Re: s390x BPF JIT failures with test_bpf

Kleber Souza
2018-06-27T12:30:38Z

On 06/27/18 12:36, Daniel Borkmann wrote:
> On 06/27/2018 12:13 PM, Kleber Souza wrote:
>> On 06/27/18 12:01, Daniel Borkmann wrote:
>>> On 06/27/2018 11:40 AM, Kleber Souza wrote:
>>> [...]
>>>> When I load the test_bpf module from mainline (v4.18-rc2) with
>>>> CONFIG_BPF_JIT_ALWAYS_ON=y on a s390x system I get the following errors:
>>>>
>>>> test_bpf: #289 BPF_MAXINSNS: Ctx heavy transformations FAIL to
>>>> prog_create err=-524 len=4096
>>>> test_bpf: #290 BPF_MAXINSNS: Call heavy transformations FAIL to
>>>> prog_create err=-524 len=4096
>>>> [...]
>>>> test_bpf: #296 BPF_MAXINSNS: exec all MSH FAIL to prog_create err=-524
>>>> len=4096
>>>> test_bpf: #297 BPF_MAXINSNS: ld_abs+get_processor_id FAIL to prog_create
>>>> err=-524 len=4096
>>>>
>>>> From a quick look at the code it seems that
>>>> arch/s390/net/bpf_jit_comp.c:bpf_int_jit_compile() is failing to JIT
>>>> compile the test code.
>>>>
>>>> Are those failures expected and could be flagged with FLAG_EXPECTED_FAIL
>>>> on lib/test_bpf.c or are those caused by some issue with the s390x JIT
>>>> compiler that needs to be fixed?
>>>
>>> JIT doesn't guarantee in general to map really all programs to native insns,
>>> so some, mostly crafted corner cases could fail. E.g. x86-64 JIT doesn't converge
>>> on some programs in test_bpf.c and thus falls back to interpreter or simply
>>> rejects the program in case of CONFIG_BPF_JIT_ALWAYS_ON=y. Above would seem
>>> likely that it's hitting the BPF_SIZE_MAX that s390 would do. I think it might
>>> make sense to either have the FLAG_EXPECTED_FAIL in lib/test_bpf.c more fine
>>> grained as a flag per arch, so we could say it's expected to fail on e.g. s390
>>> but not on x86 and the like, or just denote it as 'could potentially fail but
>>> doesn't have to be the case everywhere'.
>>
>> Thank you for your reply. I will run some more tests to make sure we are
>> hitting BPF_SIZE_MAX or what exactly is failing and send a patch to flag
>> it conditionally for s390x.
>
> Sounds good, thanks! In any case, please let us know your findings.
>
> Best,
> Daniel
>
Hi Daniel,

Your presumption was correct, all four tests are failing because they
exceed BPF_SIZE_MAX. I'll send a patch shortly.

Thanks!
Kleber