Date:   Wed, 21 Apr 2021 15:10:10 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Quentin Monnet <quentin@...valent.com>,
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Subject: Re: [PATCH bpf-next 1/2] bpf: Remove bpf_jit_enable=2 debugging mode



Le 20/04/2021 à 05:28, Alexei Starovoitov a écrit :
> On Sat, Apr 17, 2021 at 1:16 AM Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>>
>>
>>
>> Le 16/04/2021 à 01:49, Alexei Starovoitov a écrit :
>>> On Thu, Apr 15, 2021 at 8:41 AM Quentin Monnet <quentin@...valent.com> wrote:
>>>>
>>>> 2021-04-15 16:37 UTC+0200 ~ Daniel Borkmann <daniel@...earbox.net>
>>>>> On 4/15/21 11:32 AM, Jianlin Lv wrote:
>>>>>> For debugging JITs, dumping the JITed image to kernel log is discouraged,
>>>>>> "bpftool prog dump jited" is much better way to examine JITed dumps.
>>>>>> This patch get rid of the code related to bpf_jit_enable=2 mode and
>>>>>> update the proc handler of bpf_jit_enable, also added auxiliary
>>>>>> information to explain how to use bpf_jit_disasm tool after this change.
>>>>>>
>>>>>> Signed-off-by: Jianlin Lv <Jianlin.Lv@....com>
>>>>
>>>> Hello,
>>>>
>>>> For what it's worth, I have already seen people dump the JIT image in
>>>> kernel logs in Qemu VMs running with just a busybox, not for kernel
>>>> development, but in a context where buiding/using bpftool was not
>>>> possible.
>>>
>>> If building/using bpftool is not possible then majority of selftests won't
>>> be exercised. I don't think such environment is suitable for any kind
>>> of bpf development. Much so for JIT debugging.
>>> While bpf_jit_enable=2 is nothing but the debugging tool for JIT developers.
>>> I'd rather nuke that code instead of carrying it from kernel to kernel.
>>>
>>
>> When I implemented JIT for PPC32, it was extremely helpfull.
>>
>> As far as I understand, for the time being bpftool is not usable in my environment because it
>> doesn't support cross compilation when the target's endianess differs from the building host
>> endianess, see discussion at
>> https://lore.kernel.org/bpf/21e66a09-514f-f426-b9e2-13baab0b938b@csgroup.eu/
>>
>> That's right that selftests can't be exercised because they don't build.
>>
>> The question might be candid as I didn't investigate much about the replacement of "bpf_jit_enable=2
>> debugging mode" by bpftool, how do we use bpftool exactly for that ? Especially when using the BPF
>> test module ?
> 
> the kernel developers can add any amount of printk and dumps to debug
> their code,
> but such debugging aid should not be part of the production kernel.
> That sysctl was two things at once: debugging tool for kernel devs and
> introspection for users.
> bpftool jit dump solves the 2nd part. It provides JIT introspection to users.
> Debugging of the kernel can be done with any amount of auxiliary code
> including calling print_hex_dump() during jiting.
> 

I get the following message when trying the command suggested in the patch message:

root@...ip:~# ./bpftool prog dump jited
Error: No libbfd support

Christophe

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