[PATCH] pthread_getcpuclockid: Add descriptive comment to smoke test

[Florian Weimer]

* Siddhesh Poyarekar:

> On 2024-11-28 07:00, Florian Weimer wrote:
>> * Siddhesh Poyarekar:
>> 
>>> +/* The input thread descriptor to pthread_getcpuclockid needs to be valid when
>>> +   the function is called.  For the purposes of this test, this means that the
>>> +   thread should not be detached, have exited, but not joined.  There is also a
>>> +   tiny window where the TCB has been allocated but the target thread itself
>>> +   not created, where pthread_getcpuclockid could return ESRCH, but it's not
>>> +   possible to tap into that window to reliably test it.  It's not necessary
>>> +   anyway, since the window this test exploits is good enough to complete
>>> +   coverage for pthread_getcpuclockid alongside tst-clock2.  */
>> The kernel stores the TID to the TCB before the system call returns
>> (according to the clone manual page).
>> So the only race is for thread exit, not thread creation, and the
>> comment is misleading.
>
> I was thinking of a situation where the thread descriptor points to a
> TCB and the syscall has not been invoked yet; this would be
> exploitable when the thread descriptor itself is in memory shared with
> another thread.  But now that I'm saying it out loud, it doesn't sound
> like behaviour that ought to be relied upon, just something that the
> internals currently happen to allow.
>
> How about this then:
>
> /* The input thread descriptor to pthread_getcpuclockid needs to be
> valid when
>    the function is called.  For the purposes of this test, this means
>    that the
>    thread should not be detached, have exited, but not joined.  This
>    should be
>    good enough to complete coverage for pthread_getcpuclockid alongside
>    tst-clock2.  */

This looks reasonable to me (except for the line breaks).

Thanks,
Florian