[Qemu-devel] [PATCH] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD

[Brad Smith <brad@comstyle.com>]

Use MAP_STACK in qemu_alloc_stack() on OpenBSD.

Added to -current and will be in our soon to be 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: Brad Smith <brad@comstyle.com>


diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index fbd0dc8c57..51e9a012c2 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -611,7 +611,11 @@ void *qemu_alloc_stack(size_t *sz)
     *sz += pagesz;
 
     ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE,
-               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+               MAP_PRIVATE | MAP_ANONYMOUS
+#ifdef MAP_STACK
+               | MAP_STACK
+#endif
+               , -1, 0);
     if (ptr == MAP_FAILED) {
         perror("failed to allocate memory for stack");
         abort();