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

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Message-ID: <20110301190638.GA12196@host1.dyn.jankratochvil.net>
Date:	Tue, 1 Mar 2011 20:06:38 +0100
From:	Jan Kratochvil <jan.kratochvil@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [RFC] Proposal for ptrace improvements

On Tue, 01 Mar 2011 16:24:57 +0100, Tejun Heo wrote:
> P4. PTRACE_SEIZE
[...]
> Completion notification is delivered in the usual way via wait(2).  If
> the task was in jctl stop, it would report the stop signal with the
> matching siginfo.  If the task hits an existing ptrace trap condition,
> the matching SIGTRAP will be reported; otherwise, SIGTRAP will be
> reported with siginfo indicating PTRACE_SEIZE trap.

This is a change from the Roland's proposed PTRACE_ATTACH_NOSTOP.
Currently it is already a problem that apps did not / do not expect the first
waitpid after PTRACE_ATTACH may not be SIGSTOP.  And in the racy case of
a pending signal during PTRACE_ATTACH which gets delivered by first waidpid
the tracer either crashes or loses the signal etc.  (It was a problem for
older GDB, it is fixed in recent GDBs, there exist other ptrace using tools.)

And after all it gets complicated to call PTRACE_ATTACH and then postpone the
reception of SIGSTOP while other signals are being delivered first.

> WAY FORWARD (yeah, I'm feeling some marketing vibe)
> -----------
[...]
> What someone would want if one could start from the scratch is interesting
> but ultimately irrelevant.

The ugdb project provides unified debugging facility for both local and remote
debugging.  With the ptrace extensions a network protocol server needs to be
developed/used anyway and it will be just a thin userland layer mapping
read/write calls to the ptrace calls on the server side in an ineffective way.

OTOH one can look at the similar case of TUX/khttpd (also) has never been
merged into the kernel.

Regards,
Jan
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