linux-kernel - Re: [PATCH v2 bpf-next] libbpf: Add bpf_obj_get

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Date:   Fri, 29 Jul 2022 11:55:29 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>
Cc:     Joe Burton <jevburton.kernel@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Joe Burton <jevburton@...gle.com>
Subject: Re: [PATCH v2 bpf-next] libbpf: Add bpf_obj_get_opts()

On Thu, Jul 28, 2022 at 12:58 AM Roberto Sassu <roberto.sassu@...wei.com> wrote:
>
> > From: Andrii Nakryiko [mailto:andrii.nakryiko@...il.com]
> > Sent: Thursday, July 28, 2022 1:03 AM
> > On Tue, Jul 19, 2022 at 12:40 PM Joe Burton <jevburton.kernel@...il.com>
> > wrote:
> > >
> > > From: Joe Burton <jevburton@...gle.com>
> > >
> > > Add an extensible variant of bpf_obj_get() capable of setting the
> > > `file_flags` parameter.
> > >
> > > This parameter is needed to enable unprivileged access to BPF maps.
> > > Without a method like this, users must manually make the syscall.
> > >
> > > Signed-off-by: Joe Burton <jevburton@...gle.com>
> > > ---
> > >  tools/lib/bpf/bpf.c      | 10 ++++++++++
> > >  tools/lib/bpf/bpf.h      |  9 +++++++++
> > >  tools/lib/bpf/libbpf.map |  1 +
> > >  3 files changed, 20 insertions(+)
> > >
> >
> > I agree that bpf_obj_get_opts should be separate from bpf_get_fd_opts.
> > Just because both currently have file_flags in them doesn't mean that
> > they should/will always stay in sync. So two separate opts for two
> > separate APIs makes sense to me.
> >
> > So I'd accept this patch, but please see a few small things below and
> > send v3. Thanks!
>
> Should map_parse_fds() accept two opts, or just the flags
> to be set on locally-defined variables?

it's because map_parse_fds() is used with both get_fd_by_id() and
bpf_obj_get(), right? I'd pass flags and construct correct set of
options internally, based on which BPF command you need to use to get
FD

>
> Thanks
>
> Roberto
>
> > > diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> > > index 5eb0df90eb2b..5acb0e8bd13c 100644
> > > --- a/tools/lib/bpf/bpf.c
> > > +++ b/tools/lib/bpf/bpf.c
> > > @@ -578,12 +578,22 @@ int bpf_obj_pin(int fd, const char *pathname)
> > >  }
> > >
> > >  int bpf_obj_get(const char *pathname)
> > > +{
> > > +       LIBBPF_OPTS(bpf_obj_get_opts, opts);
> >
> > if you were doing it this way, here should be an empty line. But
> > really you can/should just pass NULL instead of opts in this case.
> >
> > > +       return bpf_obj_get_opts(pathname, &opts);
> > > +}
> > > +
> > > +int bpf_obj_get_opts(const char *pathname, const struct bpf_obj_get_opts
> > *opts)
> > >  {
> > >         union bpf_attr attr;
> > >         int fd;
> > >
> > > +       if (!OPTS_VALID(opts, bpf_obj_get_opts))
> > > +               return libbpf_err(-EINVAL);
> > > +
> > >         memset(&attr, 0, sizeof(attr));
> > >         attr.pathname = ptr_to_u64((void *)pathname);
> > > +       attr.file_flags = OPTS_GET(opts, file_flags, 0);
> > >
> > >         fd = sys_bpf_fd(BPF_OBJ_GET, &attr, sizeof(attr));
> > >         return libbpf_err_errno(fd);
> > > diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> > > index 88a7cc4bd76f..f31b493b5f9a 100644
> > > --- a/tools/lib/bpf/bpf.h
> > > +++ b/tools/lib/bpf/bpf.h
> > > @@ -270,8 +270,17 @@ LIBBPF_API int bpf_map_update_batch(int fd, const
> > void *keys, const void *values
> > >                                     __u32 *count,
> > >                                     const struct bpf_map_batch_opts *opts);
> > >
> > > +struct bpf_obj_get_opts {
> > > +       size_t sz; /* size of this struct for forward/backward compatibility */
> > > +
> > > +       __u32 file_flags;
> >
> > please add size_t :0; to avoid non-zero-initialized padding  (we do it
> > in a lot of other opts structs)
> >
> >
> > > +};
> > > +#define bpf_obj_get_opts__last_field file_flags
> > > +
> > >  LIBBPF_API int bpf_obj_pin(int fd, const char *pathname);
> > >  LIBBPF_API int bpf_obj_get(const char *pathname);
> > > +LIBBPF_API int bpf_obj_get_opts(const char *pathname,
> > > +                               const struct bpf_obj_get_opts *opts);
> > >
> > >  struct bpf_prog_attach_opts {
> > >         size_t sz; /* size of this struct for forward/backward compatibility */
> > > diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> > > index 0625adb9e888..119e6e1ea7f1 100644
> > > --- a/tools/lib/bpf/libbpf.map
> > > +++ b/tools/lib/bpf/libbpf.map
> > > @@ -355,6 +355,7 @@ LIBBPF_0.8.0 {
> > >
> > >  LIBBPF_1.0.0 {
> > >         global:
> > > +               bpf_obj_get_opts;
> > >                 bpf_prog_query_opts;
> > >                 bpf_program__attach_ksyscall;
> > >                 btf__add_enum64;
> > > --
> > > 2.37.0.170.g444d1eabd0-goog
> > >