fat/vfat: optionally ignore system timezone offset when reading/writing timestamps

Hello,

Here is a patch that adds a mount option named "posixtime" that, when enabled, causes the fat/vfat code to not adjust timestamps as they are read/written to/from disk. The intent of the adjustment as performed by the existing code appears to be to present correct timestamps to Windows and friends, which treat the timestamp values as local time.

However, the systems that I use to update my FAT32 filesystems are all running Linux, and as such will process POSIX timestamps correctly. (The filesystems are on disks in digital audio players, which do not process timestamps except to check if they have changed.) Due to the aforementioned adjustment on read/write, after a Daylight Savings transition I must update the file timestamps on the filesystems so that rsync does not needlessly re-copy files.

Please review and consider applying this patch.

fs/fat/dir.c +1 -1

index c16af24..d6c0a7f 100644
@@ -1064,7 +1064,7 @@ int fat_alloc_new_dir(struct inode *dir, struct timespec *ts)
 		goto error_free;
 	}
 
-	fat_date_unix2dos(ts->tv_sec, &time, &date);
+	fat_date_unix2dos(ts->tv_sec, &time, &date, sbi->options.adjust);
 
 	de = (struct msdos_dir_entry *)bhs[0]->b_data;
 	/* filling the new directory slots ("." and ".." entries) */

fs/fat/inode.c +19 -7

index a9e4688..02d9225 100644
@@ -374,17 +374,20 @@ static int fat_fill_inode(struct inode *inode, struct msdos_dir_entry *de)
 	inode->i_blocks = ((inode->i_size + (sbi->cluster_size - 1))
 			   & ~((loff_t)sbi->cluster_size - 1)) >> 9;
 	inode->i_mtime.tv_sec =
-		date_dos2unix(le16_to_cpu(de->time), le16_to_cpu(de->date));
+		date_dos2unix(le16_to_cpu(de->time), le16_to_cpu(de->date),
+			      sbi->options.adjust);
 	inode->i_mtime.tv_nsec = 0;
 	if (sbi->options.isvfat) {
 		int secs = de->ctime_cs / 100;
 		int csecs = de->ctime_cs % 100;
 		inode->i_ctime.tv_sec  =
 			date_dos2unix(le16_to_cpu(de->ctime),
-				      le16_to_cpu(de->cdate)) + secs;
+				      le16_to_cpu(de->cdate),
+				      sbi->options.adjust) + secs;
 		inode->i_ctime.tv_nsec = csecs * 10000000;
 		inode->i_atime.tv_sec =
-			date_dos2unix(0, le16_to_cpu(de->adate));
+			date_dos2unix(0, le16_to_cpu(de->adate),
+				      sbi->options.adjust);
 		inode->i_atime.tv_nsec = 0;
 	} else
 		inode->i_ctime = inode->i_atime = inode->i_mtime;

@@ -592,11 +595,14 @@ retry:
 	raw_entry->attr = fat_attr(inode);
 	raw_entry->start = cpu_to_le16(MSDOS_I(inode)->i_logstart);
 	raw_entry->starthi = cpu_to_le16(MSDOS_I(inode)->i_logstart >> 16);
-	fat_date_unix2dos(inode->i_mtime.tv_sec, &raw_entry->time, &raw_entry->date);
+	fat_date_unix2dos(inode->i_mtime.tv_sec, &raw_entry->time, &raw_entry->date,
+			  sbi->options.adjust);
 	if (sbi->options.isvfat) {
 		__le16 atime;
-		fat_date_unix2dos(inode->i_ctime.tv_sec,&raw_entry->ctime,&raw_entry->cdate);
-		fat_date_unix2dos(inode->i_atime.tv_sec,&atime,&raw_entry->adate);
+		fat_date_unix2dos(inode->i_ctime.tv_sec,&raw_entry->ctime,&raw_entry->cdate,
+				  sbi->options.adjust);
+		fat_date_unix2dos(inode->i_atime.tv_sec,&atime,&raw_entry->adate,
+				  sbi->options.adjust);
 		raw_entry->ctime_cs = (inode->i_ctime.tv_sec & 1) * 100 +
 			inode->i_ctime.tv_nsec / 10000000;
 	}

@@ -854,7 +860,7 @@ enum {
 	Opt_charset, Opt_shortname_lower, Opt_shortname_win95,
 	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_obsolate, Opt_flush, Opt_err,
+	Opt_obsolate, Opt_flush, Opt_posixtime, Opt_err,
 };
 
 static match_table_t fat_tokens = {

@@ -887,6 +893,7 @@ static match_table_t fat_tokens = {
 	{Opt_obsolate, "cvf_options=%100s"},
 	{Opt_obsolate, "posix"},
 	{Opt_flush, "flush"},
+	{Opt_posixtime, "posixtime"},
 	{Opt_err, NULL},
 };
 static match_table_t msdos_tokens = {

@@ -950,6 +957,7 @@ static int parse_options(char *options, int is_vfat, int silent, int *debug,
 	opts->utf8 = opts->unicode_xlate = 0;
 	opts->numtail = 1;
 	opts->nocase = 0;
+	opts->adjust = 1;
 	*debug = 0;
 
 	if (!options)

@@ -1032,6 +1040,10 @@ static int parse_options(char *options, int is_vfat, int silent, int *debug,
 			opts->flush = 1;
 			break;
 
+		case Opt_posixtime:
+			opts->adjust = 0;
+			break;
+
 		/* msdos specific */
 		case Opt_dots:
 			opts->dotsOK = 1;

fs/fat/misc.c +6 -4

index 308f2b6..72a1a29 100644
@@ -143,7 +143,7 @@ static int day_n[] = {
 };
 
 /* Convert a MS-DOS time/date pair to a UNIX date (seconds since 1 1 70). */
-int date_dos2unix(unsigned short time, unsigned short date)
+int date_dos2unix(unsigned short time, unsigned short date, int adjust)
 {
 	int month, year, secs;
 

@@ -157,16 +157,18 @@ int date_dos2unix(unsigned short time, unsigned short date)
 	    ((date & 31)-1+day_n[month]+(year/4)+year*365-((year & 3) == 0 &&
 	    month < 2 ? 1 : 0)+3653);
 			/* days since 1.1.70 plus 80's leap day */
-	secs += sys_tz.tz_minuteswest*60;
+	if (adjust)
+		secs += sys_tz.tz_minuteswest*60;
 	return secs;
 }
 
 /* Convert linear UNIX date to a MS-DOS time/date pair. */
-void fat_date_unix2dos(int unix_date, __le16 *time, __le16 *date)
+void fat_date_unix2dos(int unix_date, __le16 *time, __le16 *date, int adjust)
 {
 	int day, year, nl_day, month;
 
-	unix_date -= sys_tz.tz_minuteswest*60;
+	if (adjust)
+		unix_date -= sys_tz.tz_minuteswest*60;
 
 	/* Jan 1 GMT 00:00:00 1980. But what about another time zone? */
 	if (unix_date < 315532800)

fs/vfat/namei.c +1 -1

index 0afd745..7e5487b 100644
@@ -626,7 +626,7 @@ shortname:
 	memcpy(de->name, msdos_name, MSDOS_NAME);
 	de->attr = is_dir ? ATTR_DIR : ATTR_ARCH;
 	de->lcase = lcase;
-	fat_date_unix2dos(ts->tv_sec, &time, &date);
+	fat_date_unix2dos(ts->tv_sec, &time, &date, sbi->options.adjust);
 	de->time = de->ctime = time;
 	de->date = de->cdate = de->adate = date;
 	de->ctime_cs = 0;

include/linux/msdos_fs.h +4 -3

index 24a9ef1..b3eeec8 100644
@@ -205,7 +205,8 @@ struct fat_mount_options {
 		 numtail:1,       /* Does first alias have a numeric '~1' type tail? */
 		 atari:1,         /* Use Atari GEMDOS variation of MS-DOS fs */
 		 flush:1,	  /* write things quickly */
-		 nocase:1;	  /* Does this need case conversion? 0=need case conversion*/
+		 nocase:1,	  /* Does this need case conversion? 0=need case conversion*/
+		 adjust:1;	  /* Adjust time and date based on sys_tz?*/
 };
 
 #define FAT_HASH_BITS	8

@@ -421,8 +422,8 @@ extern int fat_flush_inodes(struct super_block *sb, struct inode *i1,
 extern void fat_fs_panic(struct super_block *s, const char *fmt, ...);
 extern void fat_clusters_flush(struct super_block *sb);
 extern int fat_chain_add(struct inode *inode, int new_dclus, int nr_cluster);
-extern int date_dos2unix(unsigned short time, unsigned short date);
-extern void fat_date_unix2dos(int unix_date, __le16 *time, __le16 *date);
+extern int date_dos2unix(unsigned short time, unsigned short date, int adjust);
+extern void fat_date_unix2dos(int unix_date, __le16 *time, __le16 *date, int adjust);
 extern int fat_sync_bhs(struct buffer_head **bhs, int nr_bhs);
 
 int fat_cache_init(void);