Re: [RFC bpf-next 4/5] iproute2: Allow compiling against libbpf

[Daniel Borkmann <daniel@iogearbox.net>]

On 8/22/19 12:43 PM, Toke Høiland-Jørgensen wrote:
> Daniel Borkmann <daniel@iogearbox.net> writes:
>> On 8/20/19 1:47 PM, Toke Høiland-Jørgensen wrote:
>>> This adds a configure check for libbpf and renames functions to allow
>>> lib/bpf.c to be compiled with it present. This makes it possible to
>>> port functionality piecemeal to use libbpf.
>>>
>>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>>> ---
>>>    configure          | 16 ++++++++++++++++
>>>    include/bpf_util.h |  6 +++---
>>>    ip/ipvrf.c         |  4 ++--
>>>    lib/bpf.c          | 33 +++++++++++++++++++--------------
>>>    4 files changed, 40 insertions(+), 19 deletions(-)
>>>
>>> diff --git a/configure b/configure
>>> index 45fcffb6..5a89ee9f 100755
>>> --- a/configure
>>> +++ b/configure
>>> @@ -238,6 +238,19 @@ check_elf()
>>>        fi
>>>    }
>>>    
>>> +check_libbpf()
>>> +{
>>> +    if ${PKG_CONFIG} libbpf --exists; then
>>> +	echo "HAVE_LIBBPF:=y" >>$CONFIG
>>> +	echo "yes"
>>> +
>>> +	echo 'CFLAGS += -DHAVE_LIBBPF' `${PKG_CONFIG} libbpf --cflags` >> $CONFIG
>>> +	echo 'LDLIBS += ' `${PKG_CONFIG} libbpf --libs` >>$CONFIG
>>> +    else
>>> +	echo "no"
>>> +    fi
>>> +}
>>> +
>>>    check_selinux()
>>
>> More of an implementation detail at this point in time, but want to
>> make sure this doesn't get missed along the way: as discussed at
>> bpfconf [0] best for iproute2 to handle libbpf support would be the
>> same way of integration as pahole does, that is, to integrate it via
>> submodule [1] to allow kernel and libbpf features to be in sync with
>> iproute2 releases and therefore easily consume extensions we're adding
>> to libbpf to aide iproute2 integration.
> 
> I can sorta see the point wrt keeping in sync with kernel features. But
> how will this work with distros that package libbpf as a regular
> library? Have you guys given up on regular library symbol versioning for
> libbpf?

Not at all, and I hope you know that. ;-) The reason I added lib/bpf.c
integration into iproute2 directly back then was exactly such that users
can start consuming BPF for tc and XDP via iproute2 /everywhere/ with
only a simple libelf dependency which is also available on all distros
since pretty much forever. If it was an external library, we could have
waited till hell freezes over and initial distro adoption would have pretty
much taken forever: to pick one random example here wrt the pace of some
downstream distros [0]. The main rationale is pretty much the same as with
added kernel features that land complementary iproute2 patches for that
kernel release and as libbpf is developed alongside it is reasonable to
guarantee user expectations that iproute2 released for kernel version x
can make use of BPF features added to kernel x with same loader support
from x.

   [0] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1774815

>>     [0] http://vger.kernel.org/bpfconf2019.html#session-4
> 
> Thanks for that link! Didn't manage to find any of the previous
> discussions on iproute2 compatibility.
> 
> -Toke
>