netdev - Re: [PATCH v4 bpf-next 3/9] libbpf: add ability to attach/detach BPF program to perf event

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Date:   Mon, 1 Jul 2019 14:57:57 -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 3/9] libbpf: add ability to attach/detach BPF
 program to perf event

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.

> 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;
> >