Re: [PATCH] x86/boot: Reinstate -nostdinc for CFLAGS_x86_32

["Roger Pau Monné" <roger.pau@citrix.com>]

On Tue, Sep 03, 2024 at 12:16:33PM +0100, Andrew Cooper wrote:
> On 03/09/2024 12:08 pm, Andrew Cooper wrote:
> > On 03/09/2024 11:54 am, Roger Pau Monné wrote:
> >> On Tue, Sep 03, 2024 at 11:49:40AM +0100, Andrew Cooper wrote:
> >>> Most of Xen is build using -nostdinc and a fully specified include path.
> >>> However, the makefile line:
> >>>
> >>>   $(head-bin-objs): XEN_CFLAGS := $(CFLAGS_x86_32) -fpic
> >>>
> >>> discards XEN_CFLAGS and replaces them with CFLAGS_x86_32.
> >>>
> >>> Reinstate -nostdinc, and add the arch include path to the command line.  This
> >>> is a latent bug for now, but it breaks properly with subsequent include
> >>> changes.
> >>>
> >>> Fixes: d58a509e01c4 ("build,x86: remove the need for build32.mk")
> >>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> >>> ---
> >>> CC: Jan Beulich <JBeulich@suse.com>
> >>> CC: Roger Pau Monné <roger.pau@citrix.com>
> >>> CC: Anthony PERARD <anthony.perard@vates.tech>
> >>> ---
> >>>  xen/arch/x86/boot/Makefile | 6 +++---
> >>>  1 file changed, 3 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/xen/arch/x86/boot/Makefile b/xen/arch/x86/boot/Makefile
> >>> index 03d8ce3a9e48..19eec01e176e 100644
> >>> --- a/xen/arch/x86/boot/Makefile
> >>> +++ b/xen/arch/x86/boot/Makefile
> >>> @@ -13,11 +13,11 @@ $(obj)/head.o: $(head-bin-objs:.o=.bin)
> >>>  
> >>>  CFLAGS_x86_32 := $(subst -m64,-m32 -march=i686,$(XEN_TREEWIDE_CFLAGS))
> >>>  $(call cc-options-add,CFLAGS_x86_32,CC,$(EMBEDDED_EXTRA_CFLAGS))
> >>> -CFLAGS_x86_32 += -Werror -fno-builtin -g0 -msoft-float
> >>> +CFLAGS_x86_32 += -Werror -nostdinc -fno-builtin -g0 -msoft-float
> >>>  ifneq ($(abs_objtree),$(abs_srctree))
> >>> -CFLAGS_x86_32 += -I$(objtree)/include
> >>> +CFLAGS_x86_32 += -I$(objtree)/include -I$(objtree)/arch/$(SRCARCH)/include
> >>>  endif
> >>> -CFLAGS_x86_32 += -I$(srctree)/include
> >>> +CFLAGS_x86_32 += -I$(srctree)/include -I$(srctree)/arch/$(SRCARCH)/include
> >> I think it might be best to just filter out the include paths from
> >> XEN_CFLAGS, iow:
> >>
> >> CFLAGS_x86_32 += $(filter -I%,$(XEN_CFLAGS))
> >>
> >> Instead of having to open-code the paths for the include search paths
> >> here again.  Using the filter leads to the following CC command line:
> >>
> >> clang -MMD -MP -MF arch/x86/boot/.cmdline.o.d -m32 -march=i686 -DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall -Wstrict-prototypes -Wno-unused-but-set-variable -Wno-unused-local-typedefs   -fno-pie -fno-stack-protector -fno-exceptions -fno-asynchronous-unwind-tables -Werror -fno-builtin -g0 -msoft-float -mregparm=3 -I./include -I./arch/x86/include -I./arch/x86/include/generated -I./arch/x86/include/asm/mach-generic -I./arch/x86/include/asm/mach-default -fpic '-D__OBJECT_LABEL__=arch/x86/boot/cmdline.o'    -c arch/x86/boot/cmdline.c -o arch/x86/boot/.cmdline.o.tmp -MQ arch/x86/boot/cmdline.o
> > FWIW, https://cirrus-ci.com/build/5930269490806784 shows a good build on
> > FreeBSD with this patch in place.
> >
> > I'd be happy with that approach.  It's probably less fragile, although
> > I'll probably go with:
> >
> > CFLAGS_x86_32 += -nostdinc $(filter -I%,$(XEN_CFLAGS))
> >
> > to handle all the include changes together.  Lemme spin a v2.
> 
> Actually, it's not quite that easy.  From a regular Xen object file, we
> have:
> 
>  * -Wa,-I,./include (twice, for some reason).

There's a comment next to where this is added:

# Set up the assembler include path properly for older toolchains.

But won't we also need extra -Wa,-I for the other include paths that
are passed on the command line? (./arch/x86/include for example)

>  * -include ./include/xen/config.h

Hm, we could manually add that include option.

> 
> The former can be added to the filter reasonably easily, but the latter
> cannot.  I guess we've gone this long without it...

I've been also thinking, another approach we could take is filtering
out what we don't want, but I think that might end up being more
fragile.

Thanks, Roger.