Date:   Wed, 21 Apr 2021 09:29:50 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Quentin Monnet <quentin@...valent.com>,
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        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH bpf-next 1/2] bpf: Remove bpf_jit_enable=2 debugging mode

Christophe Leroy wrote:
> 
> 
> 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

Seems your bpftool prog was built without libbfd, can you rebuild with libbfd
installed.

.John

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