Re: RFA: Need to extend x86 psABI to support thread cancellation and alternate signal stack

[Florian Weimer <fweimer at redhat dot com>]

On 03/27/2018 01:26 PM, H.J. Lu wrote:

> 2. Since shadow stack is never saved and restored by compiler, unwinder
> in libgcc counts how many stack frame it has to unwind and uses INCSSP
> to pop shadow stack.  This can't unwind the original shadow stack when
> the alternate shadow stack is used.  _URC_NO_REASON_CANCEL
> works only if longjmp will be used to finish stack unwinding, which is
> the case for thread cancellation in glibc.
>
> Here are patches for GCC:
>
> https://github.com/hjl-tools/gcc/commit/e9ff815941406e38fa629947af4d809b9129e860
>
> and glibc:
>
> https://github.com/hjl-tools/glibc/commit/1aec81528ab26aa8a8a7965317b6e1a8ba4526aa
>
> They fixed the issue.

The patches are nice and short, but: Do they really fix the issue?  They 
make cancellation work again, but they do not fix the general unwinding 
issue with alternate signal handler stacks AFAICS.

> > It may be possible to implement this without kernel changes: Patch the
> > interrupted context to continue unwinding, and then call sigreturn to switch
> > both stacks at the same time.
>
> We passed almost all 5000+ tests in glibc with shadow stack and indirect
> branch tracking enabled. The only remaining failures are thread cancellation
> with alternate signal stack and -fasynchronous-unwind-tables.  I couldn't
> find a way to unwind shadow stack by counting stack frame when exception
> happens in alternate signal stack.

I'm not sure how comprehensive these tests are, considering that no one 
expected something like shadow stacks (maybe those dual ia64 stacks are 
somewhat similar, but I don't know anything about them).

Thanks,
Florian