On Tue, Jan 30, 2024 at 6:32 AM Jose E. Marchesi
<jose.marchesi@xxxxxxxxxx> 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@xxxxxxxxxx>
> Cc: Yonghong Song <yhs@xxxxxxxx>
> Cc: Eduard Zingerman <eddyz87@xxxxxxxxx>
> Cc: David Faust <david.faust@xxxxxxxxxx>
> Cc: Cupertino Miranda <cupertino.miranda@xxxxxxxxxx>
> ---
> 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?
> endif
>
> ifneq ($(CLANG_CPUV4),)
> --
> 2.30.2
>
>
