Re: Error when loading BPF_CGROUP_INET_EGRESS program with bpftool

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Aug 12, 2019 at 1:59 AM Fejes Ferenc <fejes@inf.elte.hu> wrote:
>
> Greetings!
>
> I found a strange error when I tried to load a BPF_CGROUP_INET_EGRESS
> prog with bpftool. Loading the same program from C code with
> bpf_prog_load_xattr works without problem.
>
> The error message I got:
> bpftool prog loadall hbm_kern.o /sys/fs/bpf/hbm type cgroup/skb

You need "cgroup_skb/egress" instead of "cgroup/skb" (or try just
dropping it, bpftool will try to guess the type from program's section
name, which would be correct in this case).

> libbpf: load bpf program failed: Invalid argument
> libbpf: -- BEGIN DUMP LOG ---
> libbpf:
> ; return ALLOW_PKT | REDUCE_CW;
> 0: (b7) r0 = 3
> 1: (95) exit
> At program exit the register R0 has value (0x3; 0x0) should have been
> in (0x0; 0x1)
> processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0
> peak_states 0 mark_read 0
>
> libbpf: -- END LOG --
> libbpf: failed to load program 'cgroup_skb/egress'
> libbpf: failed to load object 'hbm_kern.o'
> Error: failed to load object file
>
>
> My environment: 5.3-rc3 / net-next master (both producing the error).
> Libbpf and bpftool installed from the kernel source (cleaned and
> reinstalled when I tried a new kernel). I compiled the program with
> Clang 8, on Ubuntu 19.10 server image, the source:
>
> #include <linux/bpf.h>
> #include "bpf_helpers.h"
>
> #define DROP_PKT        0
> #define ALLOW_PKT       1
> #define REDUCE_CW       2
>
> SEC("cgroup_skb/egress")
> int hbm(struct __sk_buff *skb)
> {
>         return ALLOW_PKT | REDUCE_CW;
> }
> char _license[] SEC("license") = "GPL";
>
>
> I also tried to trace down the bug with gdb. It seems like the
> section_names array in libbpf.c filled with garbage, especially the

I did the same, section_names appears to be correct, not sure what was
going on in your case. The problem is that "cgroup/skb", which you
provided on command line, overrides this section name and forces
bpftool to guess program type as BPF_CGROUP_INET_INGRESS, which
restricts return codes to just 0 or 1, while for
BPF_CGROUP_INET_EGRESS i is [0, 3].

> expected_attach_type fields (in my case, this contains
> BPF_CGROUP_INET_INGRESS instead of BPF_CGROUP_INET_EGRESS).
>
> Thanks!