[PATCH] exfat: introduce mount option 'sys_tz'

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

EXFAT_TZ_VALID bit in {create,modify,access}_tz is corresponding to
OffsetValid field in exfat specification [1]. When this bit isn't
set, timestamps should be treated as having the same UTC offset as
the current local time.

Currently, there is an option 'time_offset' for users to specify the
UTC offset for this issue. This patch introduces a new mount option
'sys_tz' to use system timezone as time offset.

Link: [1] https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification#74102-offsetvalid-field

Signed-off-by: Chung-Chiang Cheng <cccheng@synology.com>
---
 fs/exfat/exfat_fs.h |  1 +
 fs/exfat/misc.c     | 10 ++++++++--
 fs/exfat/super.c    |  9 ++++++++-
 3 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
index c6800b880920..82e507413291 100644
--- a/fs/exfat/exfat_fs.h
+++ b/fs/exfat/exfat_fs.h
@@ -203,6 +203,7 @@ struct exfat_mount_options {
 	/* on error: continue, panic, remount-ro */
 	enum exfat_error_mode errors;
 	unsigned utf8:1, /* Use of UTF-8 character set */
+		 sys_tz:1, /* Use local timezone */
 		 discard:1, /* Issue discard requests on deletions */
 		 keep_last_dots:1; /* Keep trailing periods in paths */
 	int time_offset; /* Offset of timestamps from UTC (in minutes) */
diff --git a/fs/exfat/misc.c b/fs/exfat/misc.c
index d5bd8e6d9741..9380e0188b55 100644
--- a/fs/exfat/misc.c
+++ b/fs/exfat/misc.c
@@ -74,6 +74,13 @@ static void exfat_adjust_tz(struct timespec64 *ts, u8 tz_off)
 		ts->tv_sec += TIMEZONE_SEC(0x80 - tz_off);
 }
 
+static inline int exfat_tz_offset(struct exfat_sb_info *sbi)
+{
+	if (sbi->options.sys_tz)
+		return -sys_tz.tz_minuteswest;
+	return sbi->options.time_offset;
+}
+
 /* Convert a EXFAT time/date pair to a UNIX date (seconds since 1 1 70). */
 void exfat_get_entry_time(struct exfat_sb_info *sbi, struct timespec64 *ts,
 		u8 tz, __le16 time, __le16 date, u8 time_cs)
@@ -96,8 +103,7 @@ void exfat_get_entry_time(struct exfat_sb_info *sbi, struct timespec64 *ts,
 		/* Adjust timezone to UTC0. */
 		exfat_adjust_tz(ts, tz & ~EXFAT_TZ_VALID);
 	else
-		/* Convert from local time to UTC using time_offset. */
-		ts->tv_sec -= sbi->options.time_offset * SECS_PER_MIN;
+		ts->tv_sec -= exfat_tz_offset(sbi) * SECS_PER_MIN;
 }
 
 /* Convert linear UNIX date to a EXFAT time/date pair. */
diff --git a/fs/exfat/super.c b/fs/exfat/super.c
index 8ca21e7917d1..3e0f67b2103e 100644
--- a/fs/exfat/super.c
+++ b/fs/exfat/super.c
@@ -170,7 +170,9 @@ static int exfat_show_options(struct seq_file *m, struct dentry *root)
 		seq_puts(m, ",discard");
 	if (opts->keep_last_dots)
 		seq_puts(m, ",keep_last_dots");
-	if (opts->time_offset)
+	if (opts->sys_tz)
+		seq_puts(m, ",sys_tz");
+	else if (opts->time_offset)
 		seq_printf(m, ",time_offset=%d", opts->time_offset);
 	return 0;
 }
@@ -214,6 +216,7 @@ enum {
 	Opt_errors,
 	Opt_discard,
 	Opt_keep_last_dots,
+	Opt_sys_tz,
 	Opt_time_offset,
 
 	/* Deprecated options */
@@ -241,6 +244,7 @@ static const struct fs_parameter_spec exfat_parameters[] = {
 	fsparam_enum("errors",			Opt_errors, exfat_param_enums),
 	fsparam_flag("discard",			Opt_discard),
 	fsparam_flag("keep_last_dots",		Opt_keep_last_dots),
+	fsparam_flag("sys_tz",			Opt_sys_tz),
 	fsparam_s32("time_offset",		Opt_time_offset),
 	__fsparam(NULL, "utf8",			Opt_utf8, fs_param_deprecated,
 		  NULL),
@@ -298,6 +302,9 @@ static int exfat_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	case Opt_keep_last_dots:
 		opts->keep_last_dots = 1;
 		break;
+	case Opt_sys_tz:
+		opts->sys_tz = 1;
+		break;
 	case Opt_time_offset:
 		/*
 		 * Make the limit 24 just in case someone invents something
-- 
2.34.1