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>
thanks
-- PMM
