Re: [PATCH bpf-next v2 3/6] libbpf: Propagate EPERM to caller on program load

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

> On Fri, Nov 08, 2019 at 02:50:43PM -0800, Andrii Nakryiko wrote:
>> On Fri, Nov 8, 2019 at 1:33 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >
>> > From: Toke Høiland-Jørgensen <toke@redhat.com>
>> >
>> > When loading an eBPF program, libbpf overrides the return code for EPERM
>> > errors instead of returning it to the caller. This makes it hard to figure
>> > out what went wrong on load.
>> >
>> > In particular, EPERM is returned when the system rlimit is too low to lock
>> > the memory required for the BPF program. Previously, this was somewhat
>> > obscured because the rlimit error would be hit on map creation (which does
>> > return it correctly). However, since maps can now be reused, object load
>> > can proceed all the way to loading programs without hitting the error;
>> > propagating it even in this case makes it possible for the caller to react
>> > appropriately (and, e.g., attempt to raise the rlimit before retrying).
>> >
>> > Acked-by: Song Liu <songliubraving@fb.com>
>> > Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>> > ---
>> >  tools/lib/bpf/libbpf.c |    4 ++--
>> >  1 file changed, 2 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> > index cea61b2ec9d3..582c0fd16697 100644
>> > --- a/tools/lib/bpf/libbpf.c
>> > +++ b/tools/lib/bpf/libbpf.c
>> > @@ -3721,7 +3721,7 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
>> >                 free(log_buf);
>> >                 goto retry_load;
>> >         }
>> > -       ret = -LIBBPF_ERRNO__LOAD;
>> > +       ret = (errno == EPERM) ? -errno : -LIBBPF_ERRNO__LOAD;
>
> ouch. so libbpf was supressing all errnos for loading and that was a commit
> from 2015. No wonder it's hard to debug. I grepped every where I could and it
> doesn't look like anyone is using this code. There are other codes that can
> come from sys_bpf(prog_load). Not sure why such decision was made back then. I
> guess noone was really paying attention. I think we better propagate all codes.
> I don't see why EPERM should be special.

Fine with me; I just assumed there was a good reason for the current
behaviour :)

Will do a v3 that just passes through the errors from the kernel.

-Toke