netdev - Re: [PATCH v2 bpf-next 2/2] btf: expose API to work with raw btf data

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Date:   Tue, 5 Feb 2019 22:24:58 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>, Alexei Starovoitov <ast@...com>,
        Martin Lau <kafai@...com>, netdev@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v2 bpf-next 2/2] btf: expose API to work with raw btf data

On Tue, Feb 05, 2019 at 09:46:14PM -0800, Andrii Nakryiko wrote:
> On Tue, Feb 5, 2019 at 7:07 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Tue, Feb 05, 2019 at 04:29:49PM -0800, Andrii Nakryiko wrote:
> > > This patch exposes two new APIs btf__get_raw_data_size() and
> > > btf__get_raw_data() that allows to get a copy of raw BTF data out of
> > > struct btf. This is useful for external programs that need to manipulate
> > > raw data, e.g., pahole using btf__dedup() to deduplicate BTF type info
> > > and then writing it back to file.
> > >
> > > Signed-off-by: Andrii Nakryiko <andriin@...com>
> > > Acked-by: Song Liu <songliubraving@...com>
> > > ---
> > >  tools/lib/bpf/btf.c      | 10 ++++++++++
> > >  tools/lib/bpf/btf.h      |  2 ++
> > >  tools/lib/bpf/libbpf.map |  2 ++
> > >  3 files changed, 14 insertions(+)
> > >
> > > diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> > > index 1c2ba7182400..34bfb3641aac 100644
> > > --- a/tools/lib/bpf/btf.c
> > > +++ b/tools/lib/bpf/btf.c
> > > @@ -437,6 +437,16 @@ int btf__fd(const struct btf *btf)
> > >       return btf->fd;
> > >  }
> > >
> > > +__u32 btf__get_raw_data_size(const struct btf *btf)
> > > +{
> > > +     return btf->data_size;
> > > +}
> > > +
> > > +void btf__get_raw_data(const struct btf *btf, char *data)
> > > +{
> > > +     memcpy(data, btf->data, btf->data_size);
> > > +}
> >
> > I cannot think of any other way to use this api,
> > but to call btf__get_raw_data_size() first,
> > then malloc that much memory and then call btf__get_raw_data()
> > to store btf into it.
> >
> > If so, may be api should be single call that allocates, copies,
> > and returns pointer to new mem and its size?
> > Probably less error prone?
> >
> 
> I don't have strong preference, but providing pointer to allocated memory
> seems more flexible and allows more clever/optimal use of memory from caller
> side. E.g., instead of doing two mallocs, you can imagine doing something
> like:
> 
> int max_size = max(btf__get_raw_data_size(btf),
>                    btf_ext__get_raw_data_size(btf_ext));
> char *m = malloc(max_size);
> btf__get_raw_data(btf, m);
> dump_btf_section_to_file(m, some_file);
> btf_ext__get_raw_data(btf_ext, m);
> dump_btf_ext_section_to_file(m, some_file);
> free(m);
> 
> Also, pointer to memory could be mmap()'ed file, for instance. In general,
> for a library it might be a good thing to not be prescriptive as to how one
> gets that piece of memory.

Plausible, but I'd like to see pahole patches to be convinced ;)