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

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Message-ID: <cf314380-a96e-eae6-6fb5-b136c50cec71@fb.com>
Date:   Mon, 1 Jul 2019 23:04:43 +0000
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.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 7/1/19 3:26 PM, Andrii Nakryiko wrote:
> 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.

Okay with me.
BTW, do you want to check whether malloc() may failure and link may be NULL?

>>
>>> +
>>> +     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;
>>>