Re: [PATCH 05/16] x86/shstk: Introduce Supervisor Shadow Stack support

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Mon, May 04, 2020 at 03:52:58PM +0200, Jan Beulich wrote:
> On 02.05.2020 00:58, Andrew Cooper wrote:
> > --- a/xen/scripts/Kconfig.include
> > +++ b/xen/scripts/Kconfig.include
> > @@ -31,6 +31,10 @@ cc-option = $(success,$(CC) -Werror $(CLANG_FLAGS) $(1) 
> > -E -x c /dev/null -o /de
> >  # Return y if the linker supports <flag>, n otherwise
> >  ld-option = $(success,$(LD) -v $(1))
> >  
> > +# $(as-instr,<instr>)
> > +# Return y if the assembler supports <instr>, n otherwise
> > +as-instr = $(success,printf "%b\n" "$(1)" | $(CC) $(CLANG_FLAGS) -c -x 
> > assembler -o /dev/null -)
> 
> CLANG_FLAGS caught my eye here, then noticing that cc-option
> also uses it. Anthony - what's the deal with this? It doesn't
> look to get defined anywhere, and I also don't see what clang-
> specific about these constructs.

It's because these constructs are gcc-specific :-). Indeed CLANG_FLAGS
probably needs to be defined as I don't think providing the full
AFLAGS/CFLAGS is going to be a good idea and may change the result of
the commands.

Linux has a few clang specific flags in CLANG_FLAGS, I have found those:
    The ones for cross compilation: --prefix, --target, --gcc-toolchain
    -no-integrated-as
    -Werror=unknown-warning-option
And that's it.

So, I think we could keep using CLANG_FLAGS in Kconfig.include and
define it in Makefile with a comment saying that it's only used by
Kconfig. It would always have -Werror=unknown-warning-option and have
-no-integrated-as when needed, the -Wunknown-warning-option is present
in clang 3.0.0, according Linux's commit 589834b3a009 ("kbuild: Add
-Werror=unknown-warning-option to CLANG_FLAGS").

The options -Werror=unknown-warning-option is to make sure that the
warning is enabled, even though it is by default but could be disabled
in a particular build of clang, see e8de12fb7cde ("kbuild: Check for
unknown options with cc-option usage in Kconfig and clang")

I'll write a patch with this new CLANG_FLAGS.

-- 
Anthony PERARD