netdev - Re: [PATCH bpf 2/2] bpf: Update bpf_design_QA.rst to clarify that attaching to functions is not ABI

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Date:   Mon, 1 Aug 2022 22:34:16 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf 2/2] bpf: Update bpf_design_QA.rst to clarify that
 attaching to functions is not ABI

On Mon, Jul 25, 2022 at 9:40 AM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Mon, Jul 25, 2022 at 01:15:49PM +0100, Matthew Wilcox wrote:
> > On Fri, Jul 22, 2022 at 02:23:46PM -0700, Paul E. McKenney wrote:
> > > On Fri, Jul 22, 2022 at 10:17:57PM +0200, Daniel Borkmann wrote:
> > > > Otherwise I think this could be a bit misunderstood, e.g. most of the networking
> > > > programs (e.g. XDP, tc, sock_addr) have a fixed framework around them where
> > > > attaching programs is part of ABI.
> > >
> > > Excellent point, thank you!
> > >
> > > Apologies for the newbie question, but does BTF_ID() mark a function as
> > > ABI from the viewpoing of a BPF program calling that function, attaching
> > > to that function, or both?  Either way, is it worth mentioning this
> > > in this QA entry?
> >
> > Not necessarily.  For example, __filemap_add_folio has a BTF_ID(), but
> > it is not ABI (it's error injection).
>
> OK, sounds like something to leave out of the QA, then.

Obviously, BTF_ID marking doesn't make the kernel function an abi
in any way. Just like EXPORT_SYMBOL_GPL doesn't do it.
Documentation/bpf/kfuncs.rst already explains it.
Probably worth repeating in the QA part of the doc.