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

[Florian Weimer]

On 03/27/2018 12:43 AM, H.J. Lu wrote:
> On Linux, when alternate signal stack is used with thread cancellation,
> _Unwind_Resume fails when it tries to unwind shadow stack from signal
> handler on alternate signal stack.  The issue is that signal handler on
> alternate signal stack uses a separate shadow stack and we must switch
> to the original shadow stack to unwind it. But frame count will be wrong
> in this case.  For thread cancellation, there is no need to unwind shadow
> stack since it will long jump back and exit.
> 
> One possibility is
> 
> 1. Add  _URC_NO_REASON_CANCEL.
> 2. unwind_stop in libpthread returns _URC_NO_REASON_CANCEL.
> 3.  _Unwind_ForcedUnwind_Phase2 sets frames to 1 for
> _URC_NO_REASON_CANCEL

I assume the sequence of events goes like this:

1. The program receives a signal with a SA_ONSTACK handler.
2. The program switches to the alternate signal stack (including an 
alternate shadow stack) and runs the handler.
3. The handler reaches a cancellation point.
4. Cancellation is acted upon.

During unwinding, INCSSP is executed as needed.  The switch from the 
alternate signal stack is implicit in the SP register restore.  But 
there is no corresponding stack switch back to the original shadow 
stack.  This means that INCSSP faults once the alternate stack is empty.

Is this description accurate?

I think this has to be fixed entirely within the libgcc unwinder. 
Otherwise, any application which throws from a (synchronous) signal 
handler will have the same issue, and I think this is something we need 
to support.

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.

Thanks,
Florian