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

[Kamil Rytarowski]

On 07.10.2018 17:37, Brad Smith 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>
> 
> 
> 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();
> 

Can we handle it differently, storing MAP_* flags in a variable:

int flags = MAP_PRIVATE | MAP_ANONYMOUS;
#ifdef MAP_STACK
flags |= MAP_STACK;
#endif

ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE, flags, -1, 0);

This way it will look nicer as we won't ifdef the middle of a function call.

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