bpf: Add link_info support for uprobe multi link

Adding support to get uprobe_link details through bpf_link_info
interface.

Adding new struct uprobe_multi to struct bpf_link_info to carry
the uprobe_multi link details.

The uprobe_multi.count is passed from user space to denote size
of array fields (offsets/ref_ctr_offsets/cookies). The actual
array size is stored back to uprobe_multi.count (allowing user
to find out the actual array size) and array fields are populated
up to the user passed size.

All the non-array fields (path/count/flags/pid) are always set.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20231125193130.834322-4-jolsa@kernel.org

include/uapi/linux/bpf.h

@@ -6562,6 +6562,16 @@ struct bpf_link_info {
 			__u32 flags;
 			__u64 missed;
 		} kprobe_multi;
+		struct {
+			__aligned_u64 path;
+			__aligned_u64 offsets;
+			__aligned_u64 ref_ctr_offsets;
+			__aligned_u64 cookies;
+			__u32 path_size; /* in/out: real path size on success, including zero byte */
+			__u32 count; /* in/out: uprobe_multi offsets/ref_ctr_offsets/cookies count */
+			__u32 flags;
+			__u32 pid;
+		} uprobe_multi;
 		struct {
 			__u32 type; /* enum bpf_perf_event_type */
 			__u32 :32;

kernel/trace/bpf_trace.c

@@ -3042,6 +3042,7 @@ struct bpf_uprobe_multi_link {
 	struct path path;
 	struct bpf_link link;
 	u32 cnt;
+	u32 flags;
 	struct bpf_uprobe *uprobes;
 	struct task_struct *task;
 };
@@ -3083,9 +3084,79 @@ static void bpf_uprobe_multi_link_dealloc(struct bpf_link *link)
 	kfree(umulti_link);
 }
 
+static int bpf_uprobe_multi_link_fill_link_info(const struct bpf_link *link,
+						struct bpf_link_info *info)
+{
+	u64 __user *uref_ctr_offsets = u64_to_user_ptr(info->uprobe_multi.ref_ctr_offsets);
+	u64 __user *ucookies = u64_to_user_ptr(info->uprobe_multi.cookies);
+	u64 __user *uoffsets = u64_to_user_ptr(info->uprobe_multi.offsets);
+	u64 __user *upath = u64_to_user_ptr(info->uprobe_multi.path);
+	u32 upath_size = info->uprobe_multi.path_size;
+	struct bpf_uprobe_multi_link *umulti_link;
+	u32 ucount = info->uprobe_multi.count;
+	int err = 0, i;
+	long left;
+
+	if (!upath ^ !upath_size)
+		return -EINVAL;
+
+	if ((uoffsets || uref_ctr_offsets || ucookies) && !ucount)
+		return -EINVAL;
+
+	umulti_link = container_of(link, struct bpf_uprobe_multi_link, link);
+	info->uprobe_multi.count = umulti_link->cnt;
+	info->uprobe_multi.flags = umulti_link->flags;
+	info->uprobe_multi.pid = umulti_link->task ?
+				 task_pid_nr_ns(umulti_link->task, task_active_pid_ns(current)) : 0;
+
+	if (upath) {
+		char *p, *buf;
+
+		upath_size = min_t(u32, upath_size, PATH_MAX);
+
+		buf = kmalloc(upath_size, GFP_KERNEL);
+		if (!buf)
+			return -ENOMEM;
+		p = d_path(&umulti_link->path, buf, upath_size);
+		if (IS_ERR(p)) {
+			kfree(buf);
+			return PTR_ERR(p);
+		}
+		upath_size = buf + upath_size - p;
+		left = copy_to_user(upath, p, upath_size);
+		kfree(buf);
+		if (left)
+			return -EFAULT;
+		info->uprobe_multi.path_size = upath_size;
+	}
+
+	if (!uoffsets && !ucookies && !uref_ctr_offsets)
+		return 0;
+
+	if (ucount < umulti_link->cnt)
+		err = -ENOSPC;
+	else
+		ucount = umulti_link->cnt;
+
+	for (i = 0; i < ucount; i++) {
+		if (uoffsets &&
+		    put_user(umulti_link->uprobes[i].offset, uoffsets + i))
+			return -EFAULT;
+		if (uref_ctr_offsets &&
+		    put_user(umulti_link->uprobes[i].ref_ctr_offset, uref_ctr_offsets + i))
+			return -EFAULT;
+		if (ucookies &&
+		    put_user(umulti_link->uprobes[i].cookie, ucookies + i))
+			return -EFAULT;
+	}
+
+	return err;
+}
+
 static const struct bpf_link_ops bpf_uprobe_multi_link_lops = {
 	.release = bpf_uprobe_multi_link_release,
 	.dealloc = bpf_uprobe_multi_link_dealloc,
+	.fill_link_info = bpf_uprobe_multi_link_fill_link_info,
 };
 
 static int uprobe_prog_run(struct bpf_uprobe *uprobe,
@@ -3274,6 +3345,7 @@ int bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
 	link->uprobes = uprobes;
 	link->path = path;
 	link->task = task;
+	link->flags = flags;
 
 	bpf_link_init(&link->link, BPF_LINK_TYPE_UPROBE_MULTI,
 		      &bpf_uprobe_multi_link_lops, prog);

tools/include/uapi/linux/bpf.h

@@ -6562,6 +6562,16 @@ struct bpf_link_info {
 			__u32 flags;
 			__u64 missed;
 		} kprobe_multi;
+		struct {
+			__aligned_u64 path;
+			__aligned_u64 offsets;
+			__aligned_u64 ref_ctr_offsets;
+			__aligned_u64 cookies;
+			__u32 path_size; /* in/out: real path size on success, including zero byte */
+			__u32 count; /* in/out: uprobe_multi offsets/ref_ctr_offsets/cookies count */
+			__u32 flags;
+			__u32 pid;
+		} uprobe_multi;
 		struct {
 			__u32 type; /* enum bpf_perf_event_type */
 			__u32 :32;