linux-kernel - Re: [PATCH v3] Make PTRACE_SEIZE set ptrace options specified in 'data'

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Date:	Fri, 9 Sep 2011 14:15:33 +0100
From:	Pedro Alves <pedro@...esourcery.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org,
	Denys Vlasenko <vda.linux@...glemail.com>
Subject: Re: [PATCH v3] Make PTRACE_SEIZE set ptrace options specified in 'data'

On Friday 09 September 2011 13:28:55, Denys Vlasenko wrote:
> On Fri, 2011-09-09 at 12:12 +0100, Pedro Alves wrote:
> > On Thursday 08 September 2011 19:22:04, Denys Vlasenko wrote:
> > > Make PTRACE_SEIZE set ptrace options specified in 'data' parameter
> > >     
> > > This can be used to close a few corner cases in strace where we get
> > > unwanted racy behavior after attach, but before we have a chance
> > > to set options (the notorious post-execve SIGTRAP comes to mind),
> > 
> > I'm still confused on why you're raising the SIGTRAP argument.  Did you see
> > 
> >  https://lkml.org/lkml/2011/9/8/7
> > 
> > From previous discussions, I understood that PTRACE_SEIZE _always_
> > disables
> > the post-execve SIGTRAP, so I don't believe that race actually exists.
> > Or is that not the case?
> 
> 
> I believe it is not the case. And I object to making it the case.

Well, if you'll remember, back in <https://lkml.org/lkml/2011/5/19/704> I
raised this exact problem with that pesky racy post-execve SIGTRAP showing
through on SEIZE, and Tejun a few replies later mentioned that the SIGTRAP
was to be removed on SEIZE.   I'm sure it was said before even, but I'm
not finding the emails now.

> My sense of taste says the approach "you need to use SEIZE to affect
> feature <foo>" for various unrelated <foo> makes ptrace API ugly.

Yes, very much agreed!

> Especially that in this case, we already have a method in API
> to suppress post-execve SIGTRAP.

Right, but we end up with no way to make the tracee _not ever
stop_ at execve if the tracer wants to, like you can make
the tracee not ever stop on forks or syscalls, by not enabling
the corresponding PTRACE_O_FOO or not PTRACE_SYSCALL.  Not
specifying PTRACE_O_TRACEEXEC coupled with `SEIZE not stopping
tracees for that magic SIGTRAP' got you that.  In a way, it looked to
me to make the API a bit less ugly.  That said I'm not seeing GDB
_not_ using PTRACE_O_TRACEEXEC...

Anyway, could you please check (or Tejun please confirm)
that that magic SIGTRAP is really still there on SEIZE?

-- 
Pedro Alves
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