[PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

[PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

[David Gow]

The things built with USER_CFLAGS don't seem to recognise it as a
compiler option, and print a warning:
clang: warning: argument unused during compilation: '-mno-global-merge' [-Wunused-command-line-argument]

Fixes: 744814d2fa ("um: Allow builds with Clang")
Signed-off-by: David Gow <davidgow@google.com>
---

This warning shows up after merging:
https://lore.kernel.org/lkml/20220227184517.504931-6-keescook@chromium.org/

I'm not 100% sure why this is necessary, but it does seem to work. All
the attempts to get rid of -mno-global-merge entirely have been met with
skepticism, but I'm guessing that it's not a problem for just the UML
"user" files, as they shouldn't(?) interact too much with modules.

 arch/um/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index f2fe63bfd819..320b09cd513c 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -75,6 +75,10 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
 		-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
 		-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__
 
+ifdef CONFIG_CC_IS_CLANG
+USER_CFLAGS := $(patsubst -mno-global-merge,,$(USER_CFLAGS))
+endif
+
 #This will adjust *FLAGS accordingly to the platform.
 include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
 
-- 
2.35.1.616.g0bdcbb4464-goog

Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

[Nathan Chancellor]

Hi David,

On Thu, Mar 03, 2022 at 05:06:42PM +0800, David Gow wrote:
> The things built with USER_CFLAGS don't seem to recognise it as a
> compiler option, and print a warning:
> clang: warning: argument unused during compilation: '-mno-global-merge' [-Wunused-command-line-argument]
> 
> Fixes: 744814d2fa ("um: Allow builds with Clang")
> Signed-off-by: David Gow <davidgow@google.com>
> ---
> 
> This warning shows up after merging:
> https://lore.kernel.org/lkml/20220227184517.504931-6-keescook@chromium.org/
> 
> I'm not 100% sure why this is necessary, but it does seem to work. All
> the attempts to get rid of -mno-global-merge entirely have been met with
> skepticism, but I'm guessing that it's not a problem for just the UML
> "user" files, as they shouldn't(?) interact too much with modules.

Thank you for the patch! I think it is correct, as this flag only works
for AArch64 and ARM, as it is only used in Clang::AddAArch64TargetArgs()
and Clang::AddARMTargetArgs() in clang/lib/Driver/ToolChains/Clang.cpp,
which are obviously never called with UML. I am not sure why we do not
see warning during regular kernel builds, maybe something about how UML
objects are compiled exposes this?

Regardless, I would definitely like to clean up this instance of the
warning because I would like to make this warning a hard error so that
we do not get cryptic cc-option failures:

https://github.com/ClangBuiltLinux/linux/issues/1587

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

One small comment below.

>  arch/um/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/um/Makefile b/arch/um/Makefile
> index f2fe63bfd819..320b09cd513c 100644
> --- a/arch/um/Makefile
> +++ b/arch/um/Makefile
> @@ -75,6 +75,10 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
>  		-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
>  		-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__
>  
> +ifdef CONFIG_CC_IS_CLANG

Is this ifdef needed?

> +USER_CFLAGS := $(patsubst -mno-global-merge,,$(USER_CFLAGS))
> +endif
> +
>  #This will adjust *FLAGS accordingly to the platform.
>  include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
>  
> -- 
> 2.35.1.616.g0bdcbb4464-goog
> 

Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

[Kees Cook]

On Thu, Mar 03, 2022 at 10:30:47AM -0700, Nathan Chancellor wrote:
> Hi David,
> 
> On Thu, Mar 03, 2022 at 05:06:42PM +0800, David Gow wrote:
> > The things built with USER_CFLAGS don't seem to recognise it as a
> > compiler option, and print a warning:
> > clang: warning: argument unused during compilation: '-mno-global-merge' [-Wunused-command-line-argument]
> > 
> > Fixes: 744814d2fa ("um: Allow builds with Clang")
> > Signed-off-by: David Gow <davidgow@google.com>
> > ---
> > 
> > This warning shows up after merging:
> > https://lore.kernel.org/lkml/20220227184517.504931-6-keescook@chromium.org/
> > 
> > I'm not 100% sure why this is necessary, but it does seem to work. All
> > the attempts to get rid of -mno-global-merge entirely have been met with
> > skepticism, but I'm guessing that it's not a problem for just the UML
> > "user" files, as they shouldn't(?) interact too much with modules.
> 
> Thank you for the patch! I think it is correct, as this flag only works
> for AArch64 and ARM, as it is only used in Clang::AddAArch64TargetArgs()
> and Clang::AddARMTargetArgs() in clang/lib/Driver/ToolChains/Clang.cpp,
> which are obviously never called with UML. I am not sure why we do not
> see warning during regular kernel builds, maybe something about how UML
> objects are compiled exposes this?

Yeah, I got really turned around when I tried to find this last week.

> 
> Regardless, I would definitely like to clean up this instance of the
> warning because I would like to make this warning a hard error so that
> we do not get cryptic cc-option failures:
> 
> https://github.com/ClangBuiltLinux/linux/issues/1587
> 
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> 
> One small comment below.
> 
> >  arch/um/Makefile | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/um/Makefile b/arch/um/Makefile
> > index f2fe63bfd819..320b09cd513c 100644
> > --- a/arch/um/Makefile
> > +++ b/arch/um/Makefile
> > @@ -75,6 +75,10 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
> >  		-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
> >  		-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__
> >  
> > +ifdef CONFIG_CC_IS_CLANG
> 
> Is this ifdef needed?
> 
> > +USER_CFLAGS := $(patsubst -mno-global-merge,,$(USER_CFLAGS))
> > +endif

How does -mno-global-merge get KBUILD_CFLAGS in the first place? If it's
arm/arm64 only, shouldn't that get relocated to those architectures?

*time travel* found it:

61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for building kernel with Clang")

So I think this may have been universally true long ago, and now only
arm/arm64 need it?


diff --git a/Makefile b/Makefile
index 2f6fbba88a0e..fbc42c3c389e 100644
--- a/Makefile
+++ b/Makefile
@@ -852,10 +852,6 @@ 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'
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index a2391b8de5a5..dcab28c44c26 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -48,6 +48,13 @@ CHECKFLAGS	+= -D__ARMEL__
 KBUILD_LDFLAGS	+= -EL
 endif
 
+ifdef CONFIG_CC_IS_CLANG
+# 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
+endif
+
 #
 # The Scalar Replacement of Aggregates (SRA) optimization pass in GCC 4.9 and
 # later may result in code being generated that handles signed short and signed
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 2f1de88651e6..2d20fb2ea664 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -36,6 +36,13 @@ ifeq ($(CONFIG_BROKEN_GAS_INST),y)
 $(warning Detected assembler with broken .inst; disassembly will be unreliable)
 endif
 
+ifdef CONFIG_CC_IS_CLANG
+# 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
+endif
+
 KBUILD_CFLAGS	+= -mgeneral-regs-only	\
 		   $(compat_vdso) $(cc_has_k_constraint)
 KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)

> > +
> >  #This will adjust *FLAGS accordingly to the platform.
> >  include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
> >  
> > -- 
> > 2.35.1.616.g0bdcbb4464-goog
> > 

-- 
Kees Cook

Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

[Nathan Chancellor]

On Thu, Mar 03, 2022 at 10:04:28AM -0800, Kees Cook wrote:
> On Thu, Mar 03, 2022 at 10:30:47AM -0700, Nathan Chancellor wrote:
> > Hi David,
> > 
> > On Thu, Mar 03, 2022 at 05:06:42PM +0800, David Gow wrote:
> > > The things built with USER_CFLAGS don't seem to recognise it as a
> > > compiler option, and print a warning:
> > > clang: warning: argument unused during compilation: '-mno-global-merge' [-Wunused-command-line-argument]
> > > 
> > > Fixes: 744814d2fa ("um: Allow builds with Clang")
> > > Signed-off-by: David Gow <davidgow@google.com>
> > > ---
> > > 
> > > This warning shows up after merging:
> > > https://lore.kernel.org/lkml/20220227184517.504931-6-keescook@chromium.org/
> > > 
> > > I'm not 100% sure why this is necessary, but it does seem to work. All
> > > the attempts to get rid of -mno-global-merge entirely have been met with
> > > skepticism, but I'm guessing that it's not a problem for just the UML
> > > "user" files, as they shouldn't(?) interact too much with modules.
> > 
> > Thank you for the patch! I think it is correct, as this flag only works
> > for AArch64 and ARM, as it is only used in Clang::AddAArch64TargetArgs()
> > and Clang::AddARMTargetArgs() in clang/lib/Driver/ToolChains/Clang.cpp,
> > which are obviously never called with UML. I am not sure why we do not
> > see warning during regular kernel builds, maybe something about how UML
> > objects are compiled exposes this?
> 
> Yeah, I got really turned around when I tried to find this last week.
> 
> > 
> > Regardless, I would definitely like to clean up this instance of the
> > warning because I would like to make this warning a hard error so that
> > we do not get cryptic cc-option failures:
> > 
> > https://github.com/ClangBuiltLinux/linux/issues/1587
> > 
> > Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> > 
> > One small comment below.
> > 
> > >  arch/um/Makefile | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > > 
> > > diff --git a/arch/um/Makefile b/arch/um/Makefile
> > > index f2fe63bfd819..320b09cd513c 100644
> > > --- a/arch/um/Makefile
> > > +++ b/arch/um/Makefile
> > > @@ -75,6 +75,10 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
> > >  		-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
> > >  		-idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__
> > >  
> > > +ifdef CONFIG_CC_IS_CLANG
> > 
> > Is this ifdef needed?
> > 
> > > +USER_CFLAGS := $(patsubst -mno-global-merge,,$(USER_CFLAGS))
> > > +endif
> 
> How does -mno-global-merge get KBUILD_CFLAGS in the first place? If it's
> arm/arm64 only, shouldn't that get relocated to those architectures?
> 
> *time travel* found it:
> 
> 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for building kernel with Clang")
> 
> So I think this may have been universally true long ago, and now only
> arm/arm64 need it?

I don't think so. -mno-global-merge was introduced in [1], which only
added the option handling in Clang::AddARMTargetArgs(). In fact, when
-mglobal-merge was added, there was a test added that checked that
-mglobal-merge/-mno-global-merge had no impact with an x86_64 triple [2].

Perhaps at the time that 61163efae020 was written, they were testing
against either ARCH=arm or ARCH=arm64, which would have shown the
problem that they mentioned? There is no context in the commit message
so *shrug*.

Regardless, I think your patch is equally viable, perhaps even better,
as I am sure this is not the only area where this warning might show up.

[1]: https://github.com/llvm/llvm-project/commit/ba3df1d3ca8569ff8f6e289594c53926e71e519a
[2]: https://github.com/llvm/llvm-project/commit/256a869d312b37ed7b5d131329050fcdadb11148

Cheers,
Nathan

> diff --git a/Makefile b/Makefile
> index 2f6fbba88a0e..fbc42c3c389e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -852,10 +852,6 @@ 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'
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index a2391b8de5a5..dcab28c44c26 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -48,6 +48,13 @@ CHECKFLAGS	+= -D__ARMEL__
>  KBUILD_LDFLAGS	+= -EL
>  endif
>  
> +ifdef CONFIG_CC_IS_CLANG
> +# 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
> +endif
> +
>  #
>  # The Scalar Replacement of Aggregates (SRA) optimization pass in GCC 4.9 and
>  # later may result in code being generated that handles signed short and signed
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 2f1de88651e6..2d20fb2ea664 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -36,6 +36,13 @@ ifeq ($(CONFIG_BROKEN_GAS_INST),y)
>  $(warning Detected assembler with broken .inst; disassembly will be unreliable)
>  endif
>  
> +ifdef CONFIG_CC_IS_CLANG
> +# 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
> +endif
> +
>  KBUILD_CFLAGS	+= -mgeneral-regs-only	\
>  		   $(compat_vdso) $(cc_has_k_constraint)
>  KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
> 
> > > +
> > >  #This will adjust *FLAGS accordingly to the platform.
> > >  include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
> > >  
> > > -- 
> > > 2.35.1.616.g0bdcbb4464-goog
> > > 
> 
> -- 
> Kees Cook

Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

[Nick Desaulniers]

On Thu, Mar 3, 2022 at 10:26 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Thu, Mar 03, 2022 at 10:04:28AM -0800, Kees Cook wrote:
> > How does -mno-global-merge get KBUILD_CFLAGS in the first place? If it's
> > arm/arm64 only, shouldn't that get relocated to those architectures?
> >
> > *time travel* found it:
> >
> > 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for building kernel with Clang")
> >
> > So I think this may have been universally true long ago, and now only
> > arm/arm64 need it?

Looks like that's the case from LLVM sources.

<snip>

> > diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> > index a2391b8de5a5..dcab28c44c26 100644
> > --- a/arch/arm/Makefile
> > +++ b/arch/arm/Makefile
> > @@ -48,6 +48,13 @@ CHECKFLAGS += -D__ARMEL__
> >  KBUILD_LDFLAGS       += -EL
> >  endif
> >
> > +ifdef CONFIG_CC_IS_CLANG
> > +# 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.

I think there's a typo in the original comment.
s/on of/one of/ ?

Also, I'm not sure what's meant by _MergedGlobals. Perhaps this is an
opportunity to make this clearer?

"Clang's "global-merge" pass (implemented only for arm and aarch64)
may break modpost Pattern 2 if symbols are renamed and thus don't
appear on modpost's allowlist.

> > +# See modpost pattern 2
> > +KBUILD_CFLAGS        += -mno-global-merge
> > +endif
> > +



-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

[Sedat Dilek]

On Thu, Mar 3, 2022 at 9:43 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Thu, Mar 3, 2022 at 10:26 AM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > On Thu, Mar 03, 2022 at 10:04:28AM -0800, Kees Cook wrote:
> > > How does -mno-global-merge get KBUILD_CFLAGS in the first place? If it's
> > > arm/arm64 only, shouldn't that get relocated to those architectures?
> > >
> > > *time travel* found it:
> > >
> > > 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for building kernel with Clang")
> > >
> > > So I think this may have been universally true long ago, and now only
> > > arm/arm64 need it?
>
> Looks like that's the case from LLVM sources.
>
> <snip>
>
> > > diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> > > index a2391b8de5a5..dcab28c44c26 100644
> > > --- a/arch/arm/Makefile
> > > +++ b/arch/arm/Makefile
> > > @@ -48,6 +48,13 @@ CHECKFLAGS += -D__ARMEL__
> > >  KBUILD_LDFLAGS       += -EL
> > >  endif
> > >
> > > +ifdef CONFIG_CC_IS_CLANG
> > > +# 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.
>
> I think there's a typo in the original comment.
> s/on of/one of/ ?
>
> Also, I'm not sure what's meant by _MergedGlobals. Perhaps this is an
> opportunity to make this clearer?
>
> "Clang's "global-merge" pass (implemented only for arm and aarch64)
> may break modpost Pattern 2 if symbols are renamed and thus don't
> appear on modpost's allowlist.
>
> > > +# See modpost pattern 2
> > > +KBUILD_CFLAGS        += -mno-global-merge
> > > +endif
> > > +
>

I can remember on x86-64 I was able to build and boot by dropping it entirely.

- Sedat -

>
>
> --
> Thanks,
> ~Nick Desaulniers

Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

[David Gow]

[-- Attachment #1: Type: text/plain, Size: 3298 bytes --]

On Fri, Mar 4, 2022 at 1:30 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Hi David,
>
> On Thu, Mar 03, 2022 at 05:06:42PM +0800, David Gow wrote:
> > The things built with USER_CFLAGS don't seem to recognise it as a
> > compiler option, and print a warning:
> > clang: warning: argument unused during compilation: '-mno-global-merge' [-Wunused-command-line-argument]
> >
> > Fixes: 744814d2fa ("um: Allow builds with Clang")
> > Signed-off-by: David Gow <davidgow@google.com>
> > ---
> >
> > This warning shows up after merging:
> > https://lore.kernel.org/lkml/20220227184517.504931-6-keescook@chromium.org/
> >
> > I'm not 100% sure why this is necessary, but it does seem to work. All
> > the attempts to get rid of -mno-global-merge entirely have been met with
> > skepticism, but I'm guessing that it's not a problem for just the UML
> > "user" files, as they shouldn't(?) interact too much with modules.
>
> Thank you for the patch! I think it is correct, as this flag only works
> for AArch64 and ARM, as it is only used in Clang::AddAArch64TargetArgs()
> and Clang::AddARMTargetArgs() in clang/lib/Driver/ToolChains/Clang.cpp,
> which are obviously never called with UML. I am not sure why we do not
> see warning during regular kernel builds, maybe something about how UML
> objects are compiled exposes this?

Yeah: this only seems to show up with UML source compiled as "user"
files (i.e., talking to the host system). Most of the kernel source
doesn't show the warning because -Qunused-arguments is set somewhere,
probably in KBUILD_CPPFLAGS, which doesn't filter down into
USER_CFLAGS:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile?id=7e57714cd0ad2d5bb90e50b5096a0e671dec1ef3#n784

Is this flag supposed to only work on ARM/AArch64, or is it just
currently not implemented anywhere else. If it's actually
ARM-specific, then moving it to the arch/arm{,64} makefile as in Kees'
patch definitely makes more sense. If it's going to show up on other
architectures at some point anyway, maybe something like this makes
sense.

>
> Regardless, I would definitely like to clean up this instance of the
> warning because I would like to make this warning a hard error so that
> we do not get cryptic cc-option failures:
>
> https://github.com/ClangBuiltLinux/linux/issues/1587
>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
>
> One small comment below.
>
> >  arch/um/Makefile | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/um/Makefile b/arch/um/Makefile
> > index f2fe63bfd819..320b09cd513c 100644
> > --- a/arch/um/Makefile
> > +++ b/arch/um/Makefile
> > @@ -75,6 +75,10 @@ USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
> >               -D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
> >               -idirafter $(objtree)/include -D__KERNEL__ -D__UM_HOST__
> >
> > +ifdef CONFIG_CC_IS_CLANG
>
> Is this ifdef needed?
>
No: it works fine without the ifdef: I just wanted to limit the damage
I could do playing with things I didn't fully understand. :-)

> > +USER_CFLAGS := $(patsubst -mno-global-merge,,$(USER_CFLAGS))
> > +endif
> > +
> >  #This will adjust *FLAGS accordingly to the platform.
> >  include $(srctree)/$(ARCH_DIR)/Makefile-os-$(OS)
> >

Cheers,
-- David

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