Re: [RFC bpf-next 0/5] Convert iproute2 to use libbpf (WIP)

Andrii Nakryiko [off-list ref] writes:

On Tue, Feb 4, 2020 at 11:19 AM Toke Høiland-Jørgensen [off-list ref] wrote:

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Andrii Nakryiko [off-list ref] writes:

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On Tue, Feb 4, 2020 at 12:25 AM Toke Høiland-Jørgensen [off-list ref] wrote:

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Andrii Nakryiko [off-list ref] writes:

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On Mon, Feb 3, 2020 at 8:53 PM David Ahern [off-list ref] wrote:

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On 2/3/20 8:41 PM, Andrii Nakryiko wrote:

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On Mon, Feb 3, 2020 at 5:46 PM David Ahern [off-list ref] wrote:

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On 2/3/20 5:56 PM, Andrii Nakryiko wrote:

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Great! Just to disambiguate and make sure we are in agreement, my hope
here is that iproute2 can completely delegate to libbpf all the ELF

iproute2 needs to compile and continue working as is when libbpf is not
available. e.g., add check in configure to define HAVE_LIBBPF and move
the existing code and move under else branch.

Wouldn't it be better to statically compile against libbpf in this
case and get rid a lot of BPF-related code and simplify the rest of
it? This can be easily done by using libbpf through submodule, the
same way as BCC and pahole do it.

iproute2 compiles today and runs on older distributions and older
distributions with newer kernels. That needs to hold true after the move
to libbpf.

And by statically compiling against libbpf, checked out as a
submodule, that will still hold true, wouldn't it? Or there is some
complications I'm missing? Libbpf is designed to handle old kernels
with no problems.

My plan was to use the same configure test I'm using for xdp-tools
(where I in turn copied the structure of the configure script from
iproute2):

https://github.com/xdp-project/xdp-tools/blob/master/configure#L59

This will look for a system libbpf install and compile against it if it
is compatible, and otherwise fall back to a statically linking against a
git submodule.

How will this work when build host has libbpf installed, but target
host doesn't? You'll get dynamic linker error when trying to run that
tool.

That's called dependency tracking; distros have various ways of going
about that :)

I'm confused, honestly. libbpf is either a dependency and thus can be
relied upon to be present in the target system, or it's not and this
whole dance with detecting libbpf presence needs to be performed.

Yes, and iproute2 is likely to be built in both sorts of environments,
so we will have to support both :)

If libbpf is optional, then I don't see how iproute2 BPF-related code
and complexity can be reduced at all, given it should still support
loading BPF programs even without libbpf. Furthermore, given libbpf
supports more features already and will probably be outpacing
iproute2's own BPF support in the future, some users will start
relying on BPF features supported only by libbpf "backend", so
iproute2's own BPF backend will just fail to load such programs,
bringing unpleasant surprises, potentially. So I still fail to see how
libbpf can be optional and what benefit does that bring.

I wasn't saying that libbpf itself should be optional; if we're porting
things, we should rip out as much of the old code as we can. I just
meant that we should support both modes of building, so distros that
*do* build libbpf as a library can link iproute2 against that with as
little friction as possible.

I'm dead set on a specific auto-detection semantic either; I guess it'll
be up to the iproute2 maintainers whether they prefer defaulting to one
or the other.

But shared or static - whatever fits iproute2 best, no preferences.

Right, cool, I think we are basically agreed, given the above :)

-Toke