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 finally managed to cross compile bpftool with libbpf, libopcodes, readline, ncurses, libcap, libz
and all needed stuff. Was not easy but I made it.
Now, how do I use it ?
Let say I want to dump the jitted code generated from a call to 'tcpdump'. How do I do that with
'bpftool prog dump jited' ?
I thought by calling this line I would then get programs dumped in a way or another just like when
setting 'bpf_jit_enable=2', but calling that line just provides me some bpftool help text.
By the way, I would be nice to have a kernel OPTION that selects all OPTIONS required for building
bpftool. Because you discover them one by one at every build failure. I had to had CONFIG_IPV6,
CONFIG_DEBUG_BTF, CONFIG_CGROUPS, ... If there could be an option like "Build a 'bpftool' ready
kernel" that selected all those, it would be great.
Christophe
