Re: [PATCH 1/4] coroutine: support SafeStack in ucontext backend

[Stefan Hajnoczi]

On Wed, Apr 29, 2020 at 03:44:17PM -0400, Daniele Buono wrote:
> diff --git a/include/qemu/coroutine_int.h b/include/qemu/coroutine_int.h
> index bd6b0468e1..2ffd75ddbe 100644
> --- a/include/qemu/coroutine_int.h
> +++ b/include/qemu/coroutine_int.h
> @@ -28,6 +28,12 @@
>  #include "qemu/queue.h"
>  #include "qemu/coroutine.h"
>  
> +#if defined(__has_feature) && __has_feature(safe_stack)
> +#define CONFIG_SAFESTACK 1

Please perform this feature check in ./configure. That way
CONFIG_SAFESTACK will be defined alongside all the other CONFIG_* values
and be available to C and Makefiles via config-host.h and
config-host.mak.

> @@ -160,6 +169,19 @@ Coroutine *qemu_coroutine_new(void)
>      /* swapcontext() in, siglongjmp() back out */
>      if (!sigsetjmp(old_env, 0)) {
>          start_switch_fiber(&fake_stack_save, co->stack, co->stack_size);
> +#ifdef CONFIG_SAFESTACK
> +        /*
> +         * Before we swap the context, set the new unsafe stack
> +         * The unsafe stack grows just like the normal stack, so start from
> +         * the last usable location of the memory area.
> +         * NOTE: we don't have to re-set it afterwards because sigsetjmp was
> +         * called with the original usp. Since we are not coming back with a
> +         * swapcontext, but with a siglongjmp, when we are back here we
> +         * already have usp restored to the valid one for this function

I don't understand this comment. __safestack_unsafe_stack_ptr is a
thread-local variable, not a CPU register. How will siglongjmp()
automatically restore it?

> +         */
> +        void *usp = co->unsafe_stack + co->unsafe_stack_size;
> +        __safestack_unsafe_stack_ptr = usp;
> +#endif
>          swapcontext(&old_uc, &uc);
>      }
>

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