bpf: Support ->fill_link_info for kprobe_multi

By adding support for ->fill_link_info to the kprobe_multi link, users will
be able to inspect it using `bpftool link show`. This enhancement will
expose both the count of probed functions and their respective addresses to
the user.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>

include/uapi/linux/bpf.h

@@ -6438,6 +6438,10 @@ struct bpf_link_info {
 			__s32 priority;
 			__u32 flags;
 		} netfilter;
+		struct {
+			__u64 addrs;
+			__u32 count;
+		} kprobe_multi;
 	};
 } __attribute__((aligned(8)));
 

kernel/trace/bpf_trace.c

@@ -2548,9 +2548,35 @@ static void bpf_kprobe_multi_link_dealloc(struct bpf_link *link)
 	kfree(kmulti_link);
 }
 
+static int bpf_kprobe_multi_link_fill_link_info(const struct bpf_link *link,
+						struct bpf_link_info *info)
+{
+	u64 *uaddrs = (u64 *)u64_to_user_ptr(info->kprobe_multi.addrs);
+	struct bpf_kprobe_multi_link *kmulti_link;
+	u32 ucount = info->kprobe_multi.count;
+
+	if (!uaddrs ^ !ucount)
+		return -EINVAL;
+
+	kmulti_link = container_of(link, struct bpf_kprobe_multi_link, link);
+	if (!uaddrs) {
+		info->kprobe_multi.count = kmulti_link->cnt;
+		return 0;
+	}
+
+	if (!ucount)
+		return 0;
+	if (ucount != kmulti_link->cnt)
+		return -EINVAL;
+	if (copy_to_user(uaddrs, kmulti_link->addrs, ucount * sizeof(u64)))
+		return -EFAULT;
+	return 0;
+}
+
 static const struct bpf_link_ops bpf_kprobe_multi_link_lops = {
 	.release = bpf_kprobe_multi_link_release,
 	.dealloc = bpf_kprobe_multi_link_dealloc,
+	.fill_link_info = bpf_kprobe_multi_link_fill_link_info,
 };
 
 static void bpf_kprobe_multi_cookie_swap(void *a, void *b, int size, const void *priv)

tools/include/uapi/linux/bpf.h

@@ -6438,6 +6438,10 @@ struct bpf_link_info {
 			__s32 priority;
 			__u32 flags;
 		} netfilter;
+		struct {
+			__u64 addrs;
+			__u32 count;
+		} kprobe_multi;
 	};
 } __attribute__((aligned(8)));