linux-kernel - Re: [PATCH v10 09/14] exfat: add misc operations

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Date:   Wed, 15 Jan 2020 11:10:45 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Namjae Jeon <namjae.jeon@...sung.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        gregkh <gregkh@...uxfoundation.org>,
        Valdis Kletnieks <valdis.kletnieks@...edu>,
        Christoph Hellwig <hch@....de>, sj1557.seo@...sung.com,
        linkinjeon@...il.com,
        Pali Rohár <pali.rohar@...il.com>
Subject: Re: [PATCH v10 09/14] exfat: add misc operations

On Wed, Jan 15, 2020 at 9:28 AM Namjae Jeon <namjae.jeon@...sung.com> wrote:

> +#define SECS_PER_MIN    (60)
> +#define TIMEZONE_SEC(x)        ((x) * 15 * SECS_PER_MIN)
> +
> +static void exfat_adjust_tz(struct timespec64 *ts, u8 tz_off)
> +{
> +       if (tz_off <= 0x3F)
> +               ts->tv_sec -= TIMEZONE_SEC(tz_off);
> +       else /* 0x40 <= (tz_off & 0x7F) <=0x7F */
> +               ts->tv_sec += TIMEZONE_SEC(0x80 - tz_off);
> +}
> +
> +static inline int exfat_tz_offset(struct exfat_sb_info *sbi)
> +{
> +       if (sbi->options.time_offset)
> +               return sbi->options.time_offset;
> +       return sys_tz.tz_minuteswest;
> +}
> +
> +/* 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,
> +               __le16 time, __le16 date, u8 tz)
> +{
> +       u16 t = le16_to_cpu(time);
> +       u16 d = le16_to_cpu(date);
> +
> +       ts->tv_sec = mktime64(1980 + (d >> 9), d >> 5 & 0x000F, d & 0x001F,
> +                             t >> 11, (t >> 5) & 0x003F, (t & 0x001F) << 1);
> +       ts->tv_nsec = 0;

This part looks good to me now.

> +       if (tz & EXFAT_TZ_VALID)
> +               /* Treat as UTC time, but need to adjust timezone to UTC0 */
> +               exfat_adjust_tz(ts, tz & ~EXFAT_TZ_VALID);
> +       else
> +               /* Treat as local time */
> +               ts->tv_sec -= exfat_tz_offset(sbi) * SECS_PER_MIN;
> +}

Whereas this seems rather complex, when it deals with three different
cases:

- timezone stored in inode
- timezone offset passed as mount option
- local time from sys_tz.tz_minuteswest

Does the exfat specification require to use some notion of 'local time' here
as the fallback? The problem with sys_tz.tz_minuteswest is that it is
not too well-defined, so if there is a choice, falling back to UTC would
be nicer.

> +/* Convert linear UNIX date to a EXFAT time/date pair. */
> +void exfat_set_entry_time(struct exfat_sb_info *sbi, struct timespec64 *ts,
> +               __le16 *time, __le16 *date, u8 *tz)
> +{
> +       struct tm tm;
> +       u16 t, d;
> +
> +       /* clamp to the range valid in the exfat on-disk representation. */
> +       time64_to_tm(clamp_t(time64_t, ts->tv_sec, EXFAT_MIN_TIMESTAMP_SECS,
> +               EXFAT_MAX_TIMESTAMP_SECS), -exfat_tz_offset(sbi) * SECS_PER_MIN,
> +               &tm);

I think you can drop the clamping here, as thes_time_min/s_time_max fields
should take care of that.

For writing out timestamps, it may be best to always encode them as UTC
and set set timezone-valid bit for that. That way, the min/max values
are known at compile time regardless of which time zone the machine
thinks it is in.

      Arnd