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@xxxxxxxxxx> 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