On Fri, Mar 22, 2024 at 9:01 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
>
> On 3/22/24 5:37 AM, Jiri Olsa wrote:
> > 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?
>
> The number of entries in /sys/kernel/tracing/available_filter_functions: 50654
> After existing filtering ('arch_cpu_idle') etc: 50513 (filtering 141)
> After above ksym_get_addr(name) check: 49437 (further filtering 1076)
>
alternatively, you could have found matching func.llvm.* for any func
in available_filter_functions. Have you considered that?
> >
> > Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> >
> > jirka
> >
> >>
> >> err = hashmap__add(map, name, 0);
> >> if (err == -EEXIST) {
> >> --
> >> 2.43.0
> >>
> >>
