Re: Error when loading BPF_CGROUP_INET_EGRESS program with bpftool

[Fejes Ferenc <fejes@inf.elte.hu>]

Thanks for the answer, I really appreciate it. I tried omitting
"cgroup/skb" to let libbpf guess the attach type, but I got the same
error. Really interesting, because I got the error
> libbpf: failed to load program 'cgroup_skb/egress'
wich is weird because it shows that libbpf guess the program type
correctly. So something definitely on my side - thank you for verifyng
that - I try to investigate it!

Ferenc
Andrii Nakryiko <andrii.nakryiko@gmail.com> ezt írta (időpont: 2019.
aug. 12., H, 20:27):
>
> 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!