Re: [PATCH] Revert Intel CET changes to __jmp_buf_tag (Bug 22743)

[Florian Weimer <fweimer at redhat dot com>]

On 01/25/2018 01:38 PM, H.J. Lu wrote:
> > I still think you are over-engineering this.  The pad array has still an
> > unused member (the last one).  Just change sigsetjmp to store the shadow
> > pointer in that location, then the old and new setjmp will work with the
> > current stack layout.  As far as I can tell, there are only 64 signals, so
> > you don't even have to change the location of the signal mask.
> No, it doesn't work.  struct pthread_unwind_buf is placed on caller's stack
> and its address is passed from applications to libpthread.   If the size of
> caller's struct pthread_unwind_buf is smaller than what libpthread expects,
> libpthread will override caller's stack.

As far as I can see, ibuf->priv.pad[3] is currently unused.  (sig)setjmp 
could save the shadow stack pointer at the right offset in jmp_buf to 
hit this place, then all these new conditionals wouldn't be necessary. 
Of course it is still a hack, but your approach is not clearer IMHO.

The patch you posted is very different from the commit link you shared 
earlier.  I still need to review it in detail.

Thanks,
Florian