[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PATCH 1/4] coroutine: support SafeStack in ucontext backend
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH 1/4] coroutine: support SafeStack in ucontext backend |
Date: |
Wed, 27 May 2020 11:34:46 +0100 |
On Fri, May 22, 2020 at 11:18:20AM -0400, Daniele Buono wrote:
> On 5/21/2020 5:44 AM, Stefan Hajnoczi wrote:
> > On Wed, Apr 29, 2020 at 03:44:17PM -0400, Daniele Buono wrote:
> > > @@ -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?
> >
> Correct, setjmp/longjmp have no visibility of the unsafe stack. What I
> meant is that it is not automatically restored by the longjmp itself,
> but by code the compiler adds around the sigsetjmp.
>
> Specifically, every sigsetjmp/sigjmp is intercepted by the compiler, the
> current value of __safestack_unsafe_stack_ptr is saved on the normal
> (safe) stack.
> Right after the sigsetjmp call it is then restored.
>
> I will change the comment to make it clearer.
>
> In practice, this is what happens:
>
> Original clang implementation in qemu_coroutine_new:
> ----
> 40130c: callq 4008d0 <__sigsetjmp@plt>
> 401311: cmp $0x0,%eax
> 401314: jne 40132d <qemu_coroutine_new+0x12d>
> ----
> Clang Implementation with safestack:
> ----
> 4027a7: mov %rdx,-0x38(%rbp) <- Save unsafe ptr onto the safe stack
> [...]
> 40289c: callq 401410 <__sigsetjmp@plt>
> 4028a1: mov 0x201738(%rip),%rdi # 603fe0
> <__safestack_unsafe_stack_ptr@@Base+0x603fe0>
> 4028a8: mov -0x38(%rbp),%rbx
> 4028ac: mov %rbx,%fs:(%rdi) <- Restore the unsafe ptr
> 4028b0: cmp $0x0,%eax
> 4028b3: jne 4028d9 <qemu_coroutine_new+0x179>
Oh, that's interesting. Thanks for explaining and updating the comment.
Stefan
signature.asc
Description: PGP signature