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Date: Fri, 9 Sep 2011 22:03:10 +0200
From: Denys Vlasenko <vda.linux@...glemail.com>
To: Pedro Alves <pedro@...esourcery.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>,
Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] Make PTRACE_SEIZE set ptrace options specified in 'data'
On Friday 09 September 2011 19:09, Pedro Alves wrote:
> > This would be a _very_ minor improvement, so tiny it's not worth
> > bothering with. Let me show you the real-world code (part of strace
> > source) which skips over unneeded PTRACE_EVENT_EXEC:
> >
> > if ((status >> 16) != 0)
> > /* Ptrace event (we ignore all of them for now) */
> > goto restart_tracee_with_sig_0;
> >
> > Yes. That is all.
> > It probably compiles into just two assembly instructions.
>
> WTH? I'm talking about _not forcing the tracee to stop_. Let
> me repeat. NOT FORCING THE TRACEE TO STOP.
No need to shout.
execve is such a rare syscall the one extra stop on it is not
going to be a problem.
> And about not needing to handle the magic unadorned SIGTRAP.
> The magic unadorned post-exec SIGTRAP does not have `status & 0xff00'
> set, it is not a ptrace event!
What SIGTRAP? With PTRACE_O_TRACEEXEC, there is no SIGTRAP.
> If we don't disable the magic SIGTRAP, there's no way for a
> tracer to do a very non-invasive SEIZE, say, a GDB mode that
> only cares to let the tracer run free to catch SIGSEGVs
> in some child, while later on during the run, the user remembers
> to set a breakpoint. At that point the tracer needs to catch
> exec events, so it'd enable TRACE_O_EVENTEXEC. Getting rid of
> the SIGTRAP gets rid of the spurious stops when TRACE_O_EVENTEXEC
> is not enabled.
This part I don't understand.
(btw, PTRACE_O_TRACEEXEC, not TRACE_O_EVENTEXEC).
--
vda
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