Commit 668ace0e authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Daniel Borkmann
Browse files

libbpf: Use BPF perf link when supported by kernel 



Detect kernel support for BPF perf link and prefer it when attaching to
perf_event, tracepoint, kprobe/uprobe. Underlying perf_event FD will be kept
open until BPF link is destroyed, at which point both perf_event FD and BPF
link FD will be closed.

This preserves current behavior in which perf_event FD is open for the
duration of bpf_link's lifetime and user is able to "disconnect" bpf_link from
underlying FD (with bpf_link__disconnect()), so that bpf_link__destroy()
doesn't close underlying perf_event FD.When BPF perf link is used, disconnect
will keep both perf_event and bpf_link FDs open, so it will be up to
(advanced) user to close them. This approach is demonstrated in bpf_cookie.c
selftests, added in this patch set.

Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210815070609.987780-10-andrii@kernel.org
parent d88b71d4

tools/lib/bpf/libbpf.c

+90 −21
Original line number Diff line number Diff line
@@ -193,6 +193,8 @@ enum kern_feature_id { 
	FEAT_MODULE_BTF,

	/* BTF_KIND_FLOAT support */

	FEAT_BTF_FLOAT,

	/* BPF perf link support */

	FEAT_PERF_LINK,

	__FEAT_CNT,

};
@@ -4337,6 +4339,37 @@ static int probe_module_btf(void) 
	return !err;

}



static int probe_perf_link(void)

{

	struct bpf_load_program_attr attr;

	struct bpf_insn insns[] = {

		BPF_MOV64_IMM(BPF_REG_0, 0),

		BPF_EXIT_INSN(),

	};

	int prog_fd, link_fd, err;



	memset(&attr, 0, sizeof(attr));

	attr.prog_type = BPF_PROG_TYPE_TRACEPOINT;

	attr.insns = insns;

	attr.insns_cnt = ARRAY_SIZE(insns);

	attr.license = "GPL";

	prog_fd = bpf_load_program_xattr(&attr, NULL, 0);

	if (prog_fd < 0)

		return -errno;



	/* use invalid perf_event FD to get EBADF, if link is supported;

	 * otherwise EINVAL should be returned

	 */

	link_fd = bpf_link_create(prog_fd, -1, BPF_PERF_EVENT, NULL);

	err = -errno; /* close() can clobber errno */



	if (link_fd >= 0)

		close(link_fd);

	close(prog_fd);



	return link_fd < 0 && err == -EBADF;

}



enum kern_feature_result {

	FEAT_UNKNOWN = 0,

	FEAT_SUPPORTED = 1,
@@ -4387,6 +4420,9 @@ static struct kern_feature_desc { 
	[FEAT_BTF_FLOAT] = {

		"BTF_KIND_FLOAT support", probe_kern_btf_float,

	},

	[FEAT_PERF_LINK] = {

		"BPF perf link support", probe_perf_link,

	},

};



static bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
@@ -8951,23 +8987,38 @@ int bpf_link__unpin(struct bpf_link *link) 
	return 0;

}



static int bpf_link__detach_perf_event(struct bpf_link *link)

struct bpf_link_perf {

	struct bpf_link link;

	int perf_event_fd;

};



static int bpf_link_perf_detach(struct bpf_link *link)

{

	int err;

	struct bpf_link_perf *perf_link = container_of(link, struct bpf_link_perf, link);

	int err = 0;



	err = ioctl(link->fd, PERF_EVENT_IOC_DISABLE, 0);

	if (err)

	if (ioctl(perf_link->perf_event_fd, PERF_EVENT_IOC_DISABLE, 0) < 0)

		err = -errno;



	if (perf_link->perf_event_fd != link->fd)

		close(perf_link->perf_event_fd);

	close(link->fd);



	return libbpf_err(err);

}



static void bpf_link_perf_dealloc(struct bpf_link *link)

{

	struct bpf_link_perf *perf_link = container_of(link, struct bpf_link_perf, link);



	free(perf_link);

}



struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog, int pfd)

{

	char errmsg[STRERR_BUFSIZE];

	struct bpf_link *link;

	int prog_fd, err;

	struct bpf_link_perf *link;

	int prog_fd, link_fd = -1, err;



	if (pfd < 0) {

		pr_warn("prog '%s': invalid perf event FD %d\n",
@@ -8984,27 +9035,45 @@ struct bpf_link *bpf_program__attach_perf_event(struct bpf_program *prog, int pf 
	link = calloc(1, sizeof(*link));

	if (!link)

		return libbpf_err_ptr(-ENOMEM);

	link->detach = &bpf_link__detach_perf_event;

	link->fd = pfd;

	link->link.detach = &bpf_link_perf_detach;

	link->link.dealloc = &bpf_link_perf_dealloc;

	link->perf_event_fd = pfd;



	if (kernel_supports(prog->obj, FEAT_PERF_LINK)) {

		link_fd = bpf_link_create(prog_fd, pfd, BPF_PERF_EVENT, NULL);

		if (link_fd < 0) {

			err = -errno;

			pr_warn("prog '%s': failed to create BPF link for perf_event FD %d: %d (%s)\n",

				prog->name, pfd,

				err, libbpf_strerror_r(err, errmsg, sizeof(errmsg)));

			goto err_out;

		}

		link->link.fd = link_fd;

	} else {

		if (ioctl(pfd, PERF_EVENT_IOC_SET_BPF, prog_fd) < 0) {

			err = -errno;

		free(link);

		pr_warn("prog '%s': failed to attach to pfd %d: %s\n",

			pr_warn("prog '%s': failed to attach to perf_event FD %d: %s\n",

				prog->name, pfd, libbpf_strerror_r(err, errmsg, sizeof(errmsg)));

			if (err == -EPROTO)

				pr_warn("prog '%s': try add PERF_SAMPLE_CALLCHAIN to or remove exclude_callchain_[kernel|user] from pfd %d\n",

					prog->name, pfd);

		return libbpf_err_ptr(err);

			goto err_out;

		}

		link->link.fd = pfd;

	}

	if (ioctl(pfd, PERF_EVENT_IOC_ENABLE, 0) < 0) {

		err = -errno;

		free(link);

		pr_warn("prog '%s': failed to enable pfd %d: %s\n",

		pr_warn("prog '%s': failed to enable perf_event FD %d: %s\n",

			prog->name, pfd, libbpf_strerror_r(err, errmsg, sizeof(errmsg)));

		return libbpf_err_ptr(err);

		goto err_out;

	}

	return link;



	return &link->link;

err_out:

	if (link_fd >= 0)

		close(link_fd);

	free(link);

	return libbpf_err_ptr(err);

}



/*