libbpf: Add support to set kprobe/uprobe attach mode

By default, libbpf will attach the kprobe/uprobe BPF program in the
latest mode that supported by kernel. In this patch, we add the support
to let users manually attach kprobe/uprobe in legacy or perf mode.

There are 3 mode that supported by the kernel to attach kprobe/uprobe:

LEGACY: create perf event in legacy way and don't use bpf_link
PERF: create perf event with perf_event_open() and don't use bpf_link

Signed-off-by: Menglong Dong <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Reviewed-by: Biao Jiang <[e-mail]>
Link: create perf event with perf_event_open() and use bpf_link
Link: https://lore.kernel.org/bpf/[e-mail]/
Link: https://lore.kernel.org/bpf/[e-mail]

Users now can manually choose the mode with
bpf_program__attach_uprobe_opts()/bpf_program__attach_kprobe_opts().

tools/lib/bpf/libbpf.c

···
 	char errmsg[STRERR_BUFSIZE];
 	struct bpf_link_perf *link;
 	int prog_fd, link_fd = -1, err;
+	bool force_ioctl_attach;
 
 	if (!OPTS_VALID(opts, bpf_perf_event_opts))
 		return libbpf_err_ptr(-EINVAL);
···
 	link->link.dealloc = &bpf_link_perf_dealloc;
 	link->perf_event_fd = pfd;
 
-	if (kernel_supports(prog->obj, FEAT_PERF_LINK)) {
+	force_ioctl_attach = OPTS_GET(opts, force_ioctl_attach, false);
+	if (kernel_supports(prog->obj, FEAT_PERF_LINK) && !force_ioctl_attach) {
 		DECLARE_LIBBPF_OPTS(bpf_link_create_opts, link_opts,
 			.perf_event.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0));
 
···
 				const struct bpf_kprobe_opts *opts)
 {
 	DECLARE_LIBBPF_OPTS(bpf_perf_event_opts, pe_opts);
+	enum probe_attach_mode attach_mode;
 	char errmsg[STRERR_BUFSIZE];
 	char *legacy_probe = NULL;
 	struct bpf_link *link;
···
 	if (!OPTS_VALID(opts, bpf_kprobe_opts))
 		return libbpf_err_ptr(-EINVAL);
 
+	attach_mode = OPTS_GET(opts, attach_mode, PROBE_ATTACH_MODE_DEFAULT);
 	retprobe = OPTS_GET(opts, retprobe, false);
 	offset = OPTS_GET(opts, offset, 0);
 	pe_opts.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0);
 
 	legacy = determine_kprobe_perf_type() < 0;
+	switch (attach_mode) {
+	case PROBE_ATTACH_MODE_LEGACY:
+		legacy = true;
+		pe_opts.force_ioctl_attach = true;
+		break;
+	case PROBE_ATTACH_MODE_PERF:
+		if (legacy)
+			return libbpf_err_ptr(-ENOTSUP);
+		pe_opts.force_ioctl_attach = true;
+		break;
+	case PROBE_ATTACH_MODE_LINK:
+		if (legacy || !kernel_supports(prog->obj, FEAT_PERF_LINK))
+			return libbpf_err_ptr(-ENOTSUP);
+		break;
+	case PROBE_ATTACH_MODE_DEFAULT:
+		break;
+	default:
+		return libbpf_err_ptr(-EINVAL);
+	}
+
 	if (!legacy) {
 		pfd = perf_event_open_probe(false /* uprobe */, retprobe,
 					    func_name, offset,
···
 	const char *archive_path = NULL, *archive_sep = NULL;
 	char errmsg[STRERR_BUFSIZE], *legacy_probe = NULL;
 	DECLARE_LIBBPF_OPTS(bpf_perf_event_opts, pe_opts);
+	enum probe_attach_mode attach_mode;
 	char full_path[PATH_MAX];
 	struct bpf_link *link;
 	size_t ref_ctr_off;
···
 	if (!OPTS_VALID(opts, bpf_uprobe_opts))
 		return libbpf_err_ptr(-EINVAL);
 
+	attach_mode = OPTS_GET(opts, attach_mode, PROBE_ATTACH_MODE_DEFAULT);
 	retprobe = OPTS_GET(opts, retprobe, false);
 	ref_ctr_off = OPTS_GET(opts, ref_ctr_offset, 0);
 	pe_opts.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0);
···
 	}
 
 	legacy = determine_uprobe_perf_type() < 0;
+	switch (attach_mode) {
+	case PROBE_ATTACH_MODE_LEGACY:
+		legacy = true;
+		pe_opts.force_ioctl_attach = true;
+		break;
+	case PROBE_ATTACH_MODE_PERF:
+		if (legacy)
+			return libbpf_err_ptr(-ENOTSUP);
+		pe_opts.force_ioctl_attach = true;
+		break;
+	case PROBE_ATTACH_MODE_LINK:
+		if (legacy || !kernel_supports(prog->obj, FEAT_PERF_LINK))
+			return libbpf_err_ptr(-ENOTSUP);
+		break;
+	case PROBE_ATTACH_MODE_DEFAULT:
+		break;
+	default:
+		return libbpf_err_ptr(-EINVAL);
+	}
+
 	if (!legacy) {
 		pfd = perf_event_open_probe(true /* uprobe */, retprobe, binary_path,
 					    func_offset, pid, ref_ctr_off);

tools/lib/bpf/libbpf.h

···
 bpf_program__attach(const struct bpf_program *prog);
 
 struct bpf_perf_event_opts {
-	/* size of this struct, for forward/backward compatiblity */
+	/* size of this struct, for forward/backward compatibility */
 	size_t sz;
 	/* custom user-provided value fetchable through bpf_get_attach_cookie() */
 	__u64 bpf_cookie;
+	/* don't use BPF link when attach BPF program */
+	bool force_ioctl_attach;
+	size_t :0;
 };
-#define bpf_perf_event_opts__last_field bpf_cookie
+#define bpf_perf_event_opts__last_field force_ioctl_attach
 
 LIBBPF_API struct bpf_link *
 bpf_program__attach_perf_event(const struct bpf_program *prog, int pfd);
···
 bpf_program__attach_perf_event_opts(const struct bpf_program *prog, int pfd,
 				    const struct bpf_perf_event_opts *opts);
 
+/**
+ * enum probe_attach_mode - the mode to attach kprobe/uprobe
+ *
+ * force libbpf to attach kprobe/uprobe in specific mode, -ENOTSUP will
+ * be returned if it is not supported by the kernel.
+ */
+enum probe_attach_mode {
+	/* attach probe in latest supported mode by kernel */
+	PROBE_ATTACH_MODE_DEFAULT = 0,
+	/* attach probe in legacy mode, using debugfs/tracefs */
+	PROBE_ATTACH_MODE_LEGACY,
+	/* create perf event with perf_event_open() syscall */
+	PROBE_ATTACH_MODE_PERF,
+	/* attach probe with BPF link */
+	PROBE_ATTACH_MODE_LINK,
+};
+
 struct bpf_kprobe_opts {
-	/* size of this struct, for forward/backward compatiblity */
+	/* size of this struct, for forward/backward compatibility */
 	size_t sz;
 	/* custom user-provided value fetchable through bpf_get_attach_cookie() */
 	__u64 bpf_cookie;
···
 	size_t offset;
 	/* kprobe is return probe */
 	bool retprobe;
+	/* kprobe attach mode */
+	enum probe_attach_mode attach_mode;
 	size_t :0;
 };
-#define bpf_kprobe_opts__last_field retprobe
+#define bpf_kprobe_opts__last_field attach_mode
 
 LIBBPF_API struct bpf_link *
 bpf_program__attach_kprobe(const struct bpf_program *prog, bool retprobe,
···
 				      const struct bpf_kprobe_multi_opts *opts);
 
 struct bpf_ksyscall_opts {
-	/* size of this struct, for forward/backward compatiblity */
+	/* size of this struct, for forward/backward compatibility */
 	size_t sz;
 	/* custom user-provided value fetchable through bpf_get_attach_cookie() */
 	__u64 bpf_cookie;
···
 			     const struct bpf_ksyscall_opts *opts);
 
 struct bpf_uprobe_opts {
-	/* size of this struct, for forward/backward compatiblity */
+	/* size of this struct, for forward/backward compatibility */
 	size_t sz;
 	/* offset of kernel reference counted USDT semaphore, added in
 	 * a6ca88b241d5 ("trace_uprobe: support reference counter in fd-based uprobe")
···
 	 * binary_path.
 	 */
 	const char *func_name;
+	/* uprobe attach mode */
+	enum probe_attach_mode attach_mode;
 	size_t :0;
 };
-#define bpf_uprobe_opts__last_field func_name
+#define bpf_uprobe_opts__last_field attach_mode
 
 /**
  * @brief **bpf_program__attach_uprobe()** attaches a BPF program
···
 			 const struct bpf_usdt_opts *opts);
 
 struct bpf_tracepoint_opts {
-	/* size of this struct, for forward/backward compatiblity */
+	/* size of this struct, for forward/backward compatibility */
 	size_t sz;
 	/* custom user-provided value fetchable through bpf_get_attach_cookie() */
 	__u64 bpf_cookie;
···
 typedef int (*ring_buffer_sample_fn)(void *ctx, void *data, size_t size);
 
 struct ring_buffer_opts {
-	size_t sz; /* size of this struct, for forward/backward compatiblity */
+	size_t sz; /* size of this struct, for forward/backward compatibility */
 };
 
 #define ring_buffer_opts__last_field sz
···
 bpf_object__destroy_subskeleton(struct bpf_object_subskeleton *s);
 
 struct gen_loader_opts {
-	size_t sz; /* size of this struct, for forward/backward compatiblity */
+	size_t sz; /* size of this struct, for forward/backward compatibility */
 	const char *data;
 	const char *insns;
 	__u32 data_sz;
···
 };
 
 struct bpf_linker_opts {
-	/* size of this struct, for forward/backward compatiblity */
+	/* size of this struct, for forward/backward compatibility */
 	size_t sz;
 };
 #define bpf_linker_opts__last_field sz
 
 struct bpf_linker_file_opts {
-	/* size of this struct, for forward/backward compatiblity */
+	/* size of this struct, for forward/backward compatibility */
 	size_t sz;
 };
 #define bpf_linker_file_opts__last_field sz
···
 				       struct bpf_link **link);
 
 struct libbpf_prog_handler_opts {
-	/* size of this struct, for forward/backward compatiblity */
+	/* size of this struct, for forward/backward compatibility */
 	size_t sz;
 	/* User-provided value that is passed to prog_setup_fn,
 	 * prog_prepare_load_fn, and prog_attach_fn callbacks. Allows user to