Re: [PATCH][GCC] arm: Add support for new frame unwinding instruction "0xb5".

[Ramana Radhakrishnan <ramana.gcc@googlemail.com>]

On Fri, Nov 18, 2022 at 9:33 AM Srinath Parvathaneni
<Srinath.Parvathaneni@arm.com> wrote:
>
> Hi,
>
> > -----Original Message-----
> > From: Ramana Radhakrishnan <ramana.gcc@googlemail.com>
> > Sent: Thursday, November 17, 2022 8:27 PM
> > To: Srinath Parvathaneni <Srinath.Parvathaneni@arm.com>
> > Cc: gcc-patches@gcc.gnu.org; Richard Earnshaw
> > <Richard.Earnshaw@arm.com>; Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
> > Subject: Re: [PATCH][GCC] arm: Add support for new frame unwinding
> > instruction "0xb5".
> >
> > On Thu, Nov 10, 2022 at 10:38 AM Srinath Parvathaneni via Gcc-patches <gcc-
> > patches@gcc.gnu.org> wrote:
> > >
> > > Hi,
> > >
> > > This patch adds support for Arm frame unwinding instruction "0xb5"
> > > [1]. When an exception is taken and "0xb5" instruction is encounter
> > > during runtime stack-unwinding, we use effective vsp as modifier in pointer
> > authentication.
> > > On completion of stack unwinding if "0xb5" instruction is not
> > > encountered then CFA will be used as modifier in pointer authentication.
> > >
> > > [1]
> > > https://github.com/ARM-software/abi-
> > aa/releases/download/2022Q3/ehabi3
> > > 2.pdf
> > >
> > > Regression tested on arm-none-eabi target and found no regressions.
> > >
> > > Ok for master?
> > >
> >
> > No, not yet.
> >
> > Presumably the logic to produce 0xb5 is in the source base and this was
> > tested with suitable options that produce said opcode ? I see no logic in place
> > to produce the said opcode in the backend in a quick read as the pacbti
> > patches still seem to be in review. ?
> >
> > So what was the test suite run actually testing ?
>
> Sorry for the late response, the patch supporting the said opcode (directive ".pacspval)" is here:
> https://gcc.gnu.org/pipermail/gcc-patches/2022-November/605524.html (still under upstream review)
>
> and the patch to encode ".pacspval" with the mentioned opcode "0xb5" in binutils is here:
> https://sourceware.org/pipermail/binutils/2022-November/124328.html (approved and committed to binutils).

Thanks for the answer but perhaps I should make my question more
explicit - are you saying that this patch was tested in combination
with those and other dependent patches on a suitable simulator with
suitable multilibs and C++ to test for this presumably for frame
unwinding ?

For the future , it would certainly be worth being explicit about this
in your patch submission :)

regards
Ramana

>
> Regards,
> Srinath.
>
> > regards
> > Ramana
> >
> >
> > > Regards,
> > > Srinath.
> > >
> > > gcc/ChangeLog:
> > >
> > > 2022-11-09  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>
> > >
> > >         * libgcc/config/arm/pr-support.c (__gnu_unwind_execute): Decode
> > opcode
> > >         "0xb5".
> > >
> > >
> > > ###############     Attachment also inlined for ease of reply
> > ###############
> > >
> > >
> > > diff --git a/libgcc/config/arm/pr-support.c
> > > b/libgcc/config/arm/pr-support.c index
> > >
> > e48854587c667a959aa66ccc4982231f63333ecc..73e4942a39b34a83c2da85de
> > f6b1
> > > 3e82ec501552 100644
> > > --- a/libgcc/config/arm/pr-support.c
> > > +++ b/libgcc/config/arm/pr-support.c
> > > @@ -107,7 +107,9 @@ __gnu_unwind_execute (_Unwind_Context *
> > context, __gnu_unwind_state * uws)
> > >    _uw op;
> > >    int set_pc;
> > >    int set_pac = 0;
> > > +  int set_pac_sp = 0;
> > >    _uw reg;
> > > +  _uw sp;
> > >
> > >    set_pc = 0;
> > >    for (;;)
> > > @@ -124,10 +126,11 @@ __gnu_unwind_execute (_Unwind_Context *
> > context,
> > > __gnu_unwind_state * uws)  #if defined(TARGET_HAVE_PACBTI)
> > >           if (set_pac)
> > >             {
> > > -             _uw sp;
> > >               _uw lr;
> > >               _uw pac;
> > > -             _Unwind_VRS_Get (context, _UVRSC_CORE, R_SP,
> > _UVRSD_UINT32, &sp);
> > > +             if (!set_pac_sp)
> > > +               _Unwind_VRS_Get (context, _UVRSC_CORE, R_SP,
> > _UVRSD_UINT32,
> > > +                                &sp);
> > >               _Unwind_VRS_Get (context, _UVRSC_CORE, R_LR, _UVRSD_UINT32,
> > &lr);
> > >               _Unwind_VRS_Get (context, _UVRSC_PAC, R_IP,
> > >                                _UVRSD_UINT32, &pac); @@ -259,7 +262,19
> > > @@ __gnu_unwind_execute (_Unwind_Context * context,
> > __gnu_unwind_state * uws)
> > >               continue;
> > >             }
> > >
> > > -         if ((op & 0xfc) == 0xb4)  /* Obsolete FPA.  */
> > > +         /* Use current VSP as modifier in PAC validation.  */
> > > +         if (op == 0xb5)
> > > +           {
> > > +             if (set_pac)
> > > +               _Unwind_VRS_Get (context, _UVRSC_CORE, R_SP,
> > _UVRSD_UINT32,
> > > +                                &sp);
> > > +             else
> > > +               return _URC_FAILURE;
> > > +             set_pac_sp = 1;
> > > +             continue;
> > > +           }
> > > +
> > > +         if ((op & 0xfd) == 0xb6)  /* Obsolete FPA.  */
> > >             return _URC_FAILURE;
> > >
> > >           /* op & 0xf8 == 0xb8.  */
> > >
> > >
> > >