Re: [BPF CO-RE clarification] Use CO-RE on older kernel versions.

[Vamsi Kodavanty <vamsi@araalinetworks.com>]

Andrii,
    Thank you so much for the quick response. You were right. I was
trying to CO-RE the `tcpconnect.py` program. It had some `.rodata`
which I removed. Now the program goes much further. It now finishes
the relocations successfully. But, fails at the next step
`bpf_object__load_progs`.

===
libbpf: prog 'tcp_v6_connect_ret': relo #3: patched insn #50
(ALU/ALU64) imm 56 -> 56
libbpf: failed to open '/sys/bus/event_source/devices/kprobe/type': No
such file or directory
libbpf: failed to determine kprobe perf type: No such file or directory
libbpf: prog 'tcp_v4_connect': failed to create kprobe
'tcp_v4_connect' perf event: No such file or directory
libbpf: failed to auto-attach program 'tcp_v4_connect': -2
===

This host has a 4.14 kernel AmazonLinux2 and does not have the above
file. It instead has this

===
$ sudo cat /sys/kernel/debug/tracing/kprobe_events
p:kprobes/p_sys_execve_bcc_2566 sys_execve
===

I am guessing this is a backward compatibility issue?. I will try to
look at an earlier version of libbpf to see how this was handled.
Meanwhile, if you have further comments they are appreciated.

And again I can't thank you enough for how helpful you have been and
your time. Cheers!.




On Thu, Jan 7, 2021 at 10:52 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Thu, Jan 7, 2021 at 10:12 AM Vamsi Kodavanty
> <vamsi@araalinetworks.com> wrote:
> >
> > First of all thank you very much for your quick response. And helpful pointers.
> > It seems like you also think what I am attempting to do should work.
> >
> > Please see inline [VAMSI-2].
> >
> > On Wed, Jan 6, 2021 at 3:55 PM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> > >
> > > On Wed, Jan 6, 2021 at 10:04 AM Vamsi Kodavanty
> > > <vamsi@araalinetworks.com> wrote:
> > > >
> > > > Had a few questions on CO-RE dependencies and usage. From what I read
> > > > CO-RE needs a supported kernel version and be compiled with
> > > > `CONFIG_DEBUG_INFO_BTF=y`.
> > > >
> > > > I also understand there are three pieces to enable CO-RE
> > > > functionality. (1) The BTF format. For efficient/compressed kernel
> > > > symbol table. (2) clang changes to emit the BTF relocations. (3)
> > >
> > > BTF is not really a symbol table, rather a type information. Like
> > > simpler and more compact DWARF.
> > >
> > > > `libbpf` changes to locate a BTF file and fix-up relocations. Once
> > > > these 3 steps are done the resulting byte code is no different from
> > > > non-CO-RE byte code.
> > > >
> > > > Given this I am hoping the knowledgeable folks on this mailer correct
> > > > and guide me if I am stating something incorrectly.
> > > >
> > > > (1) Is the kernel support requirement ONLY for the purposes of
> > > > generating and exposing the BTF file information on
> > > > `/sys/kernel/btf/vmlinux`? So that the eBPF CO-RE applications
> > > > `libbpf` can find the BTF information at a standard location?.
> > >
> > > /sys/kernel/btf/vmlinux is a standardized place, but libbpf will also
> > > try to search for vmlinux image (and BTF info within it) in a few
> > > standard locations, see [0]. Early versions of in-kernel BTF didn't
> > > even expose /sys/kernel/btf/vmlinux.
> > >
> > >   [0] https://github.com/libbpf/libbpf/blob/master/src/btf.c#L4580
> > >
> > > >
> > > > (2) If the answer to the above question is YES. Could the below
> > > > mechanism be used so that it works on all kernels whether they support
> > > > the `CONFIG_DEBUG_INFO_BTF` flag or not?.
> > > >        (a) Extract BTF generation process outside of the kernel build.
> > > > Use this to generate the equivalent BTF file for it.
> > >
> > > Yes, CONFIG_DEBUG_INFO_BTF=y is the most convenient way to add BTF
> > > info, but it's also possible to just embed BTF manually with a direct
> > > invocation of pahole -J, see [1] on how it's done for
> > > CONFIG_DEBUG_INFO_BTF. You can do that for *any* kernel image, no
> > > matter the version, and it will work with CO-RE relocations.
> > >
> > >   [1] https://github.com/torvalds/linux/blob/master/scripts/link-vmlinux.sh#L137-L170
> > >
> >
> > [VAMSI-2] Yes, this is exactly what I did. I extracted out the
> > `gen_btf` from the
> > `link-vmlinux.sh` (which uses pahole -J) and used it to generate a BTF
> > file for the
> > 4.14.0 kernel.
> >
> > > >        (b) Make changes to `libbpf` to look for BTF not only at the
> > > > standard locations but also at a user specified location. The BTF file
> > > > generated in (a) can be presented here.
> > >
> > > You can already do that, actually, though it's not very obvious. You
> > > can specify (or override) kernel BTF location by using
> > > bpf_object__load_xattr() and passing target_btf_path pointing to your
> > > BTF location (see [2]). I've been meaning to add it instead to a
> > > bpf_object_open_opts, btw, to make its use possible with a BPF
> > > skeleton. Also keep in mind that currently libbpf expects that custom
> > > BTF to be an ELF file with .BTF section, not just a raw BTF data. But
> > > we can improve that, of course.
> > >
> > >   [2] https://github.com/libbpf/libbpf/blob/master/src/libbpf.h#L136-L141
> >
> > [VAMSI-2] I took a look at this and what you suggested above does not
> > work as is.
> > Even if we used `bpf_object__load_xattr` with `target_btf_path`. It seems like
> > `bpf_object__load_vmlinux_btf` is not yet modified to use the
> > `target_btf_path` attribute.
>
> Ah, right. We used to need vmlinux BTF only for CO-RE relocations, but
> since then added a bunch more use cases. So some libbpf changes are
> needed to make this work. But it should still work for CO-RE to have a
> custom BTF.
>
> I'm not sure about making bpf_object__load_vmlinux_btf() load custom
> BTF as the real kernel BTF, because that will never work for
> fentry/fexit, struct_ops, etc. I think it is better to teach
> bpf_object__load_vmlinux_btf() to not attempt to load real kernel BTF
> if we need it only for CO-RE relocations *and* we have it overloaded
> with target_btf_path
>
> > Only, the `bpf_object__relocate` looks at the `target_btf_path`. As
> > you suggested enabling
> > use from the BPF skeleton seems useful and I can possibly help with that.
>
> Yeah, adding something like core_btf_path option to
> bpf_object_open_opts would go nicely with this change.
>
> >
> > For now, just for proof of concept I modified the search options in
> > `libbpf_find_kernel_btf` to
> > include my custom path. And on a 4.14 AmazonLinux2 VM I observe these failures.
> >
> > libbpf: loading kernel BTF '/home/ec2-user/vmlinux.btf': 0
>
> so here you successfully loaded custom BTF, which is good.
>
> > libbpf: Kernel doesn't support BTF, skipping uploading it.
>
> this just means that your BPF object's BTF won't be loaded into the
> kernel. That's no big deal, ignore this.
>
> > libbpf: kernel doesn't support global data
>
> But this means that your BPF programs rely on global variables, which
> are not supported by the kernel. So you need to change the code to not
> use global variables to make this work on very old kernels.
>
>
> > libbpf: failed to load object 'tcpconnect_bpf'
> > libbpf: failed to load BPF skeleton 'tcpconnect_bpf': -95
> > failed to load BPF object: -95
>
> This is probably OPNOTSUPP from the global data above
>
> >
> > This is the reason I had posted on the mailer. If the CO-RE executable
> > has relocations
> > resolved by the time of the BPF load. Why do we need to check for
> > kernel support?. Also,
> > does this mean what I am attempting to do will not work?.
> >
>
> it will work with minimal libbpf logic changes. Nothing in principle
> prevents this.
>
>
> > Best Regards. And again thanks a lot for your precious time.
> > - Vamsi.
> >
> > > >
> > > > This should provide us a way to enable CO-RE functionality on older
> > > > kernel versions as well. I tried to make the above changes and tried
> > > > against a 4.14 kernel and it did not work. Either I am not doing
> > > > something right or my assumptions are wrong.
> > > >
> > > > Thanks in advance for your time. And I hope someone here can guide me
> > > > in the right direction.
> > > >
> > > > Regards
> > > > Vamsi.