netdev - Re: [PATCH bpf-next] libbpf: Add libbpf_version() function to get library version at runtime

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Date:   Wed, 18 Nov 2020 09:43:25 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     daniel@...earbox.net, ast@...com, andrii@...nel.org,
        bpf@...r.kernel.org, netdev@...r.kernel.org, brouer@...hat.com,
        haliu@...hat.com, dsahern@...il.com, jbenc@...hat.com
Subject: Re: [PATCH bpf-next] libbpf: Add libbpf_version() function to get
 library version at runtime

On Wed, Nov 18, 2020 at 06:07:38PM +0100, Toke Høiland-Jørgensen wrote:
> As a response to patches adding libbpf support to iproute2, an extensive
> discussion ensued about libbpf version visibility and enforcement in tools
> using the library[0]. In particular, two problems came to light:
> 
> 1. If a tool is statically linked against libbpf, there is no way for a user
>    to discover which version of libbpf the tool is using, unless the tool
>    takes particular care to embed the library version at build time and print
>    it.
> 
> 2. If a tool is dynamically linked against libbpf, but doesn't use any
>    symbols from the latest library version, the library version used at
>    runtime can be older than the one used at compile time, and the
>    application has no way to verify the version at runtime.
> 
> To make progress on resolving this, let's add a libbpf_version() function that
> will simply return a version string which is embedded into the library at
> compile time. This makes it possible for applications to unambiguously get the
> library version at runtime, resolving (2.) above, and as an added bonus makes it
> easy for applications to print the library version, which should help with (1.).
> 
> [0] https://lore.kernel.org/bpf/20201109070802.3638167-1-haliu@redhat.com/T/#t
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>

Unless iproute2 adopts scrict libbpf.so.version == iproute2.version policy
and removes legacy bpf loader no iproute2 driven changes to libbpf will be accepted.
Just like the kernel doesn't add features for out-of-tree modules
libbpf doesn't add features for projects where libbpf is optional.