netdev - Re: [PATCH bpf-next 1/3] bpftool: add extra CO-RE mode to btf dump command

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Date:   Thu, 19 Dec 2019 14:04:03 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, 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 bpf-next 1/3] bpftool: add extra CO-RE mode to btf dump
 command

On Thu, Dec 19, 2019 at 01:07:38PM -0800, Andrii Nakryiko wrote:
> On Thu, Dec 19, 2019 at 9:06 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Wed, Dec 18, 2019 at 11:06:56PM -0800, Andrii Nakryiko wrote:
> > > +     if (core_mode) {
> > > +             printf("#if defined(__has_attribute) && __has_attribute(preserve_access_index)\n");
> > > +             printf("#define __CLANG_BPF_CORE_SUPPORTED\n");
> > > +             printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
> > > +             printf("#endif\n\n");
> >
> > I think it's dangerous to automatically opt-out when clang is not new enough.
> > bpf prog will compile fine, but it will be missing co-re relocations.
> > How about doing something like:
> >   printf("#ifdef NEEDS_CO_RE\n");
> >   printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
> >   printf("#endif\n\n");
> > and emit it always when 'format c'.
> > Then on the program side it will look:
> > #define NEEDS_CO_RE
> > #include "vmlinux.h"
> > If clang is too old there will be a compile time error which is a good thing.
> > Future features will have different NEEDS_ macros.
> 
> Wouldn't it be cleaner to separate vanilla C types dump vs
> CO-RE-specific one? I'd prefer to have them separate and not require
> every application to specify this #define NEEDS_CO_RE macro.
> Furthermore, later we probably are going to add some additional
> auto-generated types, definitions, etc, so plain C types dump and
> CO-RE-specific one will deviate quite a bit. So it feels cleaner to
> separate them now instead of polluting `format c` with irrelevant
> noise.

Say we do this 'format core' today then tomorrow another tweak to vmlinux.h
would need 'format core2' ? I think adding new format to bpftool for every
little feature will be annoying to users. I think the output should stay as
'format c' and that format should be extensible/customizable by bpf progs via
#define NEEDS_FEATURE_X. Then these features can grow without a need to keep
adding new cmd line args. This preserve_access_index feature makes up for less
than 1% difference in generated vmlinux.h. If some feature extension would
drastically change generated .h then it would justify new 'format'. This one is
just a small tweak. Also #define NEEDS_CO_RE is probably too broad. I think
#define CLANG_NEEDS_TO_EMIT_RELO would be more precise and less ambiguous.