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

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

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@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;
> 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@kernel.org>
>
> jirka
>
>>   
>>   		err = hashmap__add(map, name, 0);
>>   		if (err == -EEXIST) {
>> -- 
>> 2.43.0
>>
>>