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

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Message-ID: <20120208190250.GA23163@redhat.com>
Date:	Wed, 8 Feb 2012 20:02:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pedro Alves <palves@...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, 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.

IOW, I think that hw bp can detect the write from the kernel space,
but I didn't check.

> 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.




If only I understood why do we need CLONE_CHILD_SETTID... at least
I certainly do not understand why glibc translates fork() into
clone(CLONE_CHILD_SETTID) on my system. The child write into its
memory, the parent can't see this change. IIRC, initially
CLONE_CHILD_SETTID wrote child->pid into the parent's memory, and
even before the child was actually created.

Oleg.

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