [RFC][PATCH] signal: Store pending signal exit in tsk.jobctl not in tsk.pending Eric W. Biederman
2019-02-06 12:09 ` Dmitry Vyukov
2019-02-06 21:47 ` Eric W. Biederman
2019-02-06 18:07 ` Oleg Nesterov
2019-02-06 22:25 ` Eric W. Biederman
2019-02-07 6:42 ` [PATCH 0/2]: Fixing unkillable processes caused by SIGHUP timers Eric W. Biederman
2019-02-07 6:43 ` [PATCH 1/2] signal: Always notice exiting tasks Eric W. Biederman
2019-02-11 14:13 ` Oleg Nesterov
2019-02-12 0:42 ` Eric W. Biederman [this message]
2019-02-12 8:18 ` Eric W. Biederman
2019-02-12 16:50 ` Oleg Nesterov
2019-02-13 3:58 ` Eric W. Biederman
2019-02-13 4:09 ` [PATCH] signal: Restore the stop PTRACE_EVENT_EXIT Eric W. Biederman
2019-02-13 13:55 ` Oleg Nesterov
2019-02-13 14:38 ` Oleg Nesterov
2019-02-13 14:58 ` Eric W. Biederman
2019-02-07 6:44 ` [PATCH 2/2] signal: Better detection of synchronous signals Eric W. Biederman
2019-02-11 15:18 ` Oleg Nesterov
2019-02-12 0:01 ` Eric W. Biederman
2019-02-12 17:21 ` Oleg Nesterov
2019-02-07 11:46 ` [PATCH 0/2]: Fixing unkillable processes caused by SIGHUP timers Dmitry Vyukov
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