[ARM/FDPIC v6 13/24] [ARM] FDPIC: Force LSB bit for PC in Cortex-M architecture

[Christophe Lyon]

On Fri, 20 Sep 2019 at 14:51, Matthew Malcomson
<Matthew.Malcomson@arm.com> wrote:
>
> On 19/09/19 16:14, Kyrill Tkachov wrote:
> >
> > On 9/19/19 4:13 PM, Christophe Lyon wrote:
> >> On Tue, 17 Sep 2019 at 14:08, Christophe Lyon <christophe.lyon@st.com>
> >> wrote:
> >> >
> >> > On 17/09/2019 13:38, Wilco Dijkstra wrote:
> >> > > Hi Christophe,
> >> > >
> >> > > Can you explain this in more detail - it doesn't make sense to me
> >> to force the
> >> > > Thumb bit during unwinding since it should already be correct,
> >> even on a
> >> > > Thumb-only CPU. Perhaps the kernel code that pushes an incorrect
> >> address on
> >> > > the stack could be fixed instead?
> >> > >
> >> > >> Without this, when we are unwinding across a signal frame we can
> >> jump
> >> > >> to an even address which leads to an exception.
> >> > >>
> >> > >> This is needed in __gnu_persnality_sigframe_fdpic() when
> >> restoring the
> >> > >> PC from the signal frame since the PC saved by the kernel has the
> >> LSB
> >> > >> bit set to zero.
> >> > >
> >> > > Wilco
> >> > > .
> >> > >
> >> >
> >> > Indeed, I've noticed the problem mentioned by Matthew since I
> >> committed that patch.
> >> >
> >> > I was about to propose a fix, replacing #if (__thumb__) with #if
> >> (!__ARM_ARCH_ISA_ARM), but you are right: maybe the kernel code should
> >> be fixed instead.
> >> >
> >> > So far I haven't managed to reproduce a failure in FDPIC mode
> >> without this patch though...
> >> >
> >> > Thanks and sorry for the breakage.
> >> >
> >>
> >> I'm having problems with the board I use for testing, so I propose to
> >> revert that patch until I have a better description of the problem it
> >> fixed.
> >> OK?
> >
> > Ok by me as long as lives the fdpic toolchain in a usable state (barring
> > the potential issue here)
>
> Thanks Christophe -- reverting that patch would help our internal
> testing a lot!
> MM
>
OK, I've reverted it.

Christophe

> >
> > Thanks,
> >
> > Kyrill
> >
> >
> >>
> >> Christophe
> >>
> >> > Christophe
> >> >
>