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

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Tue, Mar 27, 2018 at 8:43 AM, Florian Weimer <fweimer@redhat.com> wrote:
> 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.

That is true.   We do support unwinding with alternate signal handler stack
using longjmp.   If there is another use case of unwinding with alternate signal
handler stack, we can investigate.  If this isn't a valid use case, we
don't want
to create a very complex scheme to support it.

>>> 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).

Glibc tests are invaluable to validate CET implementation.  I am planning to
backport CET support to glibc 2.27 so that I can enable CET in applications
under Fedora 28.

-- 
H.J.