[PATCH] lib/string: rewrite strlcat() to use sized_strscpy()

[PATCH] lib/string: rewrite strlcat() to use sized_strscpy()

[Josh Law]

Replace the hand-rolled copy logic and BUG_ON() in strlcat() with a
call to sized_strscpy(). This removes the kernel panic on caller
misuse (dsize >= count) and instead returns a truncation indicator
consistent with BSD strlcat semantics.

strlcat() is deprecated and callers should be converted to strscpy()
or similar bounded interfaces.

Signed-off-by: Josh Law <objecting@objecting.org>
---
 lib/string.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index b632c71df1a5..414016f8a6a6 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -253,18 +253,13 @@ size_t strlcat(char *dest, const char *src, size_t count)
 {
 	size_t dsize = strlen(dest);
 	size_t len = strlen(src);
-	size_t res = dsize + len;
 
-	/* This would be a bug */
-	BUG_ON(dsize >= count);
+	if (dsize >= count)
+		return count + len;
 
-	dest += dsize;
-	count -= dsize;
-	if (len >= count)
-		len = count-1;
-	__builtin_memcpy(dest, src, len);
-	dest[len] = 0;
-	return res;
+	sized_strscpy(dest + dsize, src, count - dsize);
+
+	return dsize + len;
 }
 EXPORT_SYMBOL(strlcat);
 #endif
-- 
2.34.1

Re: [PATCH] lib/string: rewrite strlcat() to use sized_strscpy()

[Andy Shevchenko]

On Mon, Mar 16, 2026 at 04:49:39PM +0000, Josh Law wrote:
> Replace the hand-rolled copy logic and BUG_ON() in strlcat() with a
> call to sized_strscpy(). This removes the kernel panic on caller
> misuse (dsize >= count) and instead returns a truncation indicator
> consistent with BSD strlcat semantics.
> 
> strlcat() is deprecated and callers should be converted to strscpy()
> or similar bounded interfaces.

I mentioned something different, id est converting users.
This function will go when ~130 or so users gone.

So, I don't see a need to touch this function (implementation) at all.

-- 
With Best Regards,
Andy Shevchenko