[PATCH bpf-next v2 4/5] selftests/bpf: Fix kprobe_multi_bench_attach test failure with LTO kernel

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

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@linux.dev>
---
 .../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;
 
 		err = hashmap__add(map, name, 0);
 		if (err == -EEXIST) {
-- 
2.43.0