[PATCH v2] kernfs: Prefer strscpy over strlcpy calls

[Azeem Shaikh <azeemshaikh38@gmail.com>]

strlcpy() reads the entire source buffer first.
This read may exceed the destination size limit.
This is both inefficient and can lead to linear read
overflows if a source string is not NUL-terminated [1].
Since strscpy() returns -E2BIG on truncate, we rely on
strlen(src) to imitate strlcpy behavior.

This is part of a tree-wide cleanup to remove the strlcpy() function
entirely from the kernel [2].

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
[2] https://github.com/KSPP/linux/issues/89

Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
---
 fs/kernfs/dir.c |   23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 45b6919903e6..0f46d7b304b0 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -51,12 +51,19 @@ static bool kernfs_lockdep(struct kernfs_node *kn)
 #endif
 }
 
+/* strscpy_mock_strlcpy - imitates strlcpy API but uses strscpy underneath. */
+static size_t strscpy_mock_strlcpy(char *dest, const char *src, size_t count)
+{
+	strscpy(dest, src, count);
+	return strlen(src);
+}
+
 static int kernfs_name_locked(struct kernfs_node *kn, char *buf, size_t buflen)
 {
 	if (!kn)
-		return strlcpy(buf, "(null)", buflen);
+		return strscpy_mock_strlcpy(buf, "(null)", buflen);
 
-	return strlcpy(buf, kn->parent ? kn->name : "/", buflen);
+	return strscpy_mock_strlcpy(buf, kn->parent ? kn->name : "/", buflen);
 }
 
 /* kernfs_node_depth - compute depth from @from to @to */
@@ -141,13 +148,13 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 	int i, j;
 
 	if (!kn_to)
-		return strlcpy(buf, "(null)", buflen);
+		return strscpy_mock_strlcpy(buf, "(null)", buflen);
 
 	if (!kn_from)
 		kn_from = kernfs_root(kn_to)->kn;
 
 	if (kn_from == kn_to)
-		return strlcpy(buf, "/", buflen);
+		return strscpy_mock_strlcpy(buf, "/", buflen);
 
 	common = kernfs_common_ancestor(kn_from, kn_to);
 	if (WARN_ON(!common))
@@ -159,16 +166,16 @@ static int kernfs_path_from_node_locked(struct kernfs_node *kn_to,
 	buf[0] = '\0';
 
 	for (i = 0; i < depth_from; i++)
-		len += strlcpy(buf + len, parent_str,
+		len += strscpy_mock_strlcpy(buf + len, parent_str,
 			       len < buflen ? buflen - len : 0);
 
 	/* Calculate how many bytes we need for the rest */
 	for (i = depth_to - 1; i >= 0; i--) {
 		for (kn = kn_to, j = 0; j < i; j++)
 			kn = kn->parent;
-		len += strlcpy(buf + len, "/",
+		len += strscpy_mock_strlcpy(buf + len, "/",
 			       len < buflen ? buflen - len : 0);
-		len += strlcpy(buf + len, kn->name,
+		len += strscpy_mock_strlcpy(buf + len, kn->name,
 			       len < buflen ? buflen - len : 0);
 	}
 
@@ -851,7 +858,7 @@ static struct kernfs_node *kernfs_walk_ns(struct kernfs_node *parent,
 
 	spin_lock_irq(&kernfs_pr_cont_lock);
 
-	len = strlcpy(kernfs_pr_cont_buf, path, sizeof(kernfs_pr_cont_buf));
+	len = strscpy_mock_strlcpy(kernfs_pr_cont_buf, path, sizeof(kernfs_pr_cont_buf));
 
 	if (len >= sizeof(kernfs_pr_cont_buf)) {
 		spin_unlock_irq(&kernfs_pr_cont_lock);
-- 
2.40.1.521.gf1e218fcd8-goog