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)
Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
jirka
err = hashmap__add(map, name, 0);
if (err == -EEXIST) {
--
2.43.0
