oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD

Use MAP_STACK in qemu_alloc_stack() on OpenBSD.

Added to our 6.4 release.

MAP_STACK      Indicate that the mapping is used as a stack.  This
               flag must be used in combination with MAP_ANON and
               MAP_PRIVATE.

Implement MAP_STACK option for mmap().  Synchronous faults (pagefault and
syscall) confirm the stack register points at MAP_STACK memory, otherwise
SIGSEGV is delivered. sigaltstack() and pthread_attr_setstack() are modified
to create a MAP_STACK sub-region which satisfies alignment requirements.
Observe that MAP_STACK can only be set/cleared by mmap(), which zeroes the
contents of the region -- there is no mprotect() equivalent operation, so
there is no MAP_STACK-adding gadget.
Signed-off-by: NBrad Smith <brad@comstyle.com>
Reviewed-by: NKamil Rytarowski <n54@gmx.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 20181019125239.GA13884@humpty.home.comstyle.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

util/oslib-posix.c

@@ -596,6 +596,7 @@ pid_t qemu_fork(Error **errp)
 void *qemu_alloc_stack(size_t *sz)
 {
     void *ptr, *guardpage;
+    int flags;
 #ifdef CONFIG_DEBUG_STACK_USAGE
     void *ptr2;
 #endif
@@ -610,8 +611,18 @@ void *qemu_alloc_stack(size_t *sz)
     /* allocate one extra page for the guard page */
     *sz += pagesz;
 
-    ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE,
-               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+    flags = MAP_PRIVATE | MAP_ANONYMOUS;
+#if defined(MAP_STACK) && defined(__OpenBSD__)
+    /* Only enable MAP_STACK on OpenBSD. Other OS's such as
+     * Linux/FreeBSD/NetBSD have a flag with the same name
+     * but have differing functionality. OpenBSD will SEGV
+     * if it spots execution with a stack pointer pointing
+     * at memory that was not allocated with MAP_STACK.
+     */
+    flags |= MAP_STACK;
+#endif
+
+    ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE, flags, -1, 0);
     if (ptr == MAP_FAILED) {
         perror("failed to allocate memory for stack");
         abort();