[PATCH 5/8] posix: Do not clobber errno by atfork handlers

[Adhemerval Zanella]

On 09/03/2021 08:01, Florian Weimer wrote:
> * Adhemerval Zanella via Libc-alpha:
> 
>> Checked on x86_64-linux-gnu.
>> ---
>>  posix/fork.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/posix/fork.c b/posix/fork.c
>> index 4c9e60f187..7f27fb8338 100644
>> --- a/posix/fork.c
>> +++ b/posix/fork.c
>> @@ -68,7 +68,7 @@ __libc_fork (void)
>>      }
>>  
>>    pid_t pid = _Fork ();
>> -
>> +  int save_errno = errno;
>>    if (pid == 0)
>>      {
>>        /* Reset the lock state in the multi-threaded case.  */
>> @@ -107,6 +107,8 @@ __libc_fork (void)
>>    __run_fork_handlers (pid == 0 ? atfork_run_child : atfork_run_parent,
>>  		       multiple_threads);
>>  
>> +  if (pid < 0)
>> +    __set_errno (save_errno);
>>    return pid;
>>  }
>>  weak_alias (__libc_fork, __fork)
> 
> Why is this condition correct?  Shouldn't it be pid >= 0?

But pid >= 0 is a valid call which should not set errno.  The idea is
to return the _Fork error value even if some atfork handler in the
parent does clobber it.

> 
> I wonder if this should be part of __run_fork_handlers, so that fork
> handlers always observe the original errno value.

I am not sure about it, checking the errno without actually calling a
function that might setting it is error-prone imho.