On 10/19/2018 7:55 AM, Peter Maydell wrote:
On 18 October 2018 at 23:10, Brad Smith <b...@comstyle.com> wrote:
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 <b...@comstyle.com>
Reviewed-by: Kamil Rytarowski <n...@gmx.com>
diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index fbd0dc8c57..7814e61114 100644
--- a/util/oslib-posix.c
+++ b/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 +611,15 @@ 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. */
+ flags |= MAP_STACK;
+#endif
I think it would be useful to also add "OpenBSD will SEGV
if it spots execution with a stack pointer pointing at
memory that was not allocated with MAP_STACK." (summarising
what the interesting bit of OpenBSD behaviour is here to save
having to go back and read the commit message in future).
Also our multiline comment format is linux-kernel style
/*
* line 1
* line 2
*/
(as noted in CODING_STYLE).
Otherwise
Reviewed-by: Peter Maydell <peter.mayd...@linaro.org>
Added the comment and adjusted the comment format.
Thanks.
