Re: [PATCH] x86: Add __sigsetjmp_cancel and __setjmp_cancel

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

On Thu, Mar 29, 2018 at 12:42 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
> * H. J. Lu:
>
>> On Thu, Mar 29, 2018 at 10:34 AM, Florian Weimer <fw@deneb.enyo.de> wrote:
>>> * H. J. Lu:
>>>
>>>> Here is the updated patch.  OK for master?
>>>
>>> If you want to backport this into Fedora 28, you will have to avoid
>>> new symbol versions.
>>
>> Should we revisit my patch to extend struct pthread_unwind_buf:
>>
>> https://sourceware.org/ml/libc-alpha/2018-02/msg00291.html
>>
>> This patch doesn't introduce the new symbol version.
>
> That patch will require recompiling a subset of static libraries in
> Fedora 28 to avoid crashes once CET is enabled.  Which should be
> doable at a technical level, but takes resources away from other work,
> obviously.

BOTH approaches NEED to recompile a subset of static libraries in Fedora
28, as stated in

https://sourceware.org/ml/libc-alpha/2018-03/msg00521.html

NB: Shared libraries built with glibc 2.27 or older are compatible with
glibc 2.28.  Relocatable objects compiled against glibc 2.27 or older
are incompatible with glibc 2.28 since they reference the older version
of __sigsetjmp.

> My preference still is a solution where we shift the stored signal
> mask to make room for the shadow stack data in CET mode (for maximum
> compatibility).  But I expect that Carlos will review your patch.

As I have pointed out before

Note: There is an unused pointer space in pthread_unwind_buf_data.  But
it isn't suitable for saving and restoring shadow stack register since
x32 is a 64-bit process with 32-bit software pointer and kernel may
place x32 shadow stack above 4GB.  We need to save and restore 64-bit
shadow stack register for x32.

-- 
H.J.