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Subject: Re: [PATCH, for 2.6.29] ptrace: fix the usage of ptrace_fork()
From: Markus Metzger <markus.t.metzger@googlemail.com>
To: Markus Metzger <markus.t.metzger@googlemail.com>
Cc: Oleg Nesterov <oleg@redhat.com>,
       "Metzger, Markus T" <markus.t.metzger@intel.com>,
       Ingo Molnar <mingo@elte.hu>, Roland McGrath <roland@redhat.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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Date: Tue, 10 Feb 2009 22:00:14 +0100
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On Tue, 2009-02-10 at 21:21 +0100, Markus Metzger wrote:
> On Tue, 2009-02-10 at 19:40 +0100, Oleg Nesterov wrote:

> > Perhaps, for 2.6.29, we can do something like the "patch" below?
> > 
> > --- a/arch/x86/kernel/ptrace.c
> > +++ b/arch/x86/kernel/ptrace.c
> > @@ -810,11 +810,15 @@ static void ptrace_bts_untrace(struct ta
> >  
> >  static void ptrace_bts_detach(struct task_struct *child)
> >  {
> > +	// We can race with de_thread/do_wait which
> > +	// can do ptrace_bts_untrace() before us
> >  	if (unlikely(child->bts)) {
> > -		ds_release_bts(child->bts);
> > -		child->bts = NULL;
> > -
> > -		ptrace_bts_free_buffer(child);
> > +		// This all will be freed by ptrace_bts_untrace()
> > +		// later, but we should update ->mm
> > +		down_write(->mmap_sem);
> > +		mm->total_vm  -= bts_size;
> > +		mm->locked_vm -= bts_size);
> > +		up_write(->mmap_sem);
> >  	}
> >  }
> >  #else
> > 
> 

There's still a race.
The kfree() is safe, now, but ptrace_bts_untrace() might have cleared
child->bts_size before we can refund the memory.

We need to make ptrace_bts_untrace() ignore child->bts_size and clear
it in ptrace_bts_detach().


regards,
markus.