netdev - Re: [PATCH 03/11] bpf: Add btf

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Date:   Fri, 19 Jun 2020 11:15:27 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Martin KaFai Lau <kafai@...com>,
        David Miller <davem@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Wenbo Zhang <ethercflow@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Andrii Nakryiko <andriin@...com>,
        Brendan Gregg <bgregg@...flix.com>,
        Florent Revest <revest@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 03/11] bpf: Add btf_ids object

On Fri, Jun 19, 2020 at 6:13 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Thu, Jun 18, 2020 at 05:56:38PM -0700, Andrii Nakryiko wrote:
> > On Tue, Jun 16, 2020 at 3:05 AM Jiri Olsa <jolsa@...nel.org> wrote:
> > >
> > > Adding support to generate .BTF_ids section that would
> > > hold various BTF IDs list for verifier.
> > >
> > > Adding macros help to define lists of BTF IDs placed in
> > > .BTF_ids section. They are initially filled with zeros
> > > (during compilation) and resolved later during the
> > > linking phase by btfid tool.
> > >
> > > Following defines list of one BTF ID that is accessible
> > > within kernel code as bpf_skb_output_btf_ids array.
> > >
> > >   extern int bpf_skb_output_btf_ids[];
> > >
> > >   BTF_ID_LIST(bpf_skb_output_btf_ids)
> > >   BTF_ID(struct, sk_buff)
> > >
> > > Suggested-by: Andrii Nakryiko <andrii.nakryiko@...il.com>
> > > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > > ---
> > >  include/asm-generic/vmlinux.lds.h |  4 ++
> > >  kernel/bpf/Makefile               |  2 +-
> > >  kernel/bpf/btf_ids.c              |  3 ++
> > >  kernel/bpf/btf_ids.h              | 70 +++++++++++++++++++++++++++++++
> > >  4 files changed, 78 insertions(+), 1 deletion(-)
> > >  create mode 100644 kernel/bpf/btf_ids.c
> > >  create mode 100644 kernel/bpf/btf_ids.h
> > >
> >
> > [...]
> >
> > > +/*
> > > + * Following macros help to define lists of BTF IDs placed
> > > + * in .BTF_ids section. They are initially filled with zeros
> > > + * (during compilation) and resolved later during the
> > > + * linking phase by btfid tool.
> > > + *
> > > + * Any change in list layout must be reflected in btfid
> > > + * tool logic.
> > > + */
> > > +
> > > +#define SECTION ".BTF_ids"
> >
> > nit: SECTION is super generic and non-greppable. BTF_IDS_SECTION?
>
> ok
>
> >
> > > +
> > > +#define ____BTF_ID(symbol)                             \
> > > +asm(                                                   \
> > > +".pushsection " SECTION ",\"a\";               \n"     \
> >
> > section should be also read-only? Either immediately here, of btfid
> > tool should mark it? Unless I missed that it's already doing it :)
>
> hm, it's there next to the .BTF section within RO_DATA macro,
> so I thought that was enough.. I'll double check

ah, linker script magic, got it

>
> >
> > > +".local " #symbol " ;                          \n"     \
> > > +".type  " #symbol ", @object;                  \n"     \
> > > +".size  " #symbol ", 4;                        \n"     \
> > > +#symbol ":                                     \n"     \
> > > +".zero 4                                       \n"     \
> > > +".popsection;                                  \n");
> > > +
> > > +#define __BTF_ID(...) \
> > > +       ____BTF_ID(__VA_ARGS__)
> >
> > why varargs, if it's always a single argument? Or it's one of those
> > macro black magic things were it works only in this particular case,
> > but not others?
>
> yea, I kind of struggled in here, because any other would not
> expand the name concat together with the unique ID bit,
> __VA_ARGS__ did it nicely ;-) I'll revisit this

it's probably not varargs, but rather nested macro call. Macros are
weird and tricky...

>
> thanks,
> jirka
>