linux-kernel - Re: Question regarding ptrace work for LInux v3.1

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Message-ID: <CALJO4zGSFCA9f4fdVteGZ8MZsk7_Ksz9-oN-HQ03DPqJGU9NPA@mail.gmail.com>
Date:	Wed, 23 Mar 2016 10:12:30 -0400
From:	Patrick Donnelly <pdonnel3@...edu>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: Question regarding ptrace work for LInux v3.1

On Mon, Mar 21, 2016 at 3:35 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 03/21, Patrick Donnelly wrote:
>> On Mon, Mar 21, 2016 at 3:07 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>> >         case SIGSTOP:
>> >         /* Black magic to get threads working on old Linux kernels... */
>> >
>> >         if(p->nsyscalls == 0) { /* stop before we begin running the process */
>> >                 debug(D_DEBUG, "suppressing bootstrap SIGSTOP for %d",pid);
>> >                 signum = 0; /* suppress delivery */
>> >                 kill(p->pid,SIGCONT);
>> >         }
>> >         break;
>> >
>> > doesn't look right. Note that kill(pid,SIGCONT) affects the whole thread-
>> > group. So if this kill() races with another thread doing clone() you can
>> > hit the problem you described.
>>
>> You're right, that should be tkill! I will give that a try and report
>> back if that solved the issue for our collaborators...
>
> Ah, sorry, I should have mentioned this...
>
> No, tkill() won't help. See prepare_signal(), SIGCONT always removes
> the SIG_KERNEL_STOP_MASK signals from all threads, not matter if it was
> sent by tkill() or kill().
>
> Perhaps you should just remove this kill(SIGCONT) ?
>
> tracer_continue(signr => 0) should equally suppress the delivery. To
> clarify this won't be right too, but without PTRACE_SEIZE you simply
> can't write the code which handles the stop/cont/etc events correctly
> anyway...

Thanks so much Oleg. Indeed this was the problem.

-- 
Patrick Donnelly