Re: s390x BPF JIT failures with test_bpf

Kleber Souza
2018-06-27T10:13:29Z

On 06/27/18 12:01, Daniel Borkmann wrote:
> Hi Kleber,
>
> 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'.

Hi Daniel,

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.

Thanks,
Kleber