Re: [PATCH] Remove cached PID/TID in clone

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 08/11/2016 18:11, Florian Weimer wrote:
> * Adhemerval Zanella:
> 
>>> It's a hotspot for incorrect/broken fork detection.
>>
>> If you mean the assert on fork.c, I review the code and it seems
>> unnecessary to remove the assert on child creation:
> 
> No, something else entirely.  OpenSSL mixes the current PID into the
> randomness pool, in an attempt to make sure that the streams generated
> by parent and child are different:
> 
>     pid_t curr_pid = getpid();
> …
>         if (curr_pid) {         /* just in the first iteration to save time */
>             if (!MD_Update(m, (unsigned char *)&curr_pid, sizeof curr_pid))
>                 goto err;
>             curr_pid = 0;
>         }
> 
> <https://github.com/openssl/openssl/blob/master/crypto/rand/md_rand.c#L283>
> 
> This happens at every invocation of RAND_bytes.  It may show up in
> profiles if all the other system calls (time, gettimeofday etc.) are
> handled by the vDSO.
> 
> But I suggest that this shouldn't block your change.  It's just
> something we should be aware of.  If the kernel provides a more
> efficient way to get the PID, we can change glibc to use it.
> 
> More comments about your revised patch tomorrow.
> 

Right, I referenced a quite old discussion about the pid caching
and the randomness provided by getpid [1].  And I think that an
portable application like OpenSSL should not rely on an specific
getpid implementation in a hotspot call, since on mostly 
implementations it will likely be a syscall.

[1] http://yarchive.net/comp/linux/getpid_caching.html