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Re: [PATCH 0/2] nptl: Update struct pthread_unwind_buf
On Sun, Feb 25, 2018 at 6:13 AM, Florian Weimer <email@example.com> wrote:
> * H. J. Lu:
>> On Sun, Feb 25, 2018 at 5:49 AM, Florian Weimer <firstname.lastname@example.org> wrote:
>>> * H. J. Lu:
>>>> On Sun, Feb 25, 2018 at 5:31 AM, Florian Weimer <email@example.com> wrote:
>>>>> * H. J. Lu:
>>>>>> libpthread cancellation implementation passes cancel_jmp_buf to
>>>>>> libgcc unwinder,
>>>>> Oh. Where does it do that? If you mean _Unwind_ForcedUnwind, I think
>>>>> that's just an opaque closure argument for the callback.
>>>> Yes. Libgcc unwinder needs to deal with it.
>>> Please point me to the code. Thanks.
>> sysdeps/nptl/unwind-forcedunwind.c has
>> _Unwind_ForcedUnwind (struct _Unwind_Exception *exc, _Unwind_Stop_Fn stop,
>> void *stop_argument)
>> if (__glibc_unlikely (libgcc_s_handle == NULL))
>> pthread_cancel_init ();
>> atomic_read_barrier ();
>> _Unwind_Reason_Code (*forcedunwind)
>> (struct _Unwind_Exception *, _Unwind_Stop_Fn, void *)
>> = libgcc_s_forcedunwind;
>> PTR_DEMANGLE (forcedunwind);
>> return forcedunwind (exc, stop, stop_argument);
> Thanks. I think stop_argument ends up in the private_2 member inside
> unwind.inc, which is only passed back to the callback (the stop
> function pointer) in _Unwind_ForcedUnwind_Phase2, and not interpreted
> by libgcc itself. So this shouldn't be a problem.
Please take a look at hjl/setjmp/cancel branch:
Functions, like LIBC_START_MAIN, START_THREAD_DEFN as well as these
with thread cancellation, call setjmp, but never return to their
callers after longjmp returns. This patch adds <bits/setjmp-cancel.h>
and <setjmp-cancelP.h> to provide a version of setjmp family functions,
__setjmp_cancel and __sigsetjmp_cancel, which are used to implement
thread cancellation. The default __setjmp_cancel and __sigsetjmp_cancel
are defined as setjmp and __sigsetjmp, respectively, which are the same
On x86, a different version is added to avoid saving and restoring
shadow stack register. __libc_longjmp, which is a private interface
for cancellation implementation in libpthread, is changed to call
__longjmp_cancel instead of __longjmp.
This leaves cancel_jmp_buf unchanged. But is it worth it?
1. I have to add __setjmp_cancel and __sigsetjmp_cancel which won't
save and restore shadow stack register.
2. I still need yet to add the new version of __sigsetjmp for older binaries.
3, Older .o files compiled against glibc 2.27 are still incompatible with