Re: [bpf-next v2] bpf: drop deprecated bpf_jit_enable == 2

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 17/01/2023 à 06:30, Tonghao Zhang a écrit :
> 
> 
>> On Jan 9, 2023, at 4:15 PM, Christophe Leroy <christophe.leroy@csgroup.eu> wrote:
>>
>>
>>
>> Le 06/01/2023 à 16:37, Daniel Borkmann a écrit :
>>> On 1/5/23 6:53 PM, Christophe Leroy wrote:
>>>> Le 05/01/2023 à 04:06, tong@infragraf.org a écrit :
>>>>> From: Tonghao Zhang <tong@infragraf.org>
>>>>>
>>>>> The x86_64 can't dump the valid insn in this way. A test BPF prog
>>>>> which include subprog:
>>>>>
>>>>> $ llvm-objdump -d subprog.o
>>>>> Disassembly of section .text:
>>>>> 0000000000000000 <subprog>:
>>>>>           0:       18 01 00 00 73 75 62 70 00 00 00 00 72 6f 67 00 r1
>>>>> = 29114459903653235 ll
>>>>>           2:       7b 1a f8 ff 00 00 00 00 *(u64 *)(r10 - 8) = r1
>>>>>           3:       bf a1 00 00 00 00 00 00 r1 = r10
>>>>>           4:       07 01 00 00 f8 ff ff ff r1 += -8
>>>>>           5:       b7 02 00 00 08 00 00 00 r2 = 8
>>>>>           6:       85 00 00 00 06 00 00 00 call 6
>>>>>           7:       95 00 00 00 00 00 00 00 exit
>>>>> Disassembly of section raw_tp/sys_enter:
>>>>> 0000000000000000 <entry>:
>>>>>           0:       85 10 00 00 ff ff ff ff call -1
>>>>>           1:       b7 00 00 00 00 00 00 00 r0 = 0
>>>>>           2:       95 00 00 00 00 00 00 00 exit
>>>>>
>>>>> kernel print message:
>>>>> [  580.775387] flen=8 proglen=51 pass=3 image=ffffffffa000c20c
>>>>> from=kprobe-load pid=1643
>>>>> [  580.777236] JIT code: 00000000: cc cc cc cc cc cc cc cc cc cc cc
>>>>> cc cc cc cc cc
>>>>> [  580.779037] JIT code: 00000010: cc cc cc cc cc cc cc cc cc cc cc
>>>>> cc cc cc cc cc
>>>>> [  580.780767] JIT code: 00000020: cc cc cc cc cc cc cc cc cc cc cc
>>>>> cc cc cc cc cc
>>>>> [  580.782568] JIT code: 00000030: cc cc cc
>>>>>
>>>>> $ bpf_jit_disasm
>>>>> 51 bytes emitted from JIT compiler (pass:3, flen:8)
>>>>> ffffffffa000c20c + <x>:
>>>>>       0:   int3
>>>>>       1:   int3
>>>>>       2:   int3
>>>>>       3:   int3
>>>>>       4:   int3
>>>>>       5:   int3
>>>>>       ...
>>>>>
>>>>> Until bpf_jit_binary_pack_finalize is invoked, we copy rw_header to
>>>>> header
>>>>> and then image/insn is valid. BTW, we can use the "bpftool prog dump"
>>>>> JITed instructions.
>>>>
>>>> NACK.
>>>>
>>>> Because the feature is buggy on x86_64, you remove it for all
>>>> architectures ?
>>>>
>>>> On powerpc bpf_jit_enable == 2 works and is very usefull.
>>>>
>>>> Last time I tried to use bpftool on powerpc/32 it didn't work. I don't
>>>> remember the details, I think it was an issue with endianess. Maybe it
>>>> is fixed now, but it needs to be verified.
>>>>
>>>> So please, before removing a working and usefull feature, make sure
>>>> there is an alternative available to it for all architectures in all
>>>> configurations.
>>>>
>>>> Also, I don't think bpftool is usable to dump kernel BPF selftests.
>>>> That's vital when a selftest fails if you want to have a chance to
>>>> understand why it fails.
>>>
>>> If this is actively used by JIT developers and considered useful, I'd be
>>> ok to leave it for the time being. Overall goal is to reach feature parity
>>> among (at least major arch) JITs and not just have most functionality only
>>> available on x86-64 JIT. Could you however check what is not working with
>>> bpftool on powerpc/32? Perhaps it's not too much effort to just fix it,
>>> but details would be useful otherwise 'it didn't work' is too fuzzy.
>>
>> Sure I will try to test bpftool again in the coming days.
>>
>> Previous discussion about that subject is here:
>> https://patchwork.kernel.org/project/linux-riscv/patch/20210415093250.3391257-1-Jianlin.Lv@arm.com/#24176847=
> Hi Christophe
> Any progress? We discuss to deprecate the bpf_jit_enable == 2 in 2021, but bpftool can not run on powerpc.
> Now can we fix this issue?

Hi Tong,

I have started to look at it but I don't have any fruitfull feedback yet.

In the meantime, were you able to confirm that bpftool can also be used 
to dump jitted tests from test_bpf.ko module on x86_64 ? In that can you 
tell me how to proceed ?

Thanks
Christophe

>>
>>>
>>> Also, with regards to the last statement that bpftool is not usable to
>>> dump kernel BPF selftests. Could you elaborate some more? I haven't used
>>> bpf_jit_enable == 2 in a long time and for debugging always relied on
>>> bpftool to dump xlated insns or JIT. Or do you mean by BPF selftests
>>> the test_bpf.ko module? Given it has a big batch with kernel-only tests,
>>> there I can see it's probably still useful.
>>
>> Yes I mean test_bpf.ko
>>
>> I used it as the test basis when I implemented eBPF for powerpc/32. And
>> not so long ago it helped decover and fix a bug, see
>> https://github.com/torvalds/linux/commit/89d21e259a94f7d5582ec675aa445f5a79f347e4
>>
>>>
>>> Cheers,
>>> Daniel
>