linux-kernel - Re: inconsistent behavior with ptrace(TRACEME) and fork/exec

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Message-ID: <8bd0f97a0810271038g4f0b7326r68c7e88841d0e6f9@mail.gmail.com>
Date:	Mon, 27 Oct 2008 13:38:16 -0400
From:	"Mike Frysinger" <vapier.adi@...il.com>
To:	"Jan Kratochvil" <jan.kratochvil@...hat.com>
Cc:	"Mike Frysinger" <vapier@...too.org>, linux-kernel@...r.kernel.org
Subject: Re: inconsistent behavior with ptrace(TRACEME) and fork/exec

On Mon, Oct 27, 2008 at 10:56, Jan Kratochvil wrote:
> On Wed, 19 Jul 2006 21:18:29 +0200, Mike Frysinger wrote:
>> my understanding is that if a parent forks and the child does
>> a ptrace(TRACEME) right before doing an exec(), the kernel should always
>> halt it and wait indefinitely for the parent to start ptracing it.
>
> Yes, just the parent must process the event (signal).  In your testcase the
> parent finished before the signal could be delivered.  If the tracer exits the
> tracee's tracing is finished and it continues freely.

no signal should have been generated.  the child should have gone
straight to the exec and waited for the parent to process it.

>> unfortunately, this behavior seems to be unreliable.
>
> Fixed the races in your code and I do not see there any problem, do you?
> The ptrace problems/testsuite is being maintained at:
>  http://sourceware.org/systemtap/wiki/utrace/tests

there is no race condition ... it's using vfork here remember ?  it is
impossible for the parent to have executed anything after the vfork()
before the child made it into the exec() and gone to sleep
-mike
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