linux-kernel - Re: ptrace() hangs on attempt to seize/attach stopped & frozen task

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Message-ID: <20151119174754.GA13949@redhat.com>
Date:	Thu, 19 Nov 2015 18:47:54 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pedro Alves <palves@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>,
	Roland McGrath <roland@...k.frob.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: ptrace() hangs on attempt to seize/attach stopped & frozen task

Thanks Pedro for your email,

I'll recheck tomorrow, but at first glance:

On 11/19, Pedro Alves wrote:
>
> Both GDB and gdbserver have special processing for attaching to already-stopped
> processes.

Yes, I am starting to recall that I have looked at this code years ago ;)

>  907 linux_attach_lwp (ptid_t ptid)
>  908 {
>  909   struct lwp_info *new_lwp;
>  910   int lwpid = ptid_get_lwp (ptid);
>  911
>  912   if (ptrace (PTRACE_ATTACH, lwpid, (PTRACE_TYPE_ARG3) 0, (PTRACE_TYPE_ARG4) 0)
>  913       != 0)
>  914     return errno;
>  915
>  916   new_lwp = add_lwp (ptid);
>  917
>  918   /* We need to wait for SIGSTOP before being able to make the next
>  919      ptrace call on this LWP.  */
>  920   new_lwp->must_set_ptrace_flags = 1;
>  921
>  922   if (linux_proc_pid_is_stopped (lwpid))

This can't happen today. Starting from v3.0 at least.

> This queuing of a SIGSTOP + PTRACE_CONT was necessary because
> otherwise when gdb attaches to a job stopped process, gdb would hang in the waitpid
> after PTRACE_ATTACH, waiting for the initial SIGSTOP which would never arrive.

Yes, because its exit code could be already cleared iirc. This was fixed
even before.

> If the proposed change makes it so that a new intermediate state can be observed
> right after PTRACE_ATTACH, and so linux_proc_pid_is_stopped can return false,
> then there's potential for breakage.

See above,

> But maybe not, if we're sure that
> that when that happens, waitpid returns for the initial
> PTRACE_ATTACH-induced SIGSTOP.

Yes. Just you can't assume that watpid(WNOHANG) will succeed. Is it OK?

Oleg.

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