linux-kernel - Re: debug registers and fork

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Message-ID: <20070301002758.GF23543@frankl.hpl.hp.com>
Date:	Wed, 28 Feb 2007 16:27:58 -0800
From:	Stephane Eranian <eranian@....hp.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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

Alan,

On Wed, Feb 28, 2007 at 07:01:17PM -0500, Alan Stern wrote:
> 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.
> 
I agree and that is how we have it on IA-64. With debugging, there is
always another process involved and no matter what I think it needs to 
be aware of the new child. I don't think autoamtic inheritance is good.
It should always be trigger by the controlling process (e.g., debugger).
There is enough support in ptrace to catch the fork/vfork/pthread_create
and decide what to do.  This is how I have coded perfmon so that hardware
performance counters are never automatically inherited.

-- 
-Stephane
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