bpf: Support ->fill_link_info for kprobe_multi

With the addition of support for fill_link_info to the kprobe_multi link,
users will gain the ability to inspect it conveniently using the
`bpftool link show`. This enhancement provides valuable information to the
user, including the count of probed functions and their respective
addresses. It's important to note that if the kptr_restrict setting is not
permitted, the probed address will not be exposed, ensuring security.
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230709025630.3735-2-laoar.shao@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -6439,6 +6439,11 @@ struct bpf_link_info {
 			__s32 priority;
 			__u32 flags;
 		} netfilter;
+		struct {
+			__aligned_u64 addrs;
+			__u32 count; /* in/out: kprobe_multi function count */
+			__u32 flags;
+		} kprobe_multi;
 	};
 } __attribute__((aligned(8)));
 

kernel/trace/bpf_trace.c

@@ -2469,6 +2469,7 @@ struct bpf_kprobe_multi_link {
 	u32 cnt;
 	u32 mods_cnt;
 	struct module **mods;
+	u32 flags;
 };
 
 struct bpf_kprobe_multi_run_ctx {
@@ -2558,9 +2559,44 @@ 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 __user *uaddrs = u64_to_user_ptr(info->kprobe_multi.addrs);
+	struct bpf_kprobe_multi_link *kmulti_link;
+	u32 ucount = info->kprobe_multi.count;
+	int err = 0, i;
+
+	if (!uaddrs ^ !ucount)
+		return -EINVAL;
+
+	kmulti_link = container_of(link, struct bpf_kprobe_multi_link, link);
+	info->kprobe_multi.count = kmulti_link->cnt;
+	info->kprobe_multi.flags = kmulti_link->flags;
+
+	if (!uaddrs)
+		return 0;
+	if (ucount < kmulti_link->cnt)
+		err = -ENOSPC;
+	else
+		ucount = kmulti_link->cnt;
+
+	if (kallsyms_show_value(current_cred())) {
+		if (copy_to_user(uaddrs, kmulti_link->addrs, ucount * sizeof(u64)))
+			return -EFAULT;
+	} else {
+		for (i = 0; i < ucount; i++) {
+			if (put_user(0, uaddrs + i))
+				return -EFAULT;
+		}
+	}
+	return err;
+}
+
 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)
@@ -2872,6 +2908,7 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
 	link->addrs = addrs;
 	link->cookies = cookies;
 	link->cnt = cnt;
+	link->flags = flags;
 
 	if (cookies) {
 		/*

tools/include/uapi/linux/bpf.h

@@ -6439,6 +6439,11 @@ struct bpf_link_info {
 			__s32 priority;
 			__u32 flags;
 		} netfilter;
+		struct {
+			__aligned_u64 addrs;
+			__u32 count; /* in/out: kprobe_multi function count */
+			__u32 flags;
+		} kprobe_multi;
 	};
 } __attribute__((aligned(8)));