bpf: Fix unpopulated path_size when uprobe_multi fields unset

Previously when retrieving `bpf_link_info.uprobe_multi` with `path` and
`path_size` fields unset, the `path_size` field is not populated
(remains 0). This behavior was inconsistent with how other input/output
string buffer fields work, as the field should be populated in cases
when:
- both buffer and length are set (currently works as expected)
- both buffer and length are unset (not working as expected)

This patch now fills the `path_size` field when `path` and `path_size`
are unset.

Fixes: e56fdbfb ("bpf: Add link_info support for uprobe multi link")
Signed-off-by: Tyrone Wu <wudevelops@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241011000803.681190-1-wudevelops@gmail.com

kernel/trace/bpf_trace.c

@@ -3133,7 +3133,8 @@ static int bpf_uprobe_multi_link_fill_link_info(const struct bpf_link *link,
 	struct bpf_uprobe_multi_link *umulti_link;
 	u32 ucount = info->uprobe_multi.count;
 	int err = 0, i;
-	long left;
+	char *p, *buf;
+	long left = 0;
 
 	if (!upath ^ !upath_size)
 		return -EINVAL;
@@ -3147,26 +3148,23 @@ static int bpf_uprobe_multi_link_fill_link_info(const struct bpf_link *link,
 	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);
+	upath_size = upath_size ? min_t(u32, upath_size, PATH_MAX) : 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);
-		if (left)
-			return -EFAULT;
-		info->uprobe_multi.path_size = upath_size;
+		return PTR_ERR(p);
 	}
+	upath_size = buf + upath_size - p;
+
+	if (upath)
+		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;