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

On Thu, Dec 7, 2017 at 10:36 AM, Florian Weimer <> wrote:
> 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.

No, shadow stack doesn't work that way.  Once it is turned on, it is on
until the process exits.   There is no such a thing of that cancellation
doesn't need shadow stack.


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