linux-kernel - Re: [PATCH] ptrace: add ability to get clear_tid

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Message-ID: <4F32B924.8080407@redhat.com>
Date:	Wed, 08 Feb 2012 18:04:20 +0000
From:	Pedro Alves <palves@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Tejun Heo <tj@...nel.org>, Andrew Vagin <avagin@...nvz.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ptrace: add ability to get clear_tid_address

On 02/08/2012 05:31 PM, Oleg Nesterov wrote:
> On 02/08, Pedro Alves wrote:
>>
>> I just tried it.  This is &pthread->tid in glibc/libpthread, so with debug
>> info it's easy to figure out where to set the watchpoint manually with gdb
>> without asking the kernel.  Doesn't work.  ptrace doesn't show any trap
>> for the kernel writes.
> 
> The tracee simply can't report this trap. it is already dead ;) and
> hw breakpoint (used by ptrace) is "pinned" to the thread.

Right, as I said.  :-)  I saw that a watchpoint trap isn't reported either
for the CLONE_CHILD_SETTID case (that is, within clone, when the kernel
writes the tid to the memory address passed in to the clone syscall).
I wouldn't have been surprised to see the trap in userspace in either
the parent or the child, though I'm not really surprised to not
see it either.

-- 
Pedro Alves
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