Re: [PATCH, for 2.6.29] ptrace: fix the usage of ptrace_fork()

[Markus Metzger <markus.t.metzger@googlemail.com>]

Oleg Nesterov wrote:
> On 02/10, Markus Metzger wrote:
>> 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
>>>>

> The goal of this patch is to avoid the crash. The memory accounting
> in ->mm is still not right. But at least, the tracer can not "steal"
> the memory above the limits. And the "good" tracer should not exit
> without detach, and it shouldn't release the tracee from sub-thread
> if this can race with detach.
> 
> So, afaics, the worst thing which can happen is: the "bad" tracer
> is punished by the "unfair" mm->xxx_vm numbers.
> 
> Except exec() can release the main thread whatever the tracer does...
> 
>> We need to make ptrace_bts_untrace() ignore child->bts_size and clear
>> it in ptrace_bts_detach().
> 
> This is worse, now we can leak the memory if the tracer doesn't
> do ptrace_detach().

I see.

If the tracer dies and bypasses detach, the next tracer to trace the tracee
would get the memory refunded when he configures branch tracing - unless we take 
care about this in ptrace_bts_configure() and only refund the memory when there 
was a buffer to free.

But this would complicate the code even more.

I think that the underlying problem is that ptrace_detach() can be bypassed.
This bypasses also arch-specific cleanup code - that's why I added 
arch_ptrace_untrace().
It would all be very simple if that were not the case.

regards,
markus.