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

["Dmitry V. Levin" <ldv at altlinux dot org>]

On Wed, Jan 24, 2018 at 05:09:39PM -0800, H.J. Lu wrote:
> 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.

I'm afraid by Monday it will be too late for 2.27 as we will get very
little testing before the release.


-- 
ldv

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