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

On Mon, Mar 26, 2018 at 11:31 PM, Florian Weimer <> wrote:
> 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
>> 2. unwind_stop in libpthread returns _URC_NO_REASON_CANCEL.
>> 3.  _Unwind_ForcedUnwind_Phase2 sets frames to 1 for
> 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?

That is correct.

> 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.

There are 2 ways to unwind shadow stack:

1. setjmp saves shadow stack register and longjmp pops shadow stack
until shadow stack register matches the saved value.   To support longjmp
from signal handler, we make a syscall to restore the original shadow stack.
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:

and glibc:

They fixed the issue.

> 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.


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