diff --git a/arch/riscv/lib/strnlen.S b/arch/riscv/lib/strnlen.S
index a8911605c248..5b3bbf0a3098 100644
--- a/arch/riscv/lib/strnlen.S
+++ b/arch/riscv/lib/strnlen.S
@@ -87,9 +87,25 @@ strnlen_zbb:
 	 * Aligned boundary.  Use the address of the last valid byte
 	 * (s + count - 1) to avoid loading a word past the count
 	 * boundary in the loop below.  count == 0 is handled above.
+	 *
+	 * Saturate the boundary when s + count would wrap around (very
+	 * large counts, e.g. SIZE_MAX passed by FORTIFY strcat/strlcat
+	 * with a destination whose size is unknown at compile time).
+	 * Without this, the wrapped boundary lands before s and the
+	 * pre-loop guard below always exits, returning a truncated
+	 * length.
+	 *
+	 * Clamping the increment to ~s (== SIZE_MAX - s) keeps the
+	 * computation branchless and wrap-free:
+	 *
+	 *     s + min(count - 1, ~s) == saturating_add(s, count - 1)
+	 *
+	 * Saturating makes the scan equivalent to strlen().
 	 */
-	add	t4, a0, a1
-	addi	t4, t4, -1
+	addi	t4, a1, -1	/* count - 1 */
+	not	t1, a0		/* SIZE_MAX - s */
+	minu	t4, t4, t1	/* clamp, so s + t4 can never wrap */
+	add	t4, a0, t4	/* saturated s + count - 1 */
 	andi	t4, t4, -SZREG
 
 	/* Get the first word.  */