Re: [Qemu-devel] [PATCH v2] oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD

[Peter Maydell]

On 18 October 2018 at 23:10, Brad Smith <address@hidden> 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 <address@hidden>
> Reviewed-by: Kamil Rytarowski <address@hidden>
>
> 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 <address@hidden>

thanks
-- PMM