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