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

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Message-ID: <4F32D042.7080907@redhat.com>
Date:	Wed, 08 Feb 2012 19:42:58 +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 07:02 PM, Oleg Nesterov wrote:
> On 02/08, Pedro Alves wrote:
>>
>> 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).
> 
> Yes. But in this case the new thread has no bps even if it is auto-
> attached.

Ah, right.  It used to be the kernel copied the debug registers from
parent->child, but they're always cleared in the child nowadays
(since 72f674d203cd230426437cdcf7dd6f681dad8b0d).

>> I wouldn't have been surprised to see the trap in userspace in either
>> the parent
> 
> It would be just wrong. Please note that it is child, not parent, who
> does the write.

Okay, I didn't know which it was that touched the memory,
hence the "either".  Thanks.  Paired with the
we-now-clear-debug-regs-on-clone thing, it makes sense.

-- 
Pedro Alves
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