fat: ignore ctime updates, and keep ctime identical to mtime in memory

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.

Link: https://lkml.kernel.org/r/20220503152536.2503003-2-cccheng@synology.com
Signed-off-by: Chung-Chiang Cheng <cccheng@synology.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index cb698a827c9a095ad168c9324689be92a034d046..d0371913f496d5727bde3dc37c7576ae1da7610e 100644 (file)
--- 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
+               inode->i_atime = fat_truncate_atime(sbi, &inode->i_mtime);
 
        return 0;
 }
@@ -888,8 +887,6 @@ retry:
                          &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 80c6f8b3dc7565a153bc13c8900566325129718b..8ebe49e315ab52aad527b66b4eef9019f47a56a6 100644 (file)
--- a/fs/fat/misc.c
+++ b/fs/fat/misc.c
@@ -347,10 +347,13 @@ int fat_truncate_time(struct inode *inode, struct timespec64 *now, int flags)
 
        if (flags & S_ATIME)
                inode->i_atime = fat_truncate_atime(sbi, now);
-       if (flags & S_CTIME)
-               inode->i_ctime = fat_truncate_crtime(sbi, now);
+       /*
+        * 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)
-               inode->i_mtime = fat_truncate_mtime(sbi, now);
+               inode->i_mtime = inode->i_ctime = fat_truncate_mtime(sbi, now);
 
        return 0;
 }