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: Brad Smith <brad@comstyle.com>
Reviewed-by: Kamil Rytarowski <n54@gmx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20181019125239.GA13884@humpty.home.comstyle.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index fbd0dc8..c1bee2a 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -596,6 +596,7 @@
void *qemu_alloc_stack(size_t *sz)
{
void *ptr, *guardpage;
+ int flags;
#ifdef CONFIG_DEBUG_STACK_USAGE
void *ptr2;
#endif
@@ -610,8 +611,18 @@
/* 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();
