libbpf: Add support to parse extra info in usdt note record

Adding support to parse extra info in usdt note record that
indicates there's nop,nop5 emitted for probe.

We detect this by checking extra zero byte placed in between
args zero termination byte and desc data end. Please see [1]
for more details.

Together with uprobe syscall feature detection we can decide
if we want to place the probe on top of nop or nop5.

[1] https://github.com/libbpf/usdt
Signed-off-by: Jiri Olsa <[email protected]>

Author: olsajiri

tools/lib/bpf/usdt.c

@@ -241,6 +241,7 @@ struct usdt_note {
 	long loc_addr;
 	long base_addr;
 	long sema_addr;
+	bool nop_combo;
 };
 
 struct usdt_target {
@@ -262,6 +263,7 @@ struct usdt_manager {
 	bool has_bpf_cookie;
 	bool has_sema_refcnt;
 	bool has_uprobe_multi;
+	bool has_uprobe_syscall;
 };
 
 struct usdt_manager *usdt_manager_new(struct bpf_object *obj)
@@ -301,6 +303,11 @@ struct usdt_manager *usdt_manager_new(struct bpf_object *obj)
 	 * usdt probes.
 	 */
 	man->has_uprobe_multi = kernel_supports(obj, FEAT_UPROBE_MULTI_LINK);
+
+	/*
+	 * Detect kernel support for uprobe syscall to be used to pick usdt attach point.
+	 */
+	man->has_uprobe_syscall = kernel_supports(obj, FEAT_UPROBE_SYSCALL);
 	return man;
 }
 
@@ -784,6 +791,15 @@ static int collect_usdt_targets(struct usdt_manager *man, Elf *elf, const char *
 		target = &targets[target_cnt];
 		memset(target, 0, sizeof(*target));
 
+		/*
+		 * We have usdt with nop,nop5 instruction and we detected uprobe syscall,
+		 * so we can place the uprobe directly on nop5 (+1) to get it optimized.
+		 */
+		if (note.nop_combo && man->has_uprobe_syscall) {
+			usdt_abs_ip++;
+			usdt_rel_ip++;
+		}
+
 		target->abs_ip = usdt_abs_ip;
 		target->rel_ip = usdt_rel_ip;
 		target->sema_off = usdt_sema_off;
@@ -1144,7 +1160,7 @@ struct bpf_link *usdt_manager_attach_usdt(struct usdt_manager *man, const struct
 static int parse_usdt_note(GElf_Nhdr *nhdr, const char *data, size_t name_off, size_t desc_off,
 			   struct usdt_note *note)
 {
-	const char *provider, *name, *args;
+	const char *provider, *name, *args, *end, *extra;
 	long addrs[3];
 	size_t len;
 
@@ -1182,6 +1198,15 @@ static int parse_usdt_note(GElf_Nhdr *nhdr, const char *data, size_t name_off, s
 	if (args >= data + len) /* missing arguments spec */
 		return -EINVAL;
 
+	extra = memchr(args, '\0', data + len - args);
+	if (!extra) /* non-zero-terminated args */
+		return -EINVAL;
+	++extra;
+	end = data + len;
+
+	/* check if we have one extra byte and if it's zero */
+	note->nop_combo = (extra + 1) == end && *extra == 0;
+
 	note->provider = provider;
 	note->name = name;
 	if (*args == '\0' || *args == ':')