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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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@xxxxxxxxxx>
>>> ---
>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>> CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
>>> ---
>>>  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).
 * -include ./include/xen/config.h

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

~Andrew