Re: [PATCH bpf-next] bpf: use -Wno-address-of-packed-member when building with GCC

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Tue, Jan 30, 2024 at 6:32 AM Jose E. Marchesi
> <jose.marchesi@oracle.com> wrote:
>>
>> GCC implements the -Wno-address-of-packed-member warning, which is
>> enabled by -Wall, that warns about taking the address of a packed
>> struct field when it can lead to an "unaligned" address.  Clang
>> doesn't support this warning.
>>
>> This triggers the following errors (-Werror) when building three
>> particular BPF selftests with GCC:
>>
>>   progs/test_cls_redirect.c
>>   986 |         if (ipv4_is_fragment((void *)&encap->ip)) {
>>   progs/test_cls_redirect_dynptr.c
>>   410 |         pkt_ipv4_checksum((void *)&encap_gre->ip);
>>   progs/test_cls_redirect.c
>>   521 |         pkt_ipv4_checksum((void *)&encap_gre->ip);
>>   progs/test_tc_tunnel.c
>>    232 |         set_ipv4_csum((void *)&h_outer.ip);
>>
>> These warnings do not signal any real problem in the tests as far as I
>> can see.
>>
>> This patch modifies selftests/bpf/Makefile to build these particular
>> selftests with -Wno-address-of-packed-member when bpf-gcc is used.
>> Note that we cannot use diagnostics pragmas (which are generally
>> preferred if I understood properly in a recent BPF office hours)
>> because Clang doesn't support these warnings.
>>
>> Tested in bpf-next master.
>> No regressions.
>>
>> Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
>> Cc: Yonghong Song <yhs@meta.com>
>> Cc: Eduard Zingerman <eddyz87@gmail.com>
>> Cc: David Faust <david.faust@oracle.com>
>> Cc: Cupertino Miranda <cupertino.miranda@oracle.com>
>> ---
>>  tools/testing/selftests/bpf/Makefile | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
>> index 1a3654bcb5dd..036473060bae 100644
>> --- a/tools/testing/selftests/bpf/Makefile
>> +++ b/tools/testing/selftests/bpf/Makefile
>> @@ -73,6 +73,12 @@ progs/btf_dump_test_case_namespacing.c-CFLAGS := -Wno-error
>>  progs/btf_dump_test_case_packing.c-CFLAGS := -Wno-error
>>  progs/btf_dump_test_case_padding.c-CFLAGS := -Wno-error
>>  progs/btf_dump_test_case_syntax.c-CFLAGS := -Wno-error
>> +
>> +# The following selftests take the address of packed struct fields in
>> +# a way that can lead to unaligned addresses.  GCC warns about this.
>> +progs/test_cls_redirect.c-CFLAGS := -Wno-address-of-packed-member
>> +progs/test_cls_redirect_dynpr.c-CFLAGS := -Wno-address-of-packed-member
>> +progs/test_tc_tunnel.c-CFLAGS := -Wno-address-of-packed-member
>
> Why Makefile additions like these are preferable to just using #pragma
> in corresponding .c file? I understand there is no #pragma equivalent
> of -Wno-error, but these diagnostics do have #pragma equivalent,
> right?

Not with this particular one, because Clang doesn't support
-W[no-]address-of-packed-member so it would lead to compilation error.

Hence:

>> Note that we cannot use diagnostics pragmas (which are generally
>> preferred if I understood properly in a recent BPF office hours)
>> because Clang doesn't support these warnings.

>
>>  endif
>>
>>  ifneq ($(CLANG_CPUV4),)
>> --
>> 2.30.2
>>
>>