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 17:03:43 +0000
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andriin@...com>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.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 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)?
This way, in bpf_link__destroy_perf_event(), we do not need to check
l->fd < 0 since it will be always nonnegative.

> +
> +	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;
>