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 11:05:14 -0700

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;
> +}
> +
> +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;
nit: maybe prog_fd instead of bpf_fd? Should be more consistent with
other places in libbf:

$ grep -Iriw bpf_fd | wc -l
2
$ grep -Iriw prog_fd | wc -l
39

> +
> +	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
>