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

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

On Wed, Jan 24, 2018 at 4:32 PM, Carlos O'Donell <carlos@redhat.com> wrote:
> On 01/24/2018 10:23 AM, Florian Weimer wrote:
>> On 01/24/2018 07:08 PM, H.J. Lu wrote:
>>> We opened a bug:
>>>
>>> https://sourceware.org/bugzilla/show_bug.cgi?id=22743
>>>
>>> Any help to track down the root cause is appreciated.
>>
>> Doesn't the bug report clearly show the root cause?  The offset of
>> priv.data.cleanup changed, and old binaries have an insufficiently
>> large stack allocation for the new offset.
>>
>> (Congratulations for tracking it down, by the way.  I know that such
>> bugs are hard.)
>>
>> You need to add a symbol version for pthread_register_cancel.  It's
>> too late for that now, so I recommend reverting the faulty commit.
>
> I have finished analyzing this and debugging the root cause myself,
> and I agree with Florian, we need to revert:
>
> commit f81ddabffd76ac9dd600b02adbf3e1dac4bb10ec
> commit cba595c350e52194e10c0006732e1991e3d0803b
>
> At a minimum. I am testing with them reverted locally.
>
> To be honest I'm surprised that this passed review and was checked
> in, because the __pthread_unwind_buf_t has only at most 4-bytes of
> space left before it is an ABI change. In the future please ping
> me if you have any doubts and I'll review.
>
> The addition of __sigset_t saved_mask moves pthread_unwind_buf's
> priv.data.cleanup forward by 124-bytes. The on-stack allocation of
> the pthread_cleanup_push's __pthread_unwind_buf_t is not that big
> and so __pthread_register_cancel writes to other structures which
> are allocated on the stack.
>
> You cannot expand struct pthread_unwind_buf because the on-stack
> allocated __pthread_unwind_buf_t is not large enough in existing
> applications.
>
> You *might* have used feature_1 to change between two different
> layouts of struct pthread_unwind_buf, but that will have to wait
> for 2.28. As Florian suggests though it is cleaner to version
> __pthread_register_cancel for x86 and the older version expects
> the smaller non-CET-enabled structure.
>

I will try to fix it by next Monday.


-- 
H.J.