Re: [Devel] [PATCH vz10 5/5] fixup! samples/bpf: fix -Wduplicate-decl-specifier and -Wmissing-declarations

["Eva Kurchatova (Virtuozzo)" <eva.kurchatova@virtuozzo.com>]

On 8/27/26 15:42, Konstantin Khorenko wrote:
> On 8/26/26 22:08, Eva Kurchatova (Virtuozzo) wrote:
>> On 8/18/26 20:00, Konstantin Khorenko wrote:
>>> i have asked to send this patch to mainstream first.
>>>
>>> Please, add a link to the mainstream version of the patch to https://virtuozzo.atlassian.net/browse/VSTOR-127585
>>>
>>> --
>>> Best regards,
>>>
>>> Konstantin Khorenko,
>>> Virtuozzo Linux Kernel Team
>>>
>>> On 6/25/26 20:16, Eva Kurchatova wrote:
>>>> BPF programs include arch-specific headers, which use address space
>>>> qualifiers for percpu variables via __seg_gs / __seg_fs, however those
>>>> are meaningless for BPF compiler and will raise a warning
>>>>
>>>> Fix this by redefining __seg_gs and __seg_fs to a no-op attribute
>>>>
>>>> Additionally, enable ms-extensions, as generated vmlinux.h uses these
>>>> for structure layout inheritance, an acceptable practice in upstream
>>>>
>>>> https://virtuozzo.atlassian.net/browse/VSTOR-127585
>>>> Feature: fix selftests
>>>>
>>>> Signed-off-by: Eva Kurchatova <eva.kurchatova@virtuozzo.com>
>>>> ---
>>>>    samples/bpf/Makefile | 4 ++++
>>>>    1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
>>>> index 2b93779b9cbf..39b20bb69bcd 100644
>>>> --- a/samples/bpf/Makefile
>>>> +++ b/samples/bpf/Makefile
>>>> @@ -339,9 +339,11 @@ CLANG_SYS_INCLUDES = $(call get_sys_includes,$(CLANG))
>>>>    
>>>>    $(obj)/xdp_router_ipv4.bpf.o: $(obj)/xdp_sample.bpf.o
>>>>    
>>>> +# Generated vmlinux.h uses ms-extensions to inherit struct layout
>>>>    $(obj)/%.bpf.o: $(src)/%.bpf.c $(obj)/vmlinux.h $(src)/xdp_sample.bpf.h $(src)/xdp_sample_shared.h
>>>>    	@echo "  CLANG-BPF " $@
>>>>    	$(Q)$(CLANG) -g -O2 --target=bpf -D__TARGET_ARCH_$(SRCARCH) \
>>>> +		-fms-extensions -Wno-microsoft-anon-tag \
>>>>    		-Wno-compare-distinct-pointer-types -I$(srctree)/include \
>>>>    		-I$(srctree)/samples/bpf -I$(srctree)/tools/include \
>>>>    		-I$(LIBBPF_INCLUDE) $(CLANG_SYS_INCLUDES) \
>>>> @@ -372,11 +374,13 @@ $(BPF_SKELS_LINKED): $(BPF_OBJS_LINKED) $(BPFTOOL)
>>>>    # with native target, e.g., x64, arm64, etc. 'opt' does bpf CORE IR builtin
>>>>    # processing (llvm12) and IR optimizations. 'llvm-dis' converts
>>>>    # 'opt' output to IR, and finally 'llc' generates bpf byte code.
>>>> +# Additionally, __seg_gs and __seg_fs are ignored in BPF code
>>>>    $(obj)/%.o: $(src)/%.c
>>>>    	@echo "  CLANG-bpf " $@
>>>>    	$(Q)$(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(BPF_EXTRA_CFLAGS) \
>>>>    		-I$(obj) -I$(srctree)/tools/testing/selftests/bpf/ \
>>>>    		-I$(LIBBPF_INCLUDE) $(CLANG_SYS_INCLUDES) \
>>>> +		-U__seg_gs -D__seg_gs= -U__seg_fs -D__seg_fs= \
>>>>    		-D__KERNEL__ -D__BPF_TRACING__ -Wno-unused-value -Wno-pointer-sign \
>>>>    		-D__TARGET_ARCH_$(SRCARCH) -Wno-compare-distinct-pointer-types \
>>>>    		-Wno-gnu-variable-sized-type-not-at-end \
>> This patch was made at a point when RHEL hardening flags leaked into
>> kselftests build, and at `-Wall -Wextra -Werror`, many build failures
>> surfaced.
>>
>> Today the build does not fail: the spec undefines _fortify_level and
>> _hardened_build for this package, so a full samples/bpf build exits 0
>> with 166 warnings and no errors, all 13 .bpf.o and 38 sample binaries
>> produced. Of those warnings, 104 are exactly the ones this patch removes.
>>
>> Does it still make sense to keep it, send upstream and backport back, or
>> should this patch be dropped? I am not sure upstream will accept this
>> patch as it stands, it was originally purely solving VZ-specific CI
>> failures.
> Not sure i understand this correctly: if we take ms kernel and compile selftests, will those warnings appear?
> If yes - then mainstream will be happy to apply the fix(es).
>
> If not, please explain what is the difference between ms vs rh - both here and in the jira bug.

Yes, these warnings will appear, but the current fix only suffices for 
an x86_64 host, where `__seg_gs`, etc attributes are used within the 
arch-specific kernel headers. If this kselftest was built on, say arm64, 
different warnings would likely surface which were out of scope. In 
general this patch is more of a workaround than the solution to the 
original problem: Arch-specific headers are used while compiling into 
BPF bytecode. That is why I originally only envisioned it as a fix for 
compile failures in Virtiozzo CI. Around the time of the submission of 
this patch, I tried to work on a more advanced solution, that would use 
arch-generic headers to build BPF programs. But soon that proved 
unsuccessful given that arch-specific headers directly alter the kernel 
structure and memory layout, which the BPF programs must understand.


>> Please also review the 4-commit patch series from Aug 21, non-backported
>> commits starting from "[PATCH vz10 2/4] selftests/bpf: run the network
>> tests in their own netns" first, so I can send a batch to upstream.
> i will review that series.


I will improve this patch description, and send it within batch of other 
BPF fixes you are currently reviewing when they are ready. Thanks.

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