Re: [PATCH v2 bpf] selftests/bpf: fix static assert compilation issue for test_cls_*.c

[Yonghong Song <yonghong.song@linux.dev>]

On 8/2/23 12:39 AM, Alan Maguire wrote:
> commit bdeeed3498c7 ("libbpf: fix offsetof() and container_of() to work with CO-RE")
> 
> ...was backported to stable trees such as 5.15. The problem is that with older
> LLVM/clang (14/15) - which is often used for older kernels - we see compilation
> failures in BPF selftests now:
> 
> In file included from progs/test_cls_redirect_subprogs.c:2:
> progs/test_cls_redirect.c:90:2: error: static assertion expression is not an integral constant expression
>          sizeof(flow_ports_t) !=
>          ^~~~~~~~~~~~~~~~~~~~~~~
> progs/test_cls_redirect.c:91:3: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
>                  offsetofend(struct bpf_sock_tuple, ipv4.dport) -
>                  ^
> progs/test_cls_redirect.c:32:3: note: expanded from macro 'offsetofend'
>          (offsetof(TYPE, MEMBER) + sizeof((((TYPE *)0)->MEMBER)))
>           ^
> tools/testing/selftests/bpf/tools/include/bpf/bpf_helpers.h:86:33: note: expanded from macro 'offsetof'
>                                   ^
> In file included from progs/test_cls_redirect_subprogs.c:2:
> progs/test_cls_redirect.c:95:2: error: static assertion expression is not an integral constant expression
>          sizeof(flow_ports_t) !=
>          ^~~~~~~~~~~~~~~~~~~~~~~
> progs/test_cls_redirect.c:96:3: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
>                  offsetofend(struct bpf_sock_tuple, ipv6.dport) -
>                  ^
> progs/test_cls_redirect.c:32:3: note: expanded from macro 'offsetofend'
>          (offsetof(TYPE, MEMBER) + sizeof((((TYPE *)0)->MEMBER)))
>           ^
> tools/testing/selftests/bpf/tools/include/bpf/bpf_helpers.h:86:33: note: expanded from macro 'offsetof'
>                                   ^
> 2 errors generated.
> make: *** [Makefile:594: tools/testing/selftests/bpf/test_cls_redirect_subprogs.bpf.o] Error 1
> 
> The problem is the new offsetof() does not play nice with static asserts.
> Given that the context is a static assert (and CO-RE relocation is not
> needed at compile time), offsetof() usage can be replaced by restoring
> the original offsetof() definition as __builtin_offsetof().
> 
> Fixes: bdeeed3498c7 ("libbpf: fix offsetof() and container_of() to work with CO-RE")
> Reported-by: Colm Harrington <colm.harrington@oracle.com>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>

Acked-by: Yonghong Song <yonghong.song@linux.dev>