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 18:32:31 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Tejun Heo <tj@...nel.org>, Roland McGrath <roland@...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, Denys Vlasenko wrote:
>
> On Monday 14 February 2011 10:03, Jan Kratochvil wrote:
> > On Mon, 14 Feb 2011 00:01:47 +0100, Denys Vlasenko wrote:
> > > * sleep runs in nanosleep
> > > * SIGSTOP arrives, strace sees it
> > > * strace logs it and allows it via ptrace(PTRACE_SYSCALL, ..., SIGSTOP)
> > > * sleep process enters group-stop
> >
> > The last point breaks the documented behavior of ptrace:
> > 	If data is nonzero and not SIGSTOP, it is interpreted as a signal to
> > 	be delivered to the child; otherwise, no signal is delivered.
>
> But SIGSTOP _is_ delivered - that's why sleep process stops.

Yes.

> > What if other signal arrives?  The tracer probably should not be notified as
> > the tracee is in a group-stop.
>
> The behavior here ideally should be the same as for non-traced process:
> the signals are remembered while process is stopped, and it sees them
> only after SIGCONT, as demonstrated by the following program

Agreed. And this is what we currently do.

> I believe it would be best if debugger sees signals immediately,
> but when it does ptrace(PTRACE_CONT/SYSCALL, ..., <sig>)
> in order to send signals to group-stopped tracee, they are queued
> to it without terminating group-stop. When SIGCONT arrives,
> ptrace(PTRACE_CONT/SYSCALL, ..., SIGCONT) terminates group-stop
> and causes all queued signals to be handled (in random order,
> not necessarily in the order of arrival. Even CONT handler is
> not guaranteed to be called first, as you see above).

Yes, personaly I think this would the best behaviour.

But, damn, again, again, again, yes this change is very noticable.
Tejun is right too.

Oleg.

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