netdev - Re: [PATCH bpf-next v8 04/11] bpf: move prog->aux->linked_prog and trampoline into bpf

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Date:   Fri, 25 Sep 2020 08:45:04 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        KP Singh <kpsingh@...omium.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v8 04/11] bpf: move prog->aux->linked_prog and
 trampoline into bpf_link on attach

On Thu, Sep 24, 2020 at 3:00 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
>
> >> +    struct mutex tgt_mutex; /* protects tgt_* pointers below, *after* prog becomes visible */
> >> +    struct bpf_prog *tgt_prog;
> >> +    struct bpf_trampoline *tgt_trampoline;
> >>      bool verifier_zext; /* Zero extensions has been inserted by verifier. */
> >>      bool offload_requested;
> >>      bool attach_btf_trace; /* true if attaching to BTF-enabled raw tp */
> > ...
> >>  struct bpf_tracing_link {
> >>      struct bpf_link link;
> >>      enum bpf_attach_type attach_type;
> >> +    struct bpf_trampoline *trampoline;
> >> +    struct bpf_prog *tgt_prog;
> >
> > imo it's confusing to have 'tgt_prog' to mean two different things.
> > In prog->aux->tgt_prog it means target prog to attach to in the future.
> > Whereas here it means the existing prog that was used to attached to.
> > They kinda both 'target progs' but would be good to disambiguate.
> > May be keep it as 'tgt_prog' here and
> > rename to 'dest_prog' and 'dest_trampoline' in prog->aux ?
>
> I started changing this as you suggested, but I think it actually makes
> the code weirder. We'll end up with a lot of 'tgt_prog =
> prog->aux->dest_prog' assignments in the verifier, unless we also rename
> all of the local variables, which I think is just code churn for very
> little gain (the existing 'target' meaning is quite clear, I think).

you mean "churn" just for this patch. that's fine.
But it will make names more accurate for everyone reading it afterwards.
Hence I prefer distinct and specific names where possible.

> I also think it's quite natural that the target moves; I mean, it's
> literally the same pointer being re-assigned from prog->aux to the link.
> We could rename the link member to 'attached_tgt_prog' or something like
> that, but I'm not sure it helps (and I don't see much of a problem in
> the first place).

'attached_tgt_prog' will not be the correct name.
There is 'prog' inside the link already. That's 'attached' prog.
Not this one. This one is the 'attached_to' prog.
But such name would be too long.
imo calling it 'dest_prog' in aux is shorter and more obvious.