linux-kernel - Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED, TASK

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Message-ID: <YrHA5UkJLornOdCz@li-4a3a4a4c-28e5-11b2-a85c-a8d192c6f089.ibm.com>
Date:   Tue, 21 Jun 2022 15:00:21 +0200
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        Oleg Nesterov <oleg@...hat.com>, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, tj@...nel.org,
        linux-pm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org, Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, linux-ia64@...r.kernel.org
Subject: Re: [PATCH v4 12/12] sched,signal,ptrace: Rework TASK_TRACED,
 TASK_STOPPED state

On Thu, May 05, 2022 at 01:26:45PM -0500, Eric W. Biederman wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> 
> Currently ptrace_stop() / do_signal_stop() rely on the special states
> TASK_TRACED and TASK_STOPPED resp. to keep unique state. That is, this
> state exists only in task->__state and nowhere else.
> 
> There's two spots of bother with this:
> 
>  - PREEMPT_RT has task->saved_state which complicates matters,
>    meaning task_is_{traced,stopped}() needs to check an additional
>    variable.
> 
>  - An alternative freezer implementation that itself relies on a
>    special TASK state would loose TASK_TRACED/TASK_STOPPED and will
>    result in misbehaviour.
> 
> As such, add additional state to task->jobctl to track this state
> outside of task->__state.
> 
> NOTE: this doesn't actually fix anything yet, just adds extra state.
> 
> --EWB
>   * didn't add a unnecessary newline in signal.h
>   * Update t->jobctl in signal_wake_up and ptrace_signal_wake_up
>     instead of in signal_wake_up_state.  This prevents the clearing
>     of TASK_STOPPED and TASK_TRACED from getting lost.
>   * Added warnings if JOBCTL_STOPPED or JOBCTL_TRACED are not cleared

Hi Eric, Peter,

On s390 this patch triggers warning at kernel/ptrace.c:272 when
kill_child testcase from strace tool is repeatedly used (the source
is attached for reference):

while :; do
	strace -f -qq -e signal=none -e trace=sched_yield,/kill ./kill_child
done

It normally takes few minutes to cause the warning in -rc3, but FWIW
it hits almost immediately for ptrace_stop-cleanup-for-v5.19 tag of
git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.

Commit 7b0fe1367ef2 ("ptrace: Document that wait_task_inactive can't
fail") suggests this WARN_ON_ONCE() is not really expected, yet we
observe a child in __TASK_TRACED state. Could you please comment here?

Thanks!

View attachment "kill_child.c" of type "text/plain" (1306 bytes)