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Message-ID: <0b46b813-05f2-5083-9f2e-82d72970dae2@csgroup.eu>
Date: Tue, 17 Jan 2023 07:30:10 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Tonghao Zhang <tong@...ragraf.org>,
Daniel Borkmann <daniel@...earbox.net>
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Subject: Re: [bpf-next v2] bpf: drop deprecated bpf_jit_enable == 2
Le 17/01/2023 à 06:30, Tonghao Zhang a écrit :
>
>
>> On Jan 9, 2023, at 4:15 PM, Christophe Leroy <christophe.leroy@...roup.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@...ragraf.org a écrit :
>>>>> From: Tonghao Zhang <tong@...ragraf.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
>
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