linux-kernel - Re: debug registers and fork

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Date:	Wed, 28 Feb 2007 19:01:17 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Roland McGrath <roland@...hat.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>, <eranian@....hp.com>,
	<linux-kernel@...r.kernel.org>, <linux-ia64@...r.kernel.org>,
	<ak@...e.de>, <tony.luck@...el.com>,
	William Cohen <wcohen@...hat.com>
Subject: Re: debug registers and fork

On Wed, 28 Feb 2007, Roland McGrath wrote:

> It is true that debug registers are inherited by fork and clone.
> I am 99% sure that this was never specifically intended, but it
> has been this way for a long time (since 2.4 at least).  It's an
> implicit consequence of the do_fork implementation style, which
> does a blind copy of the whole task_struct and then explicitly
> reinitializes some individual fields.  I suppose this has some
> benefit or other, but it is very prone to new pieces of state
> getting implicitly copied without the person adding that new state
> ever consciously deciding what its inheritance semantics should be.
> 
> Alan Stern is working on a revamp of the x86 debug register
> support.  This is a fine opportunity to clean this area up and
> decide positively what the semantics ought to be.

Absolutely.  Right now I just have a placeholder function with a note
about checking for CLONE_PTRACE.  The cleanest solution, far and away,
would be to have the child process inherit no breakpoints and no debug
register values.

Alan Stern

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