[bpf-next,3/4] selftests/bpf: Fix possible ksyms test failure with LTO kernel

Commit Message

Yonghong Song March 2, 2024, 4:50 p.m. UTC

In my locally build clang LTO kernel (enabling CONFIG_LTO and
CONFIG_LTO_CLANG_THIN), ksyms test failed like:
  test_ksyms:PASS:kallsyms_fopen 0 nsec
  test_ksyms:FAIL:ksym_find symbol 'bpf_link_fops' not found
  #118     ksyms:FAIL

The reason is that 'bpf_link_fops' is renamed to
  bpf_link_fops.llvm.8325593422554671469
Due to cross-file inlining, the static variable 'bpf_link_fops'
in syscall.c is used by a function in another file. To avoid
potential duplicated names, the llvm added suffix '.llvm.<hash>'.

To fix the failure, we can skip this test with LTO kernel
if the symbol 'bpf_link_fops' is not found in kallsyms.

After this patch, with the same LTO kernel:
  #118     ksyms:SKIP

Signed-off-by: Yonghong Song <[e-mail]>
---
 tools/testing/selftests/bpf/prog_tests/ksyms.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Comments

Alexei Starovoitov March 5, 2024, 5:37 a.m. UTC | #1

On Sat, Mar 2, 2024 at 8:50 AM Yonghong Song <[e-mail]> wrote:
>
> In my locally build clang LTO kernel (enabling CONFIG_LTO and
> CONFIG_LTO_CLANG_THIN), ksyms test failed like:
>   test_ksyms:PASS:kallsyms_fopen 0 nsec
>   test_ksyms:FAIL:ksym_find symbol 'bpf_link_fops' not found
>   #118     ksyms:FAIL
>
> The reason is that 'bpf_link_fops' is renamed to
>   bpf_link_fops.llvm.8325593422554671469
> Due to cross-file inlining, the static variable 'bpf_link_fops'
> in syscall.c is used by a function in another file. To avoid
> potential duplicated names, the llvm added suffix '.llvm.<hash>'.
>
> To fix the failure, we can skip this test with LTO kernel
> if the symbol 'bpf_link_fops' is not found in kallsyms.
>
> After this patch, with the same LTO kernel:
>   #118     ksyms:SKIP
>
> Signed-off-by: Yonghong Song <[e-mail]>
> ---
>  tools/testing/selftests/bpf/prog_tests/ksyms.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms.c b/tools/testing/selftests/bpf/prog_tests/ksyms.c
> index e081f8bf3f17..cd81f190c5d7 100644
> --- a/tools/testing/selftests/bpf/prog_tests/ksyms.c
> +++ b/tools/testing/selftests/bpf/prog_tests/ksyms.c
> @@ -21,7 +21,11 @@ void test_ksyms(void)
>                 return;
>         }
>         if (err == -ENOENT) {
> -               ASSERT_TRUE(false, "ksym_find for bpf_link_fops");
> +               /* bpf_link_fops might be renamed to bpf_link_fops.llvm.<hash> in LTO kernel. */
> +               if (check_lto_kernel() == 1)
> +                       test__skip();
> +               else
> +                       ASSERT_TRUE(false, "ksym_find for bpf_link_fops");

I'm afraid LTO breakage is bigger than this.
pid_iter program as part of bpftool is using bpf_link_fops too.
I suspect that part of bpftool feature is broken on LTO kernel.
We need to find a solution instead of 'skip' a selftest.

Yonghong Song March 5, 2024, 7:24 a.m. UTC | #2

On 3/4/24 9:37 PM, Alexei Starovoitov wrote:
> On Sat, Mar 2, 2024 at 8:50 AM Yonghong Song <[e-mail]> wrote:
>> In my locally build clang LTO kernel (enabling CONFIG_LTO and
>> CONFIG_LTO_CLANG_THIN), ksyms test failed like:
>>    test_ksyms:PASS:kallsyms_fopen 0 nsec
>>    test_ksyms:FAIL:ksym_find symbol 'bpf_link_fops' not found
>>    #118     ksyms:FAIL
>>
>> The reason is that 'bpf_link_fops' is renamed to
>>    bpf_link_fops.llvm.8325593422554671469
>> Due to cross-file inlining, the static variable 'bpf_link_fops'
>> in syscall.c is used by a function in another file. To avoid
>> potential duplicated names, the llvm added suffix '.llvm.<hash>'.
>>
>> To fix the failure, we can skip this test with LTO kernel
>> if the symbol 'bpf_link_fops' is not found in kallsyms.
>>
>> After this patch, with the same LTO kernel:
>>    #118     ksyms:SKIP
>>
>> Signed-off-by: Yonghong Song <[e-mail]>
>> ---
>>   tools/testing/selftests/bpf/prog_tests/ksyms.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms.c b/tools/testing/selftests/bpf/prog_tests/ksyms.c
>> index e081f8bf3f17..cd81f190c5d7 100644
>> --- a/tools/testing/selftests/bpf/prog_tests/ksyms.c
>> +++ b/tools/testing/selftests/bpf/prog_tests/ksyms.c
>> @@ -21,7 +21,11 @@ void test_ksyms(void)
>>                  return;
>>          }
>>          if (err == -ENOENT) {
>> -               ASSERT_TRUE(false, "ksym_find for bpf_link_fops");
>> +               /* bpf_link_fops might be renamed to bpf_link_fops.llvm.<hash> in LTO kernel. */
>> +               if (check_lto_kernel() == 1)
>> +                       test__skip();
>> +               else
>> +                       ASSERT_TRUE(false, "ksym_find for bpf_link_fops");
> I'm afraid LTO breakage is bigger than this.
> pid_iter program as part of bpftool is using bpf_link_fops too.
> I suspect that part of bpftool feature is broken on LTO kernel.
> We need to find a solution instead of 'skip' a selftest.

Thanks for pointing this out! Let me do some investigation on
how to resolve this.

Patch

diff --git a/tools/testing/selftests/bpf/prog_tests/ksyms.c b/tools/testing/selftests/bpf/prog_tests/ksyms.c
index e081f8bf3f17..cd81f190c5d7 100644
--- a/tools/testing/selftests/bpf/prog_tests/ksyms.c
+++ b/tools/testing/selftests/bpf/prog_tests/ksyms.c
@@ -21,7 +21,11 @@  void test_ksyms(void)
 		return;
 	}
 	if (err == -ENOENT) {
-		ASSERT_TRUE(false, "ksym_find for bpf_link_fops");
+		/* bpf_link_fops might be renamed to bpf_link_fops.llvm.<hash> in LTO kernel. */
+		if (check_lto_kernel() == 1)
+			test__skip();
+		else
+			ASSERT_TRUE(false, "ksym_find for bpf_link_fops");
 		return;
 	}