linux-kernel - Re: [RFC] Proposal for ptrace improvements

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Date:	Thu, 3 Mar 2011 02:30:27 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	"Indan Zupancic" <indan@....nu>
Cc:	"Oleg Nesterov" <oleg@...hat.com>, "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,
	"Michael Kerrisk" <mtk.manpages@...glemail.com>
Subject: Re: [RFC] Proposal for ptrace improvements

On Thursday 03 March 2011 01:47, Indan Zupancic wrote:
> On Wed, March 2, 2011 14:32, Oleg Nesterov wrote:
> > On 03/02, Tejun Heo wrote:
> >> On Wed, Mar 02, 2011 at 06:07:35AM +0100, Indan Zupancic wrote:
> >> > I'm not sure what Denys is talking about: Currently it's impossible to
> >> > pass along SIGSTOP to traced processes. Quoting the ptrace manpage:
> >> >
> >> >    PTRACE_CONT
> >> >           Restarts  the stopped child process.  If data is nonzero and not
> >> >           SIGSTOP, it is interpreted as a signal to be  delivered  to  the
> >> >           child;  otherwise,  no  signal is delivered.
> >>
> >> AFAICS, that's not true.  SIGSTOP isn't treated differently from other
> >> signals in the ptrace signal delivery path.  Maybe it was true in the
> >> past.
> >
> > Yes, this is not true. And it seems this was never true.
> >
> > This is the second time this manpage confuses people in this discussion,
> > probably it should be fixed...
> 
> Passing SIGSTOP does not actually stop the traced task, which is in line
> with what the manpage says. All it does is generating that second SIGSTOP
> notification, but when the task is continued it's running, not stopped.

It can be argued that after this the task is running _precisely_
because it was continued by the debugger.

> So ptraced tasks can't be stopped with SIGSTOP and continued with SIGCONT.

It can be stopped - just do not PTRACE_CONT it after second SIGSTOP
notification.

The bug is that it can't be continued with SIGCONT after that.

That's the gist of Tejun Heo's proposal.

Oleg's proposal is a bit different. It proposes that we do need
to do PTRACE_CONT after second SIGSTOP notification too,
but task will be indeed stopped after this, and resumed
when SIGCONT arrives.

-- 
vda
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