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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Message-ID: <20220802174252.GG2860372@paulmck-ThinkPad-P17-Gen-1>
Date:   Tue, 2 Aug 2022 10:42:52 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
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, Aug 01, 2022 at 10:34:16PM -0700, Alexei Starovoitov wrote:
> 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.

Like this?

							Thanx, Paul

------------------------------------------------------------------------

commit 9346b452b92fc520a59da655b55d6bc40f9d1d14
Author: Paul E. McKenney <paulmck@...nel.org>
Date:   Tue Aug 2 10:31:17 2022 -0700

    bpf: Update bpf_design_QA.rst to clarify that BTF_ID does not ABIify a function
    
    This patch updates bpf_design_QA.rst to clarify that mentioning a function
    to the BTF_ID macro does not make that function become part of the Linux
    kernel's ABI.
    
    Suggested-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst
index a06ae8a828e3d..a210b8a4df005 100644
--- a/Documentation/bpf/bpf_design_QA.rst
+++ b/Documentation/bpf/bpf_design_QA.rst
@@ -291,3 +291,10 @@ The kernel function prototypes will change, and BPF programs attaching to
 them will need to change.  The BPF compile-once-run-everywhere (CO-RE)
 should be used in order to make it easier to adapt your BPF programs to
 different versions of the kernel.
+
+Q: Marking a function with BTF_ID makes that function an ABI?
+-------------------------------------------------------------
+A: NO.
+
+The BTF_ID macro does not cause a function to become part of the ABI
+any more than does the EXPORT_SYMBOL_GPL macro.