linux-kernel - Re: [PATCH 1/5] ptrace: Prepare to fix racy accesses on task breakpoints

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Date:	Wed, 4 May 2011 08:31:06 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Will Deacon <will.deacon@....com>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	"v2.6.33.." <stable@...nel.org>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1/5] ptrace: Prepare to fix racy accesses on task
 breakpoints

(Linus and Andrew Cc:-ed as well)

* Frederic Weisbecker <fweisbec@...il.com> wrote:

> When a task is traced and is in a stopped state, the tracer
> may execute a ptrace request to examine the tracee state and
> get its task struct. Right after, the tracee can be killed
> and thus its breakpoints released.
> This can happen concurrently when the tracer is in the middle
> of reading or modifying these breakpoints, leading to dereferencing
> a freed pointer.
> 
> Hence, to prepare the fix, create a generic breakpoint reference
> holding API. When a reference on the breakpoints of a task is
> held, the breakpoints won't be released until the last reference
> is dropped. After that, no more ptrace request on the task's
> breakpoints can be serviced for the tracer.
> 
> Reported-by: Oleg Nesterov <oleg@...hat.com>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>

Ok, this series looks a bit scary - and this ptrace.h change does not have 
Oleg's Acked-by. (the arch bits all have maintaner Acked-by's)

The changes look a bit ugly as well: beyond the ugly ifdeffery, we have 
ptrace.h::ptrace_init_task(), which is only used in 
tracehook.h::tracehook_finish_clone() which is only used in 
kernel/fork.c::copy_process().

That's two levels of obfuscation to do something rather simple - i think we 
should get rid of the tracehook.h redirections, it did not work out in the end 
as a method of capturing events - ftrace TRACE_EVENT() seems better structured 
and more maintainable.

But i guess we could live with this fix for v2.6.39, if neither Oleg nor Linus 
and Andrew are hating this further complication of the ptrace mess enough to 
NAK it. Thoughts?

Plus, i'd really love it if you did some stress-testing of this change of a 
mixed ptrace breakpoints and perf breakpoints workload, on some sufficiently 
SMP box. gdb's hbreak is a very low freq way of testing thus such regressions 
take ages to be reported.

Thanks,

	Ingo
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