Re: [PATCH 1/2] Linux/x86: Update cancel_jmp_buf to match __jmp_buf_tag [BZ #22563]

[Florian Weimer <fweimer at redhat dot com>]

On 12/07/2017 06:40 PM, H.J. Lu wrote:
> On x86, padding in struct __jmp_buf_tag is used for shadow stack pointer
> to support shadow stack in Intel Control-flow Enforcemen Technology.
> Since the cancel_jmp_buf array is passed to setjmp and longjmp by
> casting it to pointer to struct __jmp_buf_tag, it should be as large
> as struct __jmp_buf_tag.  This patch adds pthread.h, pthreaddef.h and
> pthreadP.h for Linux/x86 to define a new cancel_jmp_buf to match
> struct __jmp_buf_tag.

This seems the wrong thing to do.

I don't think cancellation needs the shadow stack because none of the 
functions on the existing stack return during cancellation processing.

Furthermore, SJLJ-style cancellation appears to defeat CET anyway, so 
I'm puzzled why aren't trying to get rid of this type of cancellation 
implementation instead because it looks like a fairly significant 
weakness (similar to SEH on Windows).

If you want to preserve SJLJ-style cancellation as-is, 
__pthread_unwind_buf_t has sufficient padding, and you could simply use 
that.

Thanks,
Florian