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

[Christophe Leroy]

Le 17/01/2023 à 08:30, Christophe Leroy a écrit :
> 
> 
> Le 17/01/2023 à 06:30, Tonghao Zhang a écrit :
>>
>>
>>> On Jan 9, 2023, at 4:15 PM, Christophe Leroy 
>>> <christophe.leroy at 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 at infragraf.org a écrit :
>>>>>> From: Tonghao Zhang <tong at 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.

Hi Again,

I tested again, the problem is still the same as one year ago:

root at vgoip:~# ./bpftool prog
libbpf: elf: endianness mismatch in pid_iter_bpf.
libbpf: failed to initialize skeleton BPF object 'pid_iter_bpf': -4003
Error: failed to open PID iterator skeleton

root at vgoip:~# uname -a
Linux vgoip 6.2.0-rc3-02596-g1c2c9c13e256 #242 PREEMPT Tue Jan 17 
09:36:08 CET 2023 ppc GNU/Linux


> 
> 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
> 


Christophe