Re: [PATCH bpf-next 1/2] bpf: Remove bpf_jit_enable=2 debugging mode

[Quentin Monnet]

2021-04-21 15:10 UTC+0200 ~ Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> 
> 
> Le 20/04/2021 à 05:28, Alexei Starovoitov a écrit :
>> On Sat, Apr 17, 2021 at 1:16 AM Christophe Leroy
>> <christophe.leroy@xxxxxxxxxx> wrote:
>>>
>>>
>>>
>>> Le 16/04/2021 à 01:49, Alexei Starovoitov a écrit :
>>>> On Thu, Apr 15, 2021 at 8:41 AM Quentin Monnet
>>>> <quentin@xxxxxxxxxxxxx> wrote:
>>>>>
>>>>> 2021-04-15 16:37 UTC+0200 ~ Daniel Borkmann <daniel@xxxxxxxxxxxxx>
>>>>>> 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@xxxxxxx>
>>>>>
>>>>> 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@xxxxxxxxxx/
>>>
>>>
>>> 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@vgoip:~# ./bpftool prog dump jited
> Error: No libbfd support
> 
> Christophe

Hi Christophe,

Bpftool relies on libbfd to disassemble the JIT-ed instructions, but
this is an optional dependency and your version of bpftool has been
compiled without it.

You could try to install it on your system (it is usually shipped with
binutils, package "binutils-dev" on Ubuntu for example). If you want to
cross-compile bpftool, the libbfd version provided by your distribution
may not include support for the target architecture. In that case you
would have to build libbfd yourself to make sure it supports it.

Then you can clean up the results from the libbfd probing:

    $ make -C tools/build/feature/ clean

and recompile bpftool.

I hope this helps,
Quentin