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

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Date:   Wed, 15 Jan 2020 22:30:59 +0900
From:   Namjae Jeon <linkinjeon@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Namjae Jeon <namjae.jeon@...sung.com>,
        "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,
        Pali Rohár <pali.rohar@...il.com>
Subject: Re: [PATCH v10 09/14] exfat: add misc operations

2020-01-15 19:10 GMT+09:00, Arnd Bergmann <arnd@...db.de>:
> 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.
Thanks.
>
>> +       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,
It is not described in the specification. I don't know exactly what
the problem is because sys_tz.tz_minuteswest seems to work fine to me.
It can be random garbage value ?
> so if there is a choice, falling back to UTC would
> be nicer.
Okay.
>
>> +/* 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.
Okay.
>
> 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.
Okay, I will check it.
Thanks for your review!
>
>       Arnd
>