pthread_create: fix segfault when attr is NULL

[Paul Irofti]

Hi,

The current implementation does not check for NULL attr values and
segfaults when creating new threads if that's the case:

Program received signal SIGSEGV, Segmentation fault.
0x00000a77e9550ade in pthread_create (threadp=0x7f7ffffe5488, 
attr=0x7f7ffffe5480, 
    start_routine=0xa7539a00600 <a_thread_func>, arg=<optimized out>) at 
/usr/src/lib/librthread/rthread.c:370
370             thread->attr = attr != NULL ? *(*attr) : _rthread_attr_default;
(gdb) p attr
$1 = (const pthread_attr_t *) 0x7f7ffffe5480
(gdb) p *attr
$2 = (const pthread_attr_t) 0x0


The diff bellow fixes the issue by checking *attr and returning EINVAL
if it is NULL. OK?

(Surprisingly the manual already documents this.)

Paul


Index: rthread.c
===================================================================
RCS file: /cvs/src/lib/librthread/rthread.c,v
retrieving revision 1.99
diff -u -p -u -p -r1.99 rthread.c
--- rthread.c   4 Nov 2017 22:53:57 -0000       1.99
+++ rthread.c   6 Jul 2018 12:04:07 -0000
@@ -367,6 +367,9 @@ pthread_create(pthread_t *threadp, const
        thread->arg = arg;
        tib->tib_tid = -1;
 
+       if (attr != NULL && *attr == NULL)
+               return (EINVAL);
+
        thread->attr = attr != NULL ? *(*attr) : _rthread_attr_default;
        if (thread->attr.sched_inherit == PTHREAD_INHERIT_SCHED) {
                pthread_t self = pthread_self();