On Sat, 2023-08-26 at 13:08 -0700, Yonghong Song wrote:
> With latest clang18, I hit test_progs failures for the following test:
> #13/2 bpf_cookie/multi_kprobe_link_api:FAIL
> #13/3 bpf_cookie/multi_kprobe_attach_api:FAIL
> #13 bpf_cookie:FAIL
> #75 fentry_fexit:FAIL
> #76/1 fentry_test/fentry:FAIL
> #76 fentry_test:FAIL
> #80/1 fexit_test/fexit:FAIL
> #80 fexit_test:FAIL
> #110/1 kprobe_multi_test/skel_api:FAIL
> #110/2 kprobe_multi_test/link_api_addrs:FAIL
> #110/3 kprobe_multi_test/link_api_syms:FAIL
> #110/4 kprobe_multi_test/attach_api_pattern:FAIL
> #110/5 kprobe_multi_test/attach_api_addrs:FAIL
> #110/6 kprobe_multi_test/attach_api_syms:FAIL
> #110 kprobe_multi_test:FAIL
>
> For example, for #13/2, the error messages are
> ...
> kprobe_multi_test_run:FAIL:kprobe_test7_result unexpected kprobe_test7_result: actual 0 != expected 1
> ...
> kprobe_multi_test_run:FAIL:kretprobe_test7_result unexpected kretprobe_test7_result: actual 0 != expected 1
>
> clang17 does not have this issue.
>
> Further investigation shows that kernel func bpf_fentry_test7(), used
> in the above tests, is inlined by the compiler although it is
> marked as noinline.
>
> int noinline bpf_fentry_test7(struct bpf_fentry_test_t *arg)
> {
> return (long)arg;
> }
>
> It is known that for simple functions like the above (e.g. just returning
> a constant or an input argument), the clang compiler may still do inlining
> for a noinline function. Adding 'asm volatile ("")' in the beginning of the
> bpf_fentry_test7() can prevent inlining.
Can confirm, this patch fixes listed flaky tests for me.
(when using LLVM main 651e644595b7 from 6 days ago).
Tested-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
>
> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> ---
> net/bpf/test_run.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
> index 57a7a64b84ed..0841f8d82419 100644
> --- a/net/bpf/test_run.c
> +++ b/net/bpf/test_run.c
> @@ -543,6 +543,7 @@ struct bpf_fentry_test_t {
>
> int noinline bpf_fentry_test7(struct bpf_fentry_test_t *arg)
> {
> + asm volatile ("");
> return (long)arg;
> }
>
