Re: [PATCH] Don't return new thread ID from pthread_create.

[Rich Felker <dalias@libc.org>]

On Thu, Aug 16, 2018 at 01:09:33AM +0200, Piotr Tworek wrote:
> From: Piotr Tworek <tworaz666@gmail.com>
> 
> According to documentation pthread_create is supposed to return 0 on
> success or error code in case of failure. Unfortunately this is not
> always the case in musl. In case the application has called
> pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED),
> pthread_create will return new thread ID. To make matters worse such non
> zero value does not mean thread creation failed. Its quite the oposite.
> Thread was created succesfully and start routine passed to pthread_create
> will actually get invoked.
> 
> To fix the problem simply drop the return statement from the last
> if statement in musl's pthread_create. The value currently being returned
> from there is a return code from __clone. Negative values indicating
> failure have already been handled by previous if statement and positive
> value indicates everything went correctly and pthread_create should return 0.
> ---
>  src/thread/pthread_create.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
> index 2df2e9f9..792cb42e 100644
> --- a/src/thread/pthread_create.c
> +++ b/src/thread/pthread_create.c
> @@ -306,7 +306,6 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
>  
>  	if (do_sched) {
>  		__futexwait(&ssa.futex, -1, 1);
> -		if (ret) return ret;
>  	}
>  
>  	*res = new;

You're right that there's a bug, but the fix is not correct, and in
some sense makes the problem worse -- pthread_create will wrongly
return success with an invalid pthread_t as the result (since
__start_sched detached itself and exited).

The problem is just that the value of ret isn't being updated to match
the error returned from __start_sched. Rather than:

 	if (do_sched) {
 		__futexwait(&ssa.futex, -1, 1);
-		if (ret) return ret;
 	}

it should look like (untested):

 	if (do_sched) {
 		__futexwait(&ssa.futex, -1, 1);
+		ret = ssa.futex;
		if (ret) return ret;
 	}

Note that this is a new regression and not in any released version
yet. Let me know if you see any problems with the fix I just proposed
and if you get a chance to test that it works, and I'll make sure the
fix goes in for this release.

Rich