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

[Alexei Starovoitov]

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.