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

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

Le 18/01/2023 à 03:21, Alexei Starovoitov a écrit :
> On Tue, Jan 17, 2023 at 6:13 PM Tonghao Zhang <tong@infragraf.org> wrote:
>>
>>
>>
>>> On Jan 17, 2023, at 11:59 PM, Daniel Borkmann <daniel@iogearbox.net> wrote:
>>>
>>> On 1/17/23 3:22 PM, Tonghao Zhang wrote:
>>>>> On Jan 17, 2023, at 3:30 PM, Christophe Leroy <christophe.leroy@csgroup.eu> wrote:
>>>>>
>>>>>
>>>>>
>>>>> 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 ?
>>>> Now I do not test, but we can dump the insn after bpf_prog_select_runtime in test_bpf.ko. bpf_map_get_info_by_fd can copy the insn to userspace, but we can
>>>> dump them in test_bpf.ko in the same way.
>>>
>>> Issue is that these progs are not consumable from userspace (and therefore not bpftool).
>>> it's just simple bpf_prog_alloc + copy of test insns + bpf_prog_select_runtime() to test
>>> JITs (see generate_filter()). Some of them could be converted over to test_verifier, but
>>> not all might actually pass verifier, iirc. Don't think it's a good idea to allow exposing
>>> them via fd tbh.
>> Hi
>> I mean that, can we invoke the bpf_jit_dump in test_bpf.ko directly ?. bpf_prog_get_info_by_fd copy the insn to userspace, but we only dump insn in test_bpf.ko
>>
>>                  if (bpf_dump_raw_ok(file->f_cred)) {// code copied from bpf_prog_get_info_by_fd, not tested
>>
>>                          /* for multi-function programs, copy the JITed
>>                           * instructions for all the functions
>>                           */
>>                          if (prog->aux->func_cnt) {
>>                                  for (i = 0; i < prog->aux->func_cnt; i++) {
>>                                          len = prog->aux->func[i]->jited_len;
>>                                          img = (u8 *) prog->aux->func[i]->bpf_func;
>>                                          bpf_jit_dump(1, len, 1, img);
>>                                  }
>>                          } else {
>>                                  bpf_jit_dump(1, ulen, 1, prog->bpf_func);
>>                          }
>>                  }
> 
> Let's not reinvent the wheel.
> bpftool prog dump jited
> is our supported command.
> ppc issue with bpftool is related to endianness of embedded skeleton.
> which means that none of the bpftool prog commands work on ppc.
> It's a bigger issue to address with cross compilation of bpftool.
> 
> bpftool supports gnu and llvm disassembler. It retrieves and
> prints BTF, line info and source code along with asm.
> The user experience is at different level comparing to bpf_jit_dump.

Hi Alexei,

Fair enough, we are going to try and fix bpftool.

But for test_bpf.ko module, how do you use bpftool to dump the BPF tests 
? Even on x86 I have not been able to use bpftool for that until now. 
Can you tell me how you do ?

Thanks
Christophe