initramfs: avoid memcpy for hex header fields

newc/crc cpio headers contain a bunch of 8-character hexadecimal fields
which we convert via simple_strtoul(), following memcpy() into a
zero-terminated stack buffer. The new simple_strntoul() helper allows us
to pass in max_chars=8 to avoid zero-termination and memcpy().

Signed-off-by: David Disseldorp <ddiss@suse.de>
Link: https://lore.kernel.org/r/20250304061020.9815-5-ddiss@suse.de
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: ddiss

init/initramfs.c

@@ -189,14 +189,11 @@ static __initdata u32 hdr_csum;
 static void __init parse_header(char *s)
 {
 	unsigned long parsed[13];
-	char buf[9];
 	int i;
 
-	buf[8] = '\0';
-	for (i = 0, s += 6; i < 13; i++, s += 8) {
-		memcpy(buf, s, 8);
-		parsed[i] = simple_strtoul(buf, NULL, 16);
-	}
+	for (i = 0, s += 6; i < 13; i++, s += 8)
+		parsed[i] = simple_strntoul(s, NULL, 16, 8);
+
 	ino = parsed[0];
 	mode = parsed[1];
 	uid = parsed[2];