Re: [PATCH bpf-next v3] selftests/bpf: replace fall through comment by fallthrough pseudo-keyword

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

On 8/1/23 2:48 AM, Ruan Jinjie wrote:
> Replace the existing /* fall through */ comments with the
> new pseudo-keyword macro fallthrough[1].
> 
> [1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through
> 
> Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
> ---
> v3:
> - Update the subject prefix and fix the 'fallthrough' undeclared build error.
> ---
> v2:
> - Update the subject and commit message.
> ---
>   tools/testing/selftests/bpf/prog_tests/kfunc_call.c          | 4 ++--
>   tools/testing/selftests/bpf/progs/test_cls_redirect.c        | 2 +-
>   tools/testing/selftests/bpf/progs/test_cls_redirect_dynptr.c | 2 +-
>   tools/testing/selftests/bpf/test_verifier.c                  | 3 ++-
>   4 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/kfunc_call.c b/tools/testing/selftests/bpf/prog_tests/kfunc_call.c
> index a543742cd7bd..0fd08172965a 100644
> --- a/tools/testing/selftests/bpf/prog_tests/kfunc_call.c
> +++ b/tools/testing/selftests/bpf/prog_tests/kfunc_call.c
> @@ -101,7 +101,7 @@ static void verify_success(struct kfunc_test_params *param)
>   	case syscall_test:
>   		topts.ctx_in = &args;
>   		topts.ctx_size_in = sizeof(args);
> -		/* fallthrough */
> +		fallthrough;

This won't work for clang built kernel/selftests:

In file included from progs/test_cls_redirect_subprogs.c:2:
progs/test_cls_redirect.c:303:4: error: use of undeclared identifier 
'fallthrough'
   303 |                         fallthrough;
       |                         ^
   CLNG-BPF [test_maps] netns_cookie_prog.bpf.o
   CLNG-BPF [test_maps] test_skmsg_load_helpers.bpf.o
   CLNG-BPF [test_maps] bpf_iter_setsockopt.bpf.o
   CLNG-BPF [test_maps] timer.bpf.o
progs/test_cls_redirect.c:303:4: error: use of undeclared identifier 
'fallthrough'
   303 |                         fallthrough;
       |                         ^

Try to build the kernel with:
   make -j LLVM=1
   make headers_install

and then build the selftests with
   make -C tools/testing/selftests/bpf -j LLVM=1

[~/work/bpf-next/tools/include (master)]$ egrep -r fallthrough
egrep: warning: egrep is obsolescent; using grep -E
linux/compiler-gcc.h:#if __has_attribute(__fallthrough__)
linux/compiler-gcc.h:# define fallthrough 
__attribute__((__fallthrough__))
linux/compiler-gcc.h:# define fallthrough                    do {} while 
(0)  /* fallthrough */
[~/work/bpf-next/tools/include (master)]$

Looks like 'fallthrough' is not defined for clang build tools/selftests.

>   	case syscall_null_ctx_test:
>   		break;
>   	case tc_test:
> @@ -167,7 +167,7 @@ static void verify_fail(struct kfunc_test_params *param)
>   	case syscall_test:
>   		topts.ctx_in = &args;
>   		topts.ctx_size_in = sizeof(args);
> -		/* fallthrough */
> +		fallthrough;
>   	case syscall_null_ctx_test:
>   		break;
>   	case tc_test:
[...]