linux-kernel - Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE

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Date:	Mon, 14 Feb 2011 18:57:31 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, Roland McGrath <roland@...hat.com>,
	jan.kratochvil@...hat.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE_ATTACH

On Mon, Feb 14, 2011 at 6:39 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 02/14, Denys Vlasenko wrote:
>>
>> On Mon, Feb 14, 2011 at 4:31 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>> > On 02/13, Denys Vlasenko wrote:
>> >>
>> >> $ strace -tt sleep 30
>> >> 23:02:15.619262 execve("/bin/sleep", ["sleep", "30"], [/* 30 vars */]) = 0
>> >> ...
>> >> 23:02:15.622112 nanosleep({30, 0}, NULL) = ? ERESTART_RESTARTBLOCK (To be restarted)
>> >> 23:02:23.781165 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
>> >> 23:02:23.781251 --- SIGSTOP (Stopped (signal)) @ 0 (0) ---
>> >>     (I forgot again why we see it twice. Another quirk I guess...)
>> >
>> >      (this is correct, the tracee reports the signal=SIGSTOP, then
>> >       it reports it actually stopps with exit_code=SIGSTOP)
>>
>> Ah, I see. Is there any way debugger can distinguish between these two
>> different stops?
>
> IIRC, the (only?) way to distinguish is to check last_siginfo != NULL
> via ptrace(PTRACE_GETSIGINFO).

What do you think strace needs to do when it sees second SIGSTOP
(meaning "in theory", not "on current kernel which may be buggy")?

ptrace(PTRACE_SYSCALL, $PID, 0x1, 0)?
nothing?
something else?

-- 
vda
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