Shift top-of-stack down so that the random==0 case doesn't leave stack

pointer beyond the space.
ok stefan, tedu

diff --git a/lib/librthread/rthread_stack.c b/lib/librthread/rthread_stack.c
index f40ca00..73c13e8 100644 (file)
--- a/lib/librthread/rthread_stack.c
+++ b/lib/librthread/rthread_stack.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: rthread_stack.c,v 1.17 2017/09/05 02:40:54 guenther Exp $ */
+/* $OpenBSD: rthread_stack.c,v 1.18 2018/02/10 22:59:02 deraadt Exp $ */
 
 /* PUBLIC DOMAIN: No Rights Reserved. Marco S Hyman <marc@snafu.org> */
 
@@ -65,7 +65,7 @@ _rthread_alloc_stack(pthread_t thread)
 #ifdef MACHINE_STACK_GROWS_UP
                stack->sp = base + rnd;
 #else
-               stack->sp = base + thread->attr.stack_size - rnd;
+               stack->sp = base + thread->attr.stack_size - (_STACKALIGNBYTES+1) - rnd;
 #endif
                /*
                 * This impossible guardsize marks this stack as
@@ -103,7 +103,7 @@ _rthread_alloc_stack(pthread_t thread)
        stack->sp = base + rnd;
 #else
        guard = base;
-       stack->sp = base + size - rnd;
+       stack->sp = base + size - (_STACKALIGNBYTES+1) - rnd;
 #endif
 
        /* memory protect the guard region */