[05/18] arm64: kbuild: reserve reg x18 from general allocation by the compiler
diff mbox series

Message ID 20191018161033.261971-6-samitolvanen@google.com
State Superseded
Headers show
Series
  • add support for Clang's Shadow Call Stack
Related show

Commit Message

Sami Tolvanen Oct. 18, 2019, 4:10 p.m. UTC
From: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Before we can start using register x18 for a special purpose (as permitted
by the AAPCS64 ABI), we need to tell the compiler that it is off limits
for general allocation. So tag it as 'fixed', and remove the mention from
the LL/SC compiler flag override.

Link: https://patchwork.kernel.org/patch/9836881/
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 arch/arm64/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Nick Desaulniers Oct. 18, 2019, 5:32 p.m. UTC | #1
On Fri, Oct 18, 2019 at 9:11 AM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>
> Before we can start using register x18 for a special purpose (as permitted
> by the AAPCS64 ABI), we need to tell the compiler that it is off limits
> for general allocation. So tag it as 'fixed',

yep, but...

> and remove the mention from
> the LL/SC compiler flag override.

was that cut/dropped from this patch?

>
> Link: https://patchwork.kernel.org/patch/9836881/

^ Looks like it. Maybe it doesn't matter, but if sending a V2, maybe
the commit message to be updated?

> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

If sending a V2 with the above cleaned up, you may also include:
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

I like how this does not conditionally reserve it based on the CONFIG
for SCS.  Hopefully later patches don't wrap it, but I haven't looked
through all of them yet.

> ---
>  arch/arm64/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 2c0238ce0551..1c7b276bc7c5 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -55,7 +55,7 @@ endif
>
>  KBUILD_CFLAGS  += -mgeneral-regs-only $(lseinstr) $(brokengasinst)     \
>                    $(compat_vdso) $(cc_has_k_constraint)
> -KBUILD_CFLAGS  += -fno-asynchronous-unwind-tables
> +KBUILD_CFLAGS  += -fno-asynchronous-unwind-tables -ffixed-x18
>  KBUILD_CFLAGS  += $(call cc-disable-warning, psabi)
>  KBUILD_AFLAGS  += $(lseinstr) $(brokengasinst) $(compat_vdso)
>
> --
> 2.23.0.866.gb869b98d4c-goog
>
Sami Tolvanen Oct. 18, 2019, 7 p.m. UTC | #2
On Fri, Oct 18, 2019 at 10:32 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
> > and remove the mention from
> > the LL/SC compiler flag override.
>
> was that cut/dropped from this patch?
>
> >
> > Link: https://patchwork.kernel.org/patch/9836881/
>
> ^ Looks like it. Maybe it doesn't matter, but if sending a V2, maybe
> the commit message to be updated?

True. The original patch is from 2017 and the relevant part of
arm64/lib/Makefile no longer exists. I'll update this accordingly.

> I like how this does not conditionally reserve it based on the CONFIG
> for SCS.  Hopefully later patches don't wrap it, but I haven't looked
> through all of them yet.

In a later patch x18 is only reserved with SCS. I'm fine with dropping
that patch and reserving it always, but wouldn't mind hearing thoughts
from the maintainers about this first.

Sami
Ard Biesheuvel Oct. 21, 2019, 6:12 a.m. UTC | #3
On Fri, 18 Oct 2019 at 21:00, Sami Tolvanen <samitolvanen@google.com> wrote:
>
> On Fri, Oct 18, 2019 at 10:32 AM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@googlegroups.com> wrote:
> > > and remove the mention from
> > > the LL/SC compiler flag override.
> >
> > was that cut/dropped from this patch?
> >
> > >
> > > Link: https://patchwork.kernel.org/patch/9836881/
> >
> > ^ Looks like it. Maybe it doesn't matter, but if sending a V2, maybe
> > the commit message to be updated?
>
> True. The original patch is from 2017 and the relevant part of
> arm64/lib/Makefile no longer exists. I'll update this accordingly.
>
> > I like how this does not conditionally reserve it based on the CONFIG
> > for SCS.  Hopefully later patches don't wrap it, but I haven't looked
> > through all of them yet.
>
> In a later patch x18 is only reserved with SCS. I'm fine with dropping
> that patch and reserving it always, but wouldn't mind hearing thoughts
> from the maintainers about this first.
>

Why would you reserve x18 if SCS is disabled? Given that this is a
choice that is made at code generation time, there is no justification
for always reserving it, since it will never be used for anything. (Of
course, this applies to generated code only - .S files should simply
be updated to avoid x18 altogether)

Also, please combine this patch with the one that reserves it
conditionally, no point in having both in the same series.
Sami Tolvanen Oct. 21, 2019, 8:43 p.m. UTC | #4
On Sun, Oct 20, 2019 at 11:12 PM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> Also, please combine this patch with the one that reserves it
> conditionally, no point in having both in the same series.

Sure, I'll just drop this patch from v2 then and only reserve it with SCS.

Sami

Patch
diff mbox series

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 2c0238ce0551..1c7b276bc7c5 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -55,7 +55,7 @@  endif
 
 KBUILD_CFLAGS	+= -mgeneral-regs-only $(lseinstr) $(brokengasinst)	\
 		   $(compat_vdso) $(cc_has_k_constraint)
-KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
+KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables -ffixed-x18
 KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
 KBUILD_AFLAGS	+= $(lseinstr) $(brokengasinst) $(compat_vdso)