Re: Thread flags modified without set_thread_flag() (non atomically)

[Mathieu Desnoyers <compudj@google.com>]

Andi Kleen wrote:
>> It does seem risky.  Perhaps it is a micro-optimisation which utilises
>> knowledge that this thread_struct cannot be looked up via any path in this
>> context.
>>
>> Or perhaps it is a bug.  Andi, can you please comment?
>>     
>
> On flush_thread nobody else can mess with the thread, so yes it's a micro
> optimization.
>
>   
Hi Andi,

Here is what I think would be a counter example :

If, at the same time, we have, on x86_64 :

parent process executing :
sys_ptrace()
  (lock_kernel())
  (ptrace_get_task_struct(pid))
  arch_ptrace()
    ptrace_detach()
      ptrace_disable(child);
        clear_singlestep(child);
          clear_tsk_thread_flag(child, TIF_SINGLESTEP);
          (which clears the TIF_SINGLESTEP  flag atomically from a 
different process)
  (put_task_struct(child))
  (unlock_kernel())

And at the same time, in the child process :
sys_execve()
  do_execve()
    search_binary_handler()
      load_elf_binary()
        flush_old_exec()
          flush_thread()
            doing a non-atomic thread flag update

Is there any protection mechanism that would protect from this race 
condition
that I have missed ?

>>> And about this specific flush_thread, I am puzzled about the t->flags ^= 
>>> (_TIF_ABI_PENDING | _TIF_IA32); line. The XOR will clearly flip the 
>>> _TIF_ABI_PENDING bit to 0, and very likely set _TIF_IA32 to the opposite 
>>> of its current value. Why does this change need to be written atomically 
>>> (can other threads play with these flags ?) ?
>>>
>>>       
>> Don't know.
>>     
>
> iirc it came from DaveM originally. He just likes to write things in 
> comp^wclever ways :0) It's just a little shorter.
>
>   
>> No, I don't immediately see anything in the flush_old_exec() code path
>> which tells us that nobody else can look up this thread_info (or be holding
>> a ref to it) in this context.
>>     
>
> Normally the process flags atomicity should only matter with signals;
> i don't think you can send a signal to a process being in exec this way.
>
> -Andi
>