init: replace deprecated strncpy with strscpy_pad

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

data_page wants to be NUL-terminated and NUL-padded, use strscpy_pad to
provide both of these. data_page no longer awkwardly relies on
init_mount to perform its NUL-termination, although that sanity check is
left unchanged.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc:  <[email protected]>
Signed-off-by: Justin Stitt <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

init/do_mounts.c

@@ -159,8 +159,7 @@ static int __init do_mount_root(const char *name, const char *fs,
 		if (!p)
 			return -ENOMEM;
 		data_page = page_address(p);
-		/* zero-pad. init_mount() will make sure it's terminated */
-		strncpy(data_page, data, PAGE_SIZE);
+		strscpy_pad(data_page, data, PAGE_SIZE);
 	}
 
 	ret = init_mount(name, "/root", fs, flags, data_page);