libbpf: Allow kprobe attach using legacy debugfs interface

On some old kernels, kprobe auto-attach may fail when attach to symbols
like udp_send_skb.isra.52 . This is because the kernel has kprobe PMU
but don't allow attach to a symbol with '.' ([0]). Add a new option to
bpf_kprobe_opts to allow using the legacy kprobe attach directly.
This way, users can use bpf_program__attach_kprobe_opts in a dedicated
custom sec handler to handle such case.

  [0]: https://github.com/torvalds/linux/blob/v4.18/kernel/trace/trace_kprobe.c#L340-L343

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>

tools/lib/bpf/libbpf.c

@@ -10097,9 +10097,16 @@ static void gen_kprobe_legacy_event_name(char *buf, size_t buf_sz,
 					 const char *kfunc_name, size_t offset)
 {
 	static int index = 0;
+	int i;
 
 	snprintf(buf, buf_sz, "libbpf_%u_%s_0x%zx_%d", getpid(), kfunc_name, offset,
 		 __sync_fetch_and_add(&index, 1));
+
+	/* sanitize .isra.$n symbols */
+	for (i = 0; buf[i]; i++) {
+		if (!isalnum(buf[i]))
+			buf[i] = '_';
+	}
 }
 
 static int add_kprobe_event_legacy(const char *probe_name, bool retprobe,
@@ -10189,7 +10196,7 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
 	offset = OPTS_GET(opts, offset, 0);
 	pe_opts.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0);
 
-	legacy = determine_kprobe_perf_type() < 0;
+	legacy = OPTS_GET(opts, legacy, false) || determine_kprobe_perf_type() < 0;
 	if (!legacy) {
 		pfd = perf_event_open_probe(false /* uprobe */, retprobe,
 					    func_name, offset,

tools/lib/bpf/libbpf.h

@@ -413,9 +413,11 @@ struct bpf_kprobe_opts {
 	size_t offset;
 	/* kprobe is return probe */
 	bool retprobe;
+	/* use the legacy debugfs interface */
+	bool legacy;
 	size_t :0;
 };
-#define bpf_kprobe_opts__last_field retprobe
+#define bpf_kprobe_opts__last_field legacy
 
 LIBBPF_API struct bpf_link *
 bpf_program__attach_kprobe(const struct bpf_program *prog, bool retprobe,