[PATCH v4 2/3] fat: ignore ctime updates, and keep ctime identical to mtime in memory

[Chung-Chiang Cheng <cccheng@synology.com>]

FAT supports creation time but not change time, and there was no
corresponding timestamp for creation time in previous VFS. The
original implementation took the compromise of saving the in-memory
change time into the on-disk creation time field, but this would lead
to compatibility issues with non-linux systems.

To address this issue, this patch changes the behavior of ctime. It
will no longer be loaded and stored from the creation time on disk.
Instead of that, it'll be consistent with the in-memory mtime and share
the same on-disk field. All updates to mtime will also be applied to
ctime in memory, while all updates to ctime will be ignored.

Signed-off-by: Chung-Chiang Cheng <cccheng@synology.com>
---
 fs/fat/inode.c | 11 ++++-------
 fs/fat/misc.c  | 12 +++++++++---
 2 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index bf6051bdf1d1..f2ac55cd4ea4 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -567,12 +567,11 @@ int fat_fill_inode(struct inode *inode, struct msdos_dir_entry *de)
 			   & ~((loff_t)sbi->cluster_size - 1)) >> 9;
 
 	fat_time_fat2unix(sbi, &inode->i_mtime, de->time, de->date, 0);
-	if (sbi->options.isvfat) {
-		fat_time_fat2unix(sbi, &inode->i_ctime, de->ctime,
-				  de->cdate, de->ctime_cs);
+	inode->i_ctime = inode->i_mtime;
+	if (sbi->options.isvfat)
 		fat_time_fat2unix(sbi, &inode->i_atime, 0, de->adate, 0);
-	} else
-		fat_truncate_time(inode, &inode->i_mtime, S_ATIME|S_CTIME);
+	else
+		fat_truncate_atime(sbi, &inode->i_mtime, &inode->i_atime);
 
 	return 0;
 }
@@ -888,8 +887,6 @@ static int __fat_write_inode(struct inode *inode, int wait)
 			  &raw_entry->date, NULL);
 	if (sbi->options.isvfat) {
 		__le16 atime;
-		fat_time_unix2fat(sbi, &inode->i_ctime, &raw_entry->ctime,
-				  &raw_entry->cdate, &raw_entry->ctime_cs);
 		fat_time_unix2fat(sbi, &inode->i_atime, &atime,
 				  &raw_entry->adate, NULL);
 	}
diff --git a/fs/fat/misc.c b/fs/fat/misc.c
index c87df64f8b2b..ef09b6361602 100644
--- a/fs/fat/misc.c
+++ b/fs/fat/misc.c
@@ -341,10 +341,16 @@ int fat_truncate_time(struct inode *inode, struct timespec64 *now, int flags)
 
 	if (flags & S_ATIME)
 		fat_truncate_atime(sbi, now, &inode->i_atime);
-	if (flags & S_CTIME)
-		fat_truncate_crtime(sbi, now, &inode->i_ctime);
-	if (flags & S_MTIME)
+
+	/*
+	 * ctime and mtime share the same on-disk field, and should be
+	 * identical in memory. all mtime updates will be applied to ctime,
+	 * but ctime updates are ignored.
+	 */
+	if (flags & S_MTIME) {
 		fat_truncate_mtime(sbi, now, &inode->i_mtime);
+		inode->i_ctime = inode->i_mtime;
+	}
 
 	return 0;
 }
-- 
2.34.1