netdev - Re: [PATCH v2 bpf-next 10/17] libbpf: tighten BTF type ID rewriting with error checking

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Message-ID: <CAADnVQLkXhui3K2O4v4u1gfMVXzBdEtfuUixPhnb=n-BdUbH9Q@mail.gmail.com>
Date:   Fri, 23 Apr 2021 09:34:48 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Yonghong Song <yhs@...com>, Andrii Nakryiko <andrii@...nel.org>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 10/17] libbpf: tighten BTF type ID rewriting
 with error checking

On Fri, Apr 23, 2021 at 9:31 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
> > >
> > > static int remap_type_id(__u32 *type_id, void *ctx)
> > > {
> > >         int *id_map = ctx;
> > >         int new_id = id_map[*type_id];
> > >
> > >
> > > /* Here VOID stays VOID, that's all */
> > >
> > >         if (*type_id == 0)
> > >                 return 0;
> >
> > Does it mean that id_map[0] is a garbage value?
> > and all other code that might be doing id_map[idx] might be reading
> > garbage if it doesn't have a check for idx == 0 ?
>
> No, id_map[0] == 0 by construction (id_map is obj->btf_type_map and is
> calloc()'ed) and can be used as id_map[idx].

Ok. Then why are you insisting on this micro optimization to return 0
directly?
That's the confusing part for me.

If it was:
"if (new_id == 0 && *type_id != 0) { pr_warn"
Then it would be clear what error condition is about.
But 'return 0' messing things up in my mind,
because it's far from obvious that first check is really a combination
with the 2nd check and by itself it's a micro optimization to avoid
reading id_map[0].