linux-kernel - Re: debug registers and fork

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Message-Id: <20070227122427.bf23af2f.akpm@linux-foundation.org>
Date:	Tue, 27 Feb 2007 12:24:27 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	eranian@....hp.com
Cc:	linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org,
	ak@...e.de, tony.luck@...el.com, Roland McGrath <roland@...hat.com>
Subject: Re: debug registers and fork

> On Mon, 26 Feb 2007 15:51:54 -0800 Stephane Eranian <eranian@....hp.com> wrote:
> Hello,
> 
> I have come across an issue with a monitoring using the
> hardware debug registers on ia64/i386/x86-64.
> 
> It seems that the way debug registers are inherited across fork
> differs between ia-64 and i386/x86-64. On ia-64, the debug registers
> are NEVER inherited in the child. The copy_thread() routine clears
> the necessary thread flags to avoid reloading the debug registers in
> the child.
> 
> Now, on x86-64, it appears that the TIF_DEBUG flag is inherited via
> setup_thread_stack(). By virtue of dup_task_struct() the debug registers
> get copied into the child task on fork. So the child has active breakpoints,
> unless I am mistaken somewhere.
> 
> Given the way the ptrace() interface works, I would tend to
> think that the ia-64 way is the correct one. Any comment?
> 
> Furthermore, on i386/x86-64, when switching out from a task with TIF_DEBUG
> enabled to another which does not, it seems we do not clear the debug
> registers (at least dr7) so they become inactive.
> 

Let's cc Roland - he's totally rewritten ptrace and probably knows this
stuff.
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