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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: Oleg Nesterov <oleg@redhat.com>
Cc: "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>,
       Markus Metzger <markus.t.metzger@googlemail.com>
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Date: Tue, 10 Feb 2009 21:21:59 +0100
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On Tue, 2009-02-10 at 19:40 +0100, Oleg Nesterov wrote:

> > 2. there is a race between a thread detaching
> > and another thread releasing the same task.

I think I now see the problem. Ptrace uses the tasklist_lock to protect
against __ptrace_unlink() races.

I could either introduce a separate lock to protect bts buffer
deallocation, or I put the kfree part under the tasklist_lock,
as you suggest below.



> Perhaps, for 2.6.29, we can do something like the "patch" below?
> 
> (btw, do you agree with the change in copy_process() I sent? )

Both patches look good to me.


> --- 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
> 


You already sent out the first one. I don't have access to any
test machine from home. I could send the patch tomorrow (evening).

thanks and regards,
markus.