Subject: Re: [PATCH v3 bpf-next 3/9] libbpf: add ability to attach/detach BPF program to perf event

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 28 Jun 2019 11:14:14 -0700

On 06/28, Andrii Nakryiko wrote:
> On Fri, Jun 28, 2019 at 11:00 AM Stanislav Fomichev <[email-masked]> wrote:
> >
> > On 06/27, 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 <[email-masked]>
> > > ---
> > >  tools/lib/bpf/libbpf.c   | 58 ++++++++++++++++++++++++++++++++++++++++
> > >  tools/lib/bpf/libbpf.h   |  3 +++
> > >  tools/lib/bpf/libbpf.map |  1 +
> > >  3 files changed, 62 insertions(+)
> > >
> > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > > index 455795e6f8af..606705f878ba 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 +3959,63 @@ 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);
> > > +     close(l->fd);
> > > +     return err;
> > Why not return -errno from ioctl here (as you do elsewhere)?
> 
> Good catch, will fix, thanks!
> 
> As an aside, this whole returning error on close/destroy is a bit
> moot, as there is little one can do if any of teardown steps fail
> (except crash, which is not great response :) ). So the strategy would
> be to still free all memory and try to close all FDs, before returning
> error (again, which one, first, last? eh..).
Agreed, it's like nobody cares about close() return code. So don't
bother with this fix unless you'd do another respin for a different
reason.

> > > +}
> > > +
> > > +struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog,
> > > +                                             int pfd)
> > > +{
> > > +     char errmsg[STRERR_BUFSIZE];
> > > +     struct bpf_link_fd *link;
> > > +     int bpf_fd, err;
> > > +
> > > +     bpf_fd = bpf_program__fd(prog);
> > > +     if (bpf_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));
> > > +     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, bpf_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;
> > > --
> > > 2.17.1
> > >