netdev - Re: [PATCH v4 bpf-next 6/9] libbpf: add raw tracepoint attach API

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Date:   Mon, 1 Jul 2019 15:26:30 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "sdf@...ichev.me" <sdf@...ichev.me>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH v4 bpf-next 6/9] libbpf: add raw tracepoint attach API

On Mon, Jul 1, 2019 at 10:13 AM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 6/28/19 8:49 PM, Andrii Nakryiko wrote:
> > Add a wrapper utilizing bpf_link "infrastructure" to allow attaching BPF
> > programs to raw tracepoints.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@...com>
> > Acked-by: Song Liu <songliubraving@...com>
> > ---
> >   tools/lib/bpf/libbpf.c   | 37 +++++++++++++++++++++++++++++++++++++
> >   tools/lib/bpf/libbpf.h   |  3 +++
> >   tools/lib/bpf/libbpf.map |  1 +
> >   3 files changed, 41 insertions(+)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 8ad4f915df38..f8c7a7ecb35e 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -4263,6 +4263,43 @@ struct bpf_link *bpf_program__attach_tracepoint(struct bpf_program *prog,
> >       return link;
> >   }
> >
> > +static int bpf_link__destroy_fd(struct bpf_link *link)
> > +{
> > +     struct bpf_link_fd *l = (void *)link;
> > +
> > +     return close(l->fd);
> > +}
> > +
> > +struct bpf_link *bpf_program__attach_raw_tracepoint(struct bpf_program *prog,
> > +                                                 const char *tp_name)
> > +{
> > +     char errmsg[STRERR_BUFSIZE];
> > +     struct bpf_link_fd *link;
> > +     int prog_fd, pfd;
> > +
> > +     prog_fd = bpf_program__fd(prog);
> > +     if (prog_fd < 0) {
> > +             pr_warning("program '%s': can't attach before loaded\n",
> > +                        bpf_program__title(prog, false));
> > +             return ERR_PTR(-EINVAL);
> > +     }
> > +
> > +     link = malloc(sizeof(*link));
> > +     link->link.destroy = &bpf_link__destroy_fd;
>
> You can move the "link = malloc(...)" etc. after
> bpf_raw_tracepoint_open(). That way, you do not need to free(link)
> in the error case.

It's either `free(link)` here, or `close(pfd)` if malloc fails after
we attached program. Either way extra clean up is needed. I went with
the first one.

>
> > +
> > +     pfd = bpf_raw_tracepoint_open(tp_name, prog_fd);
> > +     if (pfd < 0) {
> > +             pfd = -errno;
> > +             free(link);
> > +             pr_warning("program '%s': failed to attach to raw tracepoint '%s': %s\n",
> > +                        bpf_program__title(prog, false), tp_name,
> > +                        libbpf_strerror_r(pfd, errmsg, sizeof(errmsg)));
> > +             return ERR_PTR(pfd);
> > +     }
> > +     link->fd = pfd;
> > +     return (struct bpf_link *)link;
> > +}
> > +
> >   enum bpf_perf_event_ret
> >   bpf_perf_event_read_simple(void *mmap_mem, size_t mmap_size, size_t page_size,
> >                          void **copy_mem, size_t *copy_size,
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index 60611f4b4e1d..f55933784f95 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -182,6 +182,9 @@ LIBBPF_API struct bpf_link *
> >   bpf_program__attach_tracepoint(struct bpf_program *prog,
> >                              const char *tp_category,
> >                              const char *tp_name);
> > +LIBBPF_API struct bpf_link *
> > +bpf_program__attach_raw_tracepoint(struct bpf_program *prog,
> > +                                const char *tp_name);
> >
> >   struct bpf_insn;
> >
> > diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> > index 3c618b75ef65..e6b7d4edbc93 100644
> > --- a/tools/lib/bpf/libbpf.map
> > +++ b/tools/lib/bpf/libbpf.map
> > @@ -171,6 +171,7 @@ LIBBPF_0.0.4 {
> >               bpf_object__load_xattr;
> >               bpf_program__attach_kprobe;
> >               bpf_program__attach_perf_event;
> > +             bpf_program__attach_raw_tracepoint;
> >               bpf_program__attach_tracepoint;
> >               bpf_program__attach_uprobe;
> >               btf_dump__dump_type;
> >