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

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Message-ID: <20110214180609.GE15847@redhat.com>
Date:	Mon, 14 Feb 2011 19:06:09 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <vda.linux@...glemail.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 02/14, Oleg Nesterov wrote:
>
> On 02/14, Denys Vlasenko wrote:
> >
> > 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)?
>
> proably this, or even ptrace(PTRACE_SYSCALL, $PID, 0x1, SIGSTOP).
> I think.
>
> (assuming that ptrace_resume() respects TASK_STOPPED)

Oh, but I forgot to mention... there is another problem, _any_
ptrace request when the tracee is stopped turns it into TASK_TRACED.

Oleg.

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