Re: [PATCH iproute2] libbpf: fix attach of prog with multiple sections

On Wed, Jul 21, 2021 at 8:25 AM Martynas Pumputis [off-list ref] wrote:

On 7/21/21 4:59 PM, David Ahern wrote:

quoted

On 7/21/21 8:47 AM, Martynas Pumputis wrote:

quoted

quoted

quoted

diff --git a/lib/bpf_libbpf.c b/lib/bpf_libbpf.c
index d05737a4..f76b90d2 100644
--- a/lib/bpf_libbpf.c
+++ b/lib/bpf_libbpf.c

@@ -267,10 +267,12 @@ static int load_bpf_object(struct bpf_cfg_in *cfg)
          }

          bpf_object__for_each_program(p, obj) {
+               bool prog_to_attach = !prog && cfg->section &&
+                       !strcmp(get_bpf_program__section_name(p),

cfg->section);

This is still problematic, because one section can have multiple BPF
programs. I.e., it's possible two define two or more XDP BPF programs
all with SEC("xdp") and libbpf works just fine with that. I suggest
moving users to specify the program name (i.e., C function name
representing the BPF program). All the xdp_mycustom_suffix namings are
a hack and will be rejected by libbpf 1.0, so it would be great to get
a head start on fixing this early on.

Thanks for bringing this up. Currently, there is no way to specify a
function name with "tc exec bpf" (only a section name via the "sec"
arg). So probably, we should just add another arg to specify the
function name.

It would be interesting to hear thoughts from iproute2 maintainers
before fixing this.

maintaining backwards compatibility is a core principle for iproute2. If
we know of a libbpf change is going to cause a breakage then it is best
to fix it before any iproute2 release is affected.

Just to avoid any confusion (if there is any), the required change we
are discussing doesn't have anything to do with my fix.

To set the context, the motivation for unifying section names is
documented and discussed in "Stricter and more uniform BPF program
section name (SEC()) handling" of [1].

Andrii: is bpftool able to load programs with multiple sections which
are named the same today?

I'm not familiar with those parts of bpftool, I've never used
bpftool's command to load BPF programs. Please go check the code.

[1]:
https://docs.google.com/document/d/1UyjTZuPFWiPFyKk1tV5an11_iaRuec6U-ZESZ54nNTY/edit#