On Thu, Mar 21, 2024 at 01:01:19PM -0700, Yonghong Song wrote:
> In my locally build clang LTO kernel (enabling CONFIG_LTO and
> CONFIG_LTO_CLANG_THIN), kprobe_multi_bench_attach/kernel subtest
> failed like:
> test_kprobe_multi_bench_attach:PASS:get_syms 0 nsec
> test_kprobe_multi_bench_attach:PASS:kprobe_multi_empty__open_and_load 0 nsec
> libbpf: prog 'test_kprobe_empty': failed to attach: No such process
> test_kprobe_multi_bench_attach:FAIL:bpf_program__attach_kprobe_multi_opts unexpected error: -3
> #117/1 kprobe_multi_bench_attach/kernel:FAIL
>
> There are multiple symbols in /sys/kernel/debug/tracing/available_filter_functions
> are renamed in /proc/kallsyms due to cross file inlining. One example is for
> static function __access_remote_vm in mm/memory.c.
> In a non-LTO kernel, we have the following call stack:
> ptrace_access_vm (global, kernel/ptrace.c)
> access_remote_vm (global, mm/memory.c)
> __access_remote_vm (static, mm/memory.c)
>
> With LTO kernel, it is possible that access_remote_vm() is inlined by
> ptrace_access_vm(). So we end up with the following call stack:
> ptrace_access_vm (global, kernel/ptrace.c)
> __access_remote_vm (static, mm/memory.c)
> The compiler renames __access_remote_vm to __access_remote_vm.llvm.<hash>
> to prevent potential name collision.
>
> The kernel bpf_kprobe_multi_link_attach() and ftrace_lookup_symbols() try
> to find addresses based on /proc/kallsyms, hence the current test failed
> with LTO kenrel.
>
> This patch removed __access_remote_vm and other similar functions from
> kprobe_multi_attach by checking if the symbol like __access_remote_vm
> does not exist in kallsyms with LTO kernel. The test succeeded after this change:
> #117/1 kprobe_multi_bench_attach/kernel:OK
>
> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
> ---
> .../selftests/bpf/prog_tests/kprobe_multi_test.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
> index 05000810e28e..f6130f4f3d88 100644
> --- a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
> +++ b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
> @@ -345,6 +345,9 @@ static int get_syms(char ***symsp, size_t *cntp, bool kernel)
> FILE *f;
> int err = 0;
>
> + if (!ASSERT_OK(load_kallsyms(), "load_kallsyms"))
> + return -EINVAL;
> +
> /*
> * The available_filter_functions contains many duplicates,
> * but other than that all symbols are usable in kprobe multi
> @@ -393,6 +396,15 @@ static int get_syms(char ***symsp, size_t *cntp, bool kernel)
> if (!strncmp(name, "__ftrace_invalid_address__",
> sizeof("__ftrace_invalid_address__") - 1))
> continue;
> + /*
> + * In certain cases, e.g., clang lto kernel, the 'name' here
> + * may be different from the one in /proc/kallsyms due to
> + * /proc/kallsyms name might be "<name>.llvm.<hash>" instead
> + * of "<name>". Exclude these 'name's since they will cause
> + * later kprobe_multi_attach failure.
> + */
> + if (ksym_get_addr(name) == 0)
> + continue;
curious how many symbols like that are there?
Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
jirka
>
> err = hashmap__add(map, name, 0);
> if (err == -EEXIST) {
> --
> 2.43.0
>
>
