On Wednesday 15 January 2020 16:03:12 Arnd Bergmann wrote: > On Wed, Jan 15, 2020 at 3:24 PM Pali Rohár <pali.rohar@xxxxxxxxx> wrote: > > On Wednesday 15 January 2020 14:50:10 Arnd Bergmann wrote: > > > On Wed, Jan 15, 2020 at 2:38 PM Pali Rohár <pali.rohar@xxxxxxxxx> wrote: > > > > On Wednesday 15 January 2020 22:30:59 Namjae Jeon wrote: > > > > > 2020-01-15 19:10 GMT+09:00, Arnd Bergmann <arnd@xxxxxxxx>: > > > However, in user space, every user may set their own timezone with > > > the 'TZ' variable, and the default timezone may be different inside of a > > > container based on the contents of /etc/timezone in its root directory. > > > > So kernel timezone is shared across all containers, right? > > Yes. > > > > You can use it to access removable media that were written in > > > a different timezone or a partition that is shared with another OS > > > running on the same machine but with different timezone settings. > > > > So... basically all userspace <--> kernel API which works with timestamp > > do not have information about timezone right? creat(), utime() or > > utimensat() just pass timestamp without timezone information. Is this > > timestamp mean to be in UTC or in local user time zone (as specified by > > user's TZ= env variable)? > > As a rule, all timekeeping in the kernel is done in terms of UTC. Ok, thanks! > You can > see what the current exceptions are using > > $ git grep -wl sys_tz > Documentation/filesystems/vfat.txt > arch/alpha/kernel/osf_sys.c > arch/nds32/kernel/vdso.c > arch/powerpc/kernel/time.c > arch/s390/kernel/time.c > arch/sparc/kernel/vdso.c > drivers/media/platform/vivid/vivid-rds-gen.c > drivers/media/platform/vivid/vivid-vbi-gen.c > drivers/net/ethernet/broadcom/bnxt/bnxt_ethtool.c > drivers/scsi/3w-9xxx.c > drivers/scsi/3w-sas.c > drivers/scsi/aacraid/commsup.c > drivers/scsi/arcmsr/arcmsr_hba.c > drivers/scsi/mvumi.c > drivers/scsi/mvumi.h > drivers/scsi/smartpqi/smartpqi_init.c > fs/affs/amigaffs.c > fs/affs/inode.c > fs/affs/super.c > fs/fat/misc.c > fs/hfs/hfs_fs.h > fs/hfs/inode.c > fs/hfs/sysdep.c > fs/hpfs/hpfs_fn.h > fs/udf/udftime.c > include/linux/time.h > kernel/debug/kdb/kdb_main.c > kernel/time/ntp.c > kernel/time/time.c > kernel/time/timekeeping.c > kernel/time/vsyscall.c > net/netfilter/nft_meta.c > net/netfilter/xt_time.c > tools/testing/selftests/x86/test_vdso.c > > The vdso and kernel/time/ code are for maintaining the timezone through > settimeofday()/gettimeofday(), and the drivers should probably all be changed > to use UTC. The file systems (affs, fat, hfs, hpfs and udf) do this for > compatibility with other operating systems that store the metadata in > localtime. Ok. But situation for exFAT is quite different. exFAT timestamp structure contains also timezone information. Other filesystems do not store timezone into their metadata (IIRC udf is exception and also stores timezone). So question is in which timezone we should store to exFAT timestamps. This is not for compatibility with legacy systems, but because of fact that filesystem supports feature which is not common for other filesystems. Also, to make it more complicated exFAT supports storing timestamps also in "unspecified" (local user) timezone, which matches other linux filesystems. So for storing timestamp we have options: * Store them without timezone * Store them in sys_tz timezone * Store them in timezone specified in mount option * Store them in UTC timezone And when reading timestamp from exFAT we need to handle both: * Timestamps without timezone * Timestamps with timezone So what is the best way to handle timezone/timestamps? For me it looks sane: When storing use: mount option timezone. When not available then use sys_tz. And when sys_tz is not set (it is possible?), do not specify timezone at all. Maybe there should be a mount option which says that timestamps on exfat are stored without timezone. When reading timestamp with timezone: Convert timestamp to timezone specified in mount option (or fallback to sys_tz or fallback to UTC). And when reading timestamp without timezone: Pass it as is without conversion, ignoring all timezone mount options and sys_tz. Arnd, what do you think about it? -- Pali Rohár pali.rohar@xxxxxxxxx
