selftests/bpf: Fix possible kprobe_multi_bench_attach test failure with LTO kernel

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
  #114/1   kprobe_multi_bench_attach/kernel:FAIL

There are multiple symbols in /sys/kernel/debug/tracing/available_filter_functions
are renamed in 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.

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:
  #114/1   kprobe_multi_bench_attach/kernel:OK

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>

Author: Yonghong Song

tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c

@@ -341,10 +341,15 @@ static int get_syms(char ***symsp, size_t *cntp, bool kernel)
 	size_t cap = 0, cnt = 0, i;
 	char *name = NULL, **syms = NULL;
 	struct hashmap *map;
+	bool lto_kernel;
 	char buf[256];
 	FILE *f;
 	int err = 0;
 
+	lto_kernel = kernel && check_lto_kernel() == 1;
+	if (lto_kernel && !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 +398,8 @@ static int get_syms(char ***symsp, size_t *cntp, bool kernel)
 		if (!strncmp(name, "__ftrace_invalid_address__",
 			     sizeof("__ftrace_invalid_address__") - 1))
 			continue;
+		if (lto_kernel && ksym_get_addr(name) == 0)
+			continue;
 
 		err = hashmap__add(map, name, 0);
 		if (err == -EEXIST) {