Bpf_link_create support issue in the Android kernel 5.7.0 and later

[SajjadPourali]

It appears that Android with kernel 5.7.0 and later still does not support bpf_link_create properly, which causes attaching cgroup programs using Aya to be impossible.

I use Android 16 and 15 in the Android emulator. A potential solution is to add a fallback method that switches ProgAttachLink::attach in case bpf_link_create is unsupported, which can resolve the issue on Android.

aya/aya/src/programs/cgroup_sock_addr.rs

    pub fn attach<T: AsFd>(
        &mut self,
        cgroup: T,
        mode: CgroupAttachMode,
    ) -> Result<CgroupSockAddrLinkId, ProgramError> {
        let prog_fd = self.fd()?;
        let prog_fd = prog_fd.as_fd();
        let cgroup_fd = cgroup.as_fd();
        let attach_type = self.data.expected_attach_type.unwrap();
        if KernelVersion::current().unwrap() >= KernelVersion::new(5, 7, 0) {
            let link_fd = bpf_link_create(
                prog_fd,
                LinkTarget::Fd(cgroup_fd),
                attach_type,
                None,
                mode.into(),
                None,
            );
            match link_fd {
                Ok(lfd) => {
                    return self.data.links.insert(CgroupSockAddrLink::new(
                        CgroupSockAddrLinkInner::Fd(FdLink::new(lfd)),
                    ));
                }
                Err(e) => {
                    warn!(
                        "failed to create link using bpf_link_create: {}, switching to fallback",
                        e.1.to_string()
                    );
                }
            };
        }
        let link = ProgAttachLink::attach(prog_fd, cgroup_fd, attach_type, mode)?;

        self.data.links.insert(CgroupSockAddrLink::new(
            CgroupSockAddrLinkInner::ProgAttach(link),
        ))
    }

[alessandrod]

I think that instead of hardcoding a version, we should do a probe at startup (we already probe other features). I'd be happy to assist if you wanted to try to work on a PR!

[SajjadPourali]

Probing in this case might not be very useful, as it seems to be an Android-related glitch. bpf_link_create is already implemented on Android. However, when passing the file descriptor (fd) of a program loaded using bpf_load_program with the BPF_PROG_TYPE_CGROUP_SOCK_ADDR type, it throws a "permission denied" error, as far as I understand. The same program, with the same fd, can be loaded using bpf_prog_attach.

I also tested another program with the BPF_PROG_TYPE_SOCK_OPS type, which works well with bpf_link_create.

Any suggestions for resolving this issue?