Re: [ARM/FDPIC v5 13/21] [ARM] FDPIC: Force LSB bit for PC in Cortex-M architecture

[Ian Lance Taylor <ian@airs.com>]

Christophe Lyon <christophe.lyon@linaro.org> writes:

> Sorry, I forgot again to cc: Ian.

As far as I'm concerned, it's fine for architecture maintainers to
approve changes to architecture-specific files in libgcc.

Ian



> On Thu, 5 Sep 2019 at 10:30, Christophe Lyon <christophe.lyon@linaro.org> wrote:
>>
>> On Thu, 29 Aug 2019 at 17:32, Kyrill Tkachov
>> <kyrylo.tkachov@foss.arm.com> wrote:
>> >
>> > Hi Christophe,
>> >
>> > On 5/15/19 1:39 PM, Christophe Lyon wrote:
>> > > 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.
>> > >
>> > > 2019-XX-XX  Christophe Lyon  <christophe.lyon@st.com>
>> > >         Mickaël Guêné <mickael.guene@st.com>
>> > >
>> > >         libgcc/
>> > >         * config/arm/unwind-arm.c (_Unwind_VRS_Set): Handle v7m
>> > >         architecture.
>> > >
>> > > Change-Id: Ie84de548226bcf1751e19a09e8f091fb3013ccea
>> > >
>> > > diff --git a/libgcc/config/arm/unwind-arm.c
>> > > b/libgcc/config/arm/unwind-arm.c
>> > > index 9ba73e7..ba47150 100644
>> > > --- a/libgcc/config/arm/unwind-arm.c
>> > > +++ b/libgcc/config/arm/unwind-arm.c
>> > > @@ -199,6 +199,11 @@ _Unwind_VRS_Result _Unwind_VRS_Set
>> > > (_Unwind_Context *context,
>> > >          return _UVRSR_FAILED;
>> > >
>> > >        vrs->core.r[regno] = *(_uw *) valuep;
>> > > +#if defined(__ARM_ARCH_7M__)
>> > > +      /* Force LSB bit since we always run thumb code.  */
>> > > +      if (regno == 15)
>> > > +       vrs->core.r[regno] |= 1;
>> > > +#endif
>> >
>> > Hmm, this looks quite specific. There are other architectures that are
>> > thumb-only too (6-M, 7E-M etc).
>> >
>> > Would checking for __thumb__ be better?
>> >
>> Right.
>> The attached updated patch also uses R_PC instead of 15.
>>
>> Christophe
>>
>> > Thanks,
>> >
>> > Kyrill
>> >
>> >
>> > >        return _UVRSR_OK;
>> > >
>> > >      case _UVRSC_VFP:
>> > > --
>> > > 2.6.3
>> > >