fat: provide option for setting timezone offset

So far FAT either offsets time stamps by sys_tz.minuteswest or leaves them
as they are (when tz=UTC mount option is used).  However in some cases it
is useful if one can specify time stamp offset on his own (e.g.  when time
zone of the camera connected is different from time zone of the computer,
or when HW clock is in UTC and thus sys_tz.minuteswest == 0).

So provide a mount option time_offset= which allows user to specify offset
in minutes that should be applied to time stamps on the filesystem.

akpm: this code would work incorrectly when used via `mount -o remount',
because cached inodes would not be updated.  But fatfs's fat_remount() is
basically a no-op anyway.
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/vfat.txt

@@ -111,6 +111,15 @@ tz=UTC        -- Interpret timestamps as UTC rather than local time.
                  useful when mounting devices (like digital cameras)
                  that are set to UTC in order to avoid the pitfalls of
                  local time.
+time_offset=minutes
+	      -- Set offset for conversion of timestamps from local time
+		 used by FAT to UTC. I.e. <minutes> minutes will be subtracted
+		 from each timestamp to convert it to UTC used internally by
+		 Linux. This is useful when time zone set in sys_tz is
+		 not the time zone used by the filesystem. Note that this
+		 option still does not provide correct time stamps in all
+		 cases in presence of DST - time stamps in a different DST
+		 setting will be off by one hour.
 
 showexec      -- If set, the execute permission bits of the file will be
 		 allowed only if the extension part of the name is .EXE,

fs/fat/fat.h

@@ -29,6 +29,7 @@ struct fat_mount_options {
 	unsigned short fs_fmask;
 	unsigned short fs_dmask;
 	unsigned short codepage;   /* Codepage for shortname conversions */
+	int time_offset;	   /* Offset of timestamps from UTC (in minutes) */
 	char *iocharset;           /* Charset used for filename input/display */
 	unsigned short shortname;  /* flags for shortname display/create rule */
 	unsigned char name_check;  /* r = relaxed, n = normal, s = strict */
@@ -45,7 +46,7 @@ struct fat_mount_options {
 		 flush:1,	   /* write things quickly */
 		 nocase:1,	   /* Does this need case conversion? 0=need case conversion*/
 		 usefree:1,	   /* Use free_clusters for FAT32 */
-		 tz_utc:1,	   /* Filesystem timestamps are in UTC */
+		 tz_set:1,	   /* Filesystem timestamps' offset set */
 		 rodir:1,	   /* allow ATTR_RO for directory */
 		 discard:1,	   /* Issue discard requests on deletions */
 		 nfs:1;		   /* Do extra work needed for NFS export */

fs/fat/inode.c

@@ -778,8 +778,12 @@ static int fat_show_options(struct seq_file *m, struct dentry *root)
 	}
 	if (opts->flush)
 		seq_puts(m, ",flush");
-	if (opts->tz_utc)
+	if (opts->tz_set) {
+		if (opts->time_offset)
+			seq_printf(m, ",time_offset=%d", opts->time_offset);
+		else
 			seq_puts(m, ",tz=UTC");
+	}
 	if (opts->errors == FAT_ERRORS_CONT)
 		seq_puts(m, ",errors=continue");
 	else if (opts->errors == FAT_ERRORS_PANIC)
@@ -801,7 +805,8 @@ enum {
 	Opt_shortname_winnt, Opt_shortname_mixed, Opt_utf8_no, Opt_utf8_yes,
 	Opt_uni_xl_no, Opt_uni_xl_yes, Opt_nonumtail_no, Opt_nonumtail_yes,
 	Opt_obsolete, Opt_flush, Opt_tz_utc, Opt_rodir, Opt_err_cont,
-	Opt_err_panic, Opt_err_ro, Opt_discard, Opt_nfs, Opt_err,
+	Opt_err_panic, Opt_err_ro, Opt_discard, Opt_nfs, Opt_time_offset,
+	Opt_err,
 };
 
 static const match_table_t fat_tokens = {
@@ -826,6 +831,7 @@ static const match_table_t fat_tokens = {
 	{Opt_immutable, "sys_immutable"},
 	{Opt_flush, "flush"},
 	{Opt_tz_utc, "tz=UTC"},
+	{Opt_time_offset, "time_offset=%d"},
 	{Opt_err_cont, "errors=continue"},
 	{Opt_err_panic, "errors=panic"},
 	{Opt_err_ro, "errors=remount-ro"},
@@ -910,7 +916,7 @@ static int parse_options(struct super_block *sb, char *options, int is_vfat,
 	opts->utf8 = opts->unicode_xlate = 0;
 	opts->numtail = 1;
 	opts->usefree = opts->nocase = 0;
-	opts->tz_utc = 0;
+	opts->tz_set = 0;
 	opts->nfs = 0;
 	opts->errors = FAT_ERRORS_RO;
 	*debug = 0;
@@ -1006,8 +1012,17 @@ static int parse_options(struct super_block *sb, char *options, int is_vfat,
 		case Opt_flush:
 			opts->flush = 1;
 			break;
+		case Opt_time_offset:
+			if (match_int(&args[0], &option))
+				return 0;
+			if (option < -12 * 60 || option > 12 * 60)
+				return 0;
+			opts->tz_set = 1;
+			opts->time_offset = option;
+			break;
 		case Opt_tz_utc:
-			opts->tz_utc = 1;
+			opts->tz_set = 1;
+			opts->time_offset = 0;
 			break;
 		case Opt_err_cont:
 			opts->errors = FAT_ERRORS_CONT;

fs/fat/misc.c

@@ -212,8 +212,10 @@ void fat_time_fat2unix(struct msdos_sb_info *sbi, struct timespec *ts,
 		   + days_in_year[month] + day
 		   + DAYS_DELTA) * SECS_PER_DAY;
 
-	if (!sbi->options.tz_utc)
+	if (!sbi->options.tz_set)
 		second += sys_tz.tz_minuteswest * SECS_PER_MIN;
+	else
+		second -= sbi->options.time_offset * SECS_PER_MIN;
 
 	if (time_cs) {
 		ts->tv_sec = second + (time_cs / 100);
@@ -229,8 +231,9 @@ void fat_time_unix2fat(struct msdos_sb_info *sbi, struct timespec *ts,
 		       __le16 *time, __le16 *date, u8 *time_cs)
 {
 	struct tm tm;
-	time_to_tm(ts->tv_sec, sbi->options.tz_utc ? 0 :
-		   -sys_tz.tz_minuteswest * 60, &tm);
+	time_to_tm(ts->tv_sec,
+		   (sbi->options.tz_set ? sbi->options.time_offset :
+		   -sys_tz.tz_minuteswest) * SECS_PER_MIN, &tm);
 
 	/*  FAT can only support year between 1980 to 2107 */
 	if (tm.tm_year < 1980 - 1900) {