ext4: replace deprecated strncpy with alternatives

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

in file.c:
s_last_mounted is marked as __nonstring meaning it does not need to be
NUL-terminated. Let's instead use strtomem_pad() to copy bytes from the
string source to the byte array destination -- while also ensuring to
pad with zeroes.

in ioctl.c:
We can drop the memset and size argument in favor of using the new
2-argument version of strscpy_pad() -- which was introduced with Commit
e6584c3 ("string: Allow 2-argument strscpy()"). This guarantees
NUL-termination and NUL-padding on the destination buffer -- which seems
to be a requirement judging from this comment:

|	static int ext4_ioctl_getlabel(struct ext4_sb_info *sbi, char __user *user_label)
|	{
|		char label[EXT4_LABEL_MAX + 1];
|
|		/*
|		 * EXT4_LABEL_MAX must always be smaller than FSLABEL_MAX because
|		 * FSLABEL_MAX must include terminating null byte, while s_volume_name
|		 * does not have to.
|		 */

in super.c:
s_first_error_func is marked as __nonstring meaning we can take the same
approach as in file.c; just use strtomem_pad()

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: KSPP#90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240321-strncpy-fs-ext4-file-c-v1-1-36a6a09fef0c@google.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: JustinStitt

fs/ext4/file.c

@@ -844,8 +844,7 @@ static int ext4_sample_last_mounted(struct super_block *sb,
 	if (err)
 		goto out_journal;
 	lock_buffer(sbi->s_sbh);
-	strncpy(sbi->s_es->s_last_mounted, cp,
-		sizeof(sbi->s_es->s_last_mounted));
+	strtomem_pad(sbi->s_es->s_last_mounted, cp, 0);
 	ext4_superblock_csum_set(sb);
 	unlock_buffer(sbi->s_sbh);
 	ext4_handle_dirty_metadata(handle, NULL, sbi->s_sbh);

fs/ext4/ioctl.c

@@ -1150,9 +1150,8 @@ static int ext4_ioctl_getlabel(struct ext4_sb_info *sbi, char __user *user_label
 	 */
 	BUILD_BUG_ON(EXT4_LABEL_MAX >= FSLABEL_MAX);
 
-	memset(label, 0, sizeof(label));
 	lock_buffer(sbi->s_sbh);
-	strncpy(label, sbi->s_es->s_volume_name, EXT4_LABEL_MAX);
+	strscpy_pad(label, sbi->s_es->s_volume_name);
 	unlock_buffer(sbi->s_sbh);
 
 	if (copy_to_user(user_label, label, sizeof(label)))

fs/ext4/super.c

@@ -6129,8 +6129,8 @@ static void ext4_update_super(struct super_block *sb)
 			__ext4_update_tstamp(&es->s_first_error_time,
 					     &es->s_first_error_time_hi,
 					     sbi->s_first_error_time);
-			strncpy(es->s_first_error_func, sbi->s_first_error_func,
-				sizeof(es->s_first_error_func));
+			strtomem_pad(es->s_first_error_func,
+				     sbi->s_first_error_func, 0);
 			es->s_first_error_line =
 				cpu_to_le32(sbi->s_first_error_line);
 			es->s_first_error_ino =
@@ -6143,8 +6143,7 @@ static void ext4_update_super(struct super_block *sb)
 		__ext4_update_tstamp(&es->s_last_error_time,
 				     &es->s_last_error_time_hi,
 				     sbi->s_last_error_time);
-		strncpy(es->s_last_error_func, sbi->s_last_error_func,
-			sizeof(es->s_last_error_func));
+		strtomem_pad(es->s_last_error_func, sbi->s_last_error_func, 0);
 		es->s_last_error_line = cpu_to_le32(sbi->s_last_error_line);
 		es->s_last_error_ino = cpu_to_le32(sbi->s_last_error_ino);
 		es->s_last_error_block = cpu_to_le64(sbi->s_last_error_block);