* Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'
2022-03-30 23:45 ` [PATCH 1/2] kbuild: Remove '-mno-global-merge' Nathan Chancellor
@ 2022-03-31 1:59 ` David Gow
2022-03-31 4:57 ` Kees Cook
2022-03-31 7:11 ` Sedat Dilek
2 siblings, 0 replies; 11+ messages in thread
From: David Gow @ 2022-03-31 1:59 UTC (permalink / raw)
To: Nathan Chancellor
Cc: Masahiro Yamada, Nick Desaulniers, Richard Weinberger,
Anton Ivanov, Johannes Berg, Kees Cook, linux-kbuild,
Linux Kernel Mailing List, linux-um, llvm, patches
On Thu, Mar 31, 2022 at 7:46 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> This flag is specific to clang, where it is only used by the 32-bit and
> 64-bit ARM backends. In certain situations, the presence of this flag
> will cause a warning, as shown by commit 6580c5c18fb3 ("um: clang: Strip
> out -mno-global-merge from USER_CFLAGS").
>
> Since commit 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for
> building kernel with Clang") that added this flag back in 2014, there
> have been quite a few changes to the GlobalMerge pass in LLVM. Building
> several different ARCH=arm and ARCH=arm64 configurations with LLVM 11
> (minimum) and 15 (current main version) with this flag removed (i.e.,
> with the default of '-mglobal-merge') reveals no modpost warnings, so it
> is likely that the issue noted in the comment is no longer relevant due
> to changes in LLVM or modpost, meaning this flag can be removed.
>
> If any new warnings show up that are a result of the removal of this
> flag, it can be added back under arch/arm{,64}/Makefile to avoid
> warnings on other architectures.
>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
This seems to work fine here under the KUnit tooling, with both x86_64
and arm64. That admittedly doesn't do anything with modules, so
wouldn't reveal any of those issues, but at least the warnings about
--mno-global-merge existing remain gone.
Tested-by: David Gow <davidgow@google.com>
Cheers,
-- David
^ permalink raw reply [flat|nested] 11+ messages in thread* Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'
2022-03-30 23:45 ` [PATCH 1/2] kbuild: Remove '-mno-global-merge' Nathan Chancellor
2022-03-31 1:59 ` David Gow
@ 2022-03-31 4:57 ` Kees Cook
2022-03-31 7:11 ` Sedat Dilek
2 siblings, 0 replies; 11+ messages in thread
From: Kees Cook @ 2022-03-31 4:57 UTC (permalink / raw)
To: Nathan Chancellor
Cc: Masahiro Yamada, Nick Desaulniers, Richard Weinberger,
Anton Ivanov, Johannes Berg, linux-kbuild, linux-kernel, linux-um,
llvm, patches
On Wed, Mar 30, 2022 at 04:45:27PM -0700, Nathan Chancellor wrote:
> This flag is specific to clang, where it is only used by the 32-bit and
> 64-bit ARM backends. In certain situations, the presence of this flag
> will cause a warning, as shown by commit 6580c5c18fb3 ("um: clang: Strip
> out -mno-global-merge from USER_CFLAGS").
>
> Since commit 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for
> building kernel with Clang") that added this flag back in 2014, there
> have been quite a few changes to the GlobalMerge pass in LLVM. Building
> several different ARCH=arm and ARCH=arm64 configurations with LLVM 11
> (minimum) and 15 (current main version) with this flag removed (i.e.,
> with the default of '-mglobal-merge') reveals no modpost warnings, so it
> is likely that the issue noted in the comment is no longer relevant due
> to changes in LLVM or modpost, meaning this flag can be removed.
>
> If any new warnings show up that are a result of the removal of this
> flag, it can be added back under arch/arm{,64}/Makefile to avoid
> warnings on other architectures.
>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Yeah, this looks right -- the history of this option seems to show it
is no longer needed.
Reviewed-by: Kees Cook <keescook@chromium.org>
--
Kees Cook
^ permalink raw reply [flat|nested] 11+ messages in thread* Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'
2022-03-30 23:45 ` [PATCH 1/2] kbuild: Remove '-mno-global-merge' Nathan Chancellor
2022-03-31 1:59 ` David Gow
2022-03-31 4:57 ` Kees Cook
@ 2022-03-31 7:11 ` Sedat Dilek
2022-03-31 15:37 ` Nathan Chancellor
2 siblings, 1 reply; 11+ messages in thread
From: Sedat Dilek @ 2022-03-31 7:11 UTC (permalink / raw)
To: Nathan Chancellor
Cc: Masahiro Yamada, Nick Desaulniers, Richard Weinberger,
Anton Ivanov, Johannes Berg, Kees Cook, linux-kbuild,
linux-kernel, linux-um, llvm, patches
On Thu, Mar 31, 2022 at 5:27 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> This flag is specific to clang, where it is only used by the 32-bit and
> 64-bit ARM backends. In certain situations, the presence of this flag
> will cause a warning, as shown by commit 6580c5c18fb3 ("um: clang: Strip
> out -mno-global-merge from USER_CFLAGS").
>
> Since commit 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for
> building kernel with Clang") that added this flag back in 2014, there
> have been quite a few changes to the GlobalMerge pass in LLVM. Building
> several different ARCH=arm and ARCH=arm64 configurations with LLVM 11
> (minimum) and 15 (current main version) with this flag removed (i.e.,
> with the default of '-mglobal-merge') reveals no modpost warnings, so it
> is likely that the issue noted in the comment is no longer relevant due
> to changes in LLVM or modpost, meaning this flag can be removed.
>
> If any new warnings show up that are a result of the removal of this
> flag, it can be added back under arch/arm{,64}/Makefile to avoid
> warnings on other architectures.
>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> Makefile | 4 ----
> 1 file changed, 4 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index daeb5c88b50b..f2723d9bfca4 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG
> KBUILD_CPPFLAGS += -Qunused-arguments
> # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> KBUILD_CFLAGS += -Wno-gnu
> -# CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> -# source of a reference will be _MergedGlobals and not on of the whitelisted names.
> -# See modpost pattern 2
> -KBUILD_CFLAGS += -mno-global-merge
> else
>
> # gcc inanely warns about local variables called 'main'
> --
> 2.35.1
>
I have tested this several times and was able to boot into bar metal -
no problems with building and/or booting my kernel-modules.
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
Just as a side-note:
As with Linux v5.18-rc1 and -std=gnu11 we change the above comment ...?
# The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
KBUILD_CFLAGS += -Wno-gnu
- Sedat -
^ permalink raw reply [flat|nested] 11+ messages in thread* Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'
2022-03-31 7:11 ` Sedat Dilek
@ 2022-03-31 15:37 ` Nathan Chancellor
2022-03-31 18:52 ` Sedat Dilek
0 siblings, 1 reply; 11+ messages in thread
From: Nathan Chancellor @ 2022-03-31 15:37 UTC (permalink / raw)
To: Sedat Dilek
Cc: Masahiro Yamada, Nick Desaulniers, Richard Weinberger,
Anton Ivanov, Johannes Berg, Kees Cook, linux-kbuild,
linux-kernel, linux-um, llvm, patches
On Thu, Mar 31, 2022 at 09:11:12AM +0200, Sedat Dilek wrote:
> On Thu, Mar 31, 2022 at 5:27 AM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > This flag is specific to clang, where it is only used by the 32-bit and
> > 64-bit ARM backends. In certain situations, the presence of this flag
> > will cause a warning, as shown by commit 6580c5c18fb3 ("um: clang: Strip
> > out -mno-global-merge from USER_CFLAGS").
> >
> > Since commit 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for
> > building kernel with Clang") that added this flag back in 2014, there
> > have been quite a few changes to the GlobalMerge pass in LLVM. Building
> > several different ARCH=arm and ARCH=arm64 configurations with LLVM 11
> > (minimum) and 15 (current main version) with this flag removed (i.e.,
> > with the default of '-mglobal-merge') reveals no modpost warnings, so it
> > is likely that the issue noted in the comment is no longer relevant due
> > to changes in LLVM or modpost, meaning this flag can be removed.
> >
> > If any new warnings show up that are a result of the removal of this
> > flag, it can be added back under arch/arm{,64}/Makefile to avoid
> > warnings on other architectures.
> >
> > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > ---
> > Makefile | 4 ----
> > 1 file changed, 4 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index daeb5c88b50b..f2723d9bfca4 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG
> > KBUILD_CPPFLAGS += -Qunused-arguments
> > # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> > KBUILD_CFLAGS += -Wno-gnu
> > -# CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> > -# source of a reference will be _MergedGlobals and not on of the whitelisted names.
> > -# See modpost pattern 2
> > -KBUILD_CFLAGS += -mno-global-merge
> > else
> >
> > # gcc inanely warns about local variables called 'main'
> > --
> > 2.35.1
> >
>
> I have tested this several times and was able to boot into bar metal -
> no problems with building and/or booting my kernel-modules.
>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
I would be very concerned if you did see any impact, given this flag is
ARM specific :) thanks as always for verifying!
> Just as a side-note:
> As with Linux v5.18-rc1 and -std=gnu11 we change the above comment ...?
>
> # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> KBUILD_CFLAGS += -Wno-gnu
It was updated as part of the shift to '-std=gnu11':
https://git.kernel.org/linus/e8c07082a810fbb9db303a2b66b66b8d7e588b53
The UML tree is based on 5.17-rc6, which does not have that change.
There should not be a merge conflict though.
Cheers,
Nathan
^ permalink raw reply [flat|nested] 11+ messages in thread* Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'
2022-03-31 15:37 ` Nathan Chancellor
@ 2022-03-31 18:52 ` Sedat Dilek
0 siblings, 0 replies; 11+ messages in thread
From: Sedat Dilek @ 2022-03-31 18:52 UTC (permalink / raw)
To: Nathan Chancellor
Cc: Masahiro Yamada, Nick Desaulniers, Richard Weinberger,
Anton Ivanov, Johannes Berg, Kees Cook, linux-kbuild,
linux-kernel, linux-um, llvm, patches
On Thu, Mar 31, 2022 at 5:37 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Thu, Mar 31, 2022 at 09:11:12AM +0200, Sedat Dilek wrote:
> > On Thu, Mar 31, 2022 at 5:27 AM Nathan Chancellor <nathan@kernel.org> wrote:
> > >
> > > This flag is specific to clang, where it is only used by the 32-bit and
> > > 64-bit ARM backends. In certain situations, the presence of this flag
> > > will cause a warning, as shown by commit 6580c5c18fb3 ("um: clang: Strip
> > > out -mno-global-merge from USER_CFLAGS").
> > >
> > > Since commit 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for
> > > building kernel with Clang") that added this flag back in 2014, there
> > > have been quite a few changes to the GlobalMerge pass in LLVM. Building
> > > several different ARCH=arm and ARCH=arm64 configurations with LLVM 11
> > > (minimum) and 15 (current main version) with this flag removed (i.e.,
> > > with the default of '-mglobal-merge') reveals no modpost warnings, so it
> > > is likely that the issue noted in the comment is no longer relevant due
> > > to changes in LLVM or modpost, meaning this flag can be removed.
> > >
> > > If any new warnings show up that are a result of the removal of this
> > > flag, it can be added back under arch/arm{,64}/Makefile to avoid
> > > warnings on other architectures.
> > >
> > > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > > ---
> > > Makefile | 4 ----
> > > 1 file changed, 4 deletions(-)
> > >
> > > diff --git a/Makefile b/Makefile
> > > index daeb5c88b50b..f2723d9bfca4 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG
> > > KBUILD_CPPFLAGS += -Qunused-arguments
> > > # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> > > KBUILD_CFLAGS += -Wno-gnu
> > > -# CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> > > -# source of a reference will be _MergedGlobals and not on of the whitelisted names.
> > > -# See modpost pattern 2
> > > -KBUILD_CFLAGS += -mno-global-merge
> > > else
> > >
> > > # gcc inanely warns about local variables called 'main'
> > > --
> > > 2.35.1
> > >
> >
> > I have tested this several times and was able to boot into bar metal -
> > no problems with building and/or booting my kernel-modules.
> >
> > Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> > Reviewed-by: Sedat Dilek <sedat.dilek@gmail.com>
>
> I would be very concerned if you did see any impact, given this flag is
> ARM specific :) thanks as always for verifying!
>
> > Just as a side-note:
> > As with Linux v5.18-rc1 and -std=gnu11 we change the above comment ...?
> >
> > # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> > KBUILD_CFLAGS += -Wno-gnu
>
> It was updated as part of the shift to '-std=gnu11':
>
> https://git.kernel.org/linus/e8c07082a810fbb9db303a2b66b66b8d7e588b53
>
> The UML tree is based on 5.17-rc6, which does not have that change.
> There should not be a merge conflict though.
>
Ah OK, a different base for UML.
- Sedat -
^ permalink raw reply [flat|nested] 11+ messages in thread