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Date: Mon, 1 Jul 2019 23:02:10 +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 3/9] libbpf: add ability to attach/detach BPF
program to perf event
On 7/1/19 2:57 PM, Andrii Nakryiko wrote:
> On Mon, Jul 1, 2019 at 10:03 AM Yonghong Song <yhs@...com> wrote:
>>
>>
>>
>> On 6/28/19 8:49 PM, Andrii Nakryiko wrote:
>>> bpf_program__attach_perf_event allows to attach BPF program to existing
>>> perf event hook, providing most generic and most low-level way to attach BPF
>>> programs. It returns struct bpf_link, which should be passed to
>>> bpf_link__destroy to detach and free resources, associated with a link.
>>>
>>> Signed-off-by: Andrii Nakryiko <andriin@...com>
>>> ---
>>> tools/lib/bpf/libbpf.c | 61 ++++++++++++++++++++++++++++++++++++++++
>>> tools/lib/bpf/libbpf.h | 3 ++
>>> tools/lib/bpf/libbpf.map | 1 +
>>> 3 files changed, 65 insertions(+)
>>>
>>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>>> index 455795e6f8af..98c155ec3bfa 100644
>>> --- a/tools/lib/bpf/libbpf.c
>>> +++ b/tools/lib/bpf/libbpf.c
>>> @@ -32,6 +32,7 @@
>>> #include <linux/limits.h>
>>> #include <linux/perf_event.h>
>>> #include <linux/ring_buffer.h>
>>> +#include <sys/ioctl.h>
>>> #include <sys/stat.h>
>>> #include <sys/types.h>
>>> #include <sys/vfs.h>
>>> @@ -3958,6 +3959,66 @@ int bpf_link__destroy(struct bpf_link *link)
>>> return err;
>>> }
>>>
>>> +struct bpf_link_fd {
>>> + struct bpf_link link; /* has to be at the top of struct */
>>> + int fd; /* hook FD */
>>> +};
>>> +
>>> +static int bpf_link__destroy_perf_event(struct bpf_link *link)
>>> +{
>>> + struct bpf_link_fd *l = (void *)link;
>>> + int err;
>>> +
>>> + if (l->fd < 0)
>>> + return 0;
>>> +
>>> + err = ioctl(l->fd, PERF_EVENT_IOC_DISABLE, 0);
>>> + if (err)
>>> + err = -errno;
>>> +
>>> + close(l->fd);
>>> + return err;
>>> +}
>>> +
>>> +struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
>>> + int pfd)
>>> +{
>>> + char errmsg[STRERR_BUFSIZE];
>>> + struct bpf_link_fd *link;
>>> + int prog_fd, err;
>>> +
>>> + 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);
>>> + }
>>
>> should we check validity of pfd here?
>> If pfd < 0, we just return ERR_PTR(-EINVAL)?
>
> I can add that. I didn't do it, because in general, you can provide fd
>> = 0 which is still not a valid FD for PERF_EVENT_IOC_SET_BPF and
> PERF_EVENT_IOC_ENABLE, so in general we can't detect this reliably.
I just want a check for validity of input parameter which will failure
later with dedicated error message. But the same negative pfd will
be printed in ioctl error message. I am okay with this.
>
>> This way, in bpf_link__destroy_perf_event(), we do not need to check
>> l->fd < 0 since it will be always nonnegative.
>
> That check is not needed anyway, because even if pfd < 0, ioctl should
> fail and return error. I'll remove that check.
>
>>
>>> +
>>> + link = malloc(sizeof(*link));
>>> + if (!link)
>>> + return ERR_PTR(-ENOMEM);
>>> + link->link.destroy = &bpf_link__destroy_perf_event;
>>> + link->fd = pfd;
>>> +
>>> + if (ioctl(pfd, PERF_EVENT_IOC_SET_BPF, prog_fd) < 0) {
>>> + err = -errno;
>>> + free(link);
>>> + pr_warning("program '%s': failed to attach to pfd %d: %s\n",
>>> + bpf_program__title(prog, false), pfd,
>>> + libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
>>> + return ERR_PTR(err);
>>> + }
>>> + if (ioctl(pfd, PERF_EVENT_IOC_ENABLE, 0) < 0) {
>>> + err = -errno;
>>> + free(link);
>>> + pr_warning("program '%s': failed to enable pfd %d: %s\n",
>>> + bpf_program__title(prog, false), pfd,
>>> + libbpf_strerror_r(err, errmsg, sizeof(errmsg)));
>>> + return ERR_PTR(err);
>>> + }
>>> + 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 5082a5ebb0c2..1bf66c4a9330 100644
>>> --- a/tools/lib/bpf/libbpf.h
>>> +++ b/tools/lib/bpf/libbpf.h
>>> @@ -169,6 +169,9 @@ struct bpf_link;
>>>
>>> LIBBPF_API int bpf_link__destroy(struct bpf_link *link);
>>>
>>> +LIBBPF_API struct bpf_link *
>>> +bpf_program__attach_perf_event(struct bpf_program *prog, int pfd);
>>> +
>>> struct bpf_insn;
>>>
>>> /*
>>> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
>>> index 3cde850fc8da..756f5aa802e9 100644
>>> --- a/tools/lib/bpf/libbpf.map
>>> +++ b/tools/lib/bpf/libbpf.map
>>> @@ -169,6 +169,7 @@ LIBBPF_0.0.4 {
>>> global:
>>> bpf_link__destroy;
>>> bpf_object__load_xattr;
>>> + bpf_program__attach_perf_event;
>>> btf_dump__dump_type;
>>> btf_dump__free;
>>> btf_dump__new;
>>>
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