linux-kernel - Re: [PATCH 1/3] hfs: stop using timespec based interfaces

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Date:   Wed, 20 Jun 2018 09:55:08 -0700
From:   Viacheslav Dubeyko <slava@...eyko.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        Jeff Layton <jlayton@...hat.com>, Jan Kara <jack@...e.cz>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] hfs: stop using timespec based interfaces

On Tue, 2018-06-19 at 21:42 +0200, Arnd Bergmann wrote:
> On Tue, Jun 19, 2018 at 7:03 PM, Viacheslav Dubeyko <slava@...eyko.co
> m> wrote:
> > 
> > On Tue, 2018-06-19 at 18:02 +0200, Arnd Bergmann wrote:
> > > 
> > > The native HFS timestamps overflow in year 2040, two years after
> > > the
> > > Unix
> > > y2038 overflow. However, the way that the conversion between on-
> > > disk
> > > timestamps and in-kernel timestamps was implemented, 64-bit
> > > machines
> > > actually ended up converting negative UTC timestamps (1902
> > > through
> > > 1969)
> > > into times between 2038 and 2106.
> > > 
> > > Rather than making all machines faithfully represent timestamps
> > > in
> > > the
> > > ancient past but break after 2040, this changes the file system
> > > to
> > > always use the unsigned UTC interpretation, reading back times
> > > between
> > > 1970 and 2106.
> > > 
> > The trouble with HFS and HFS+ that the specification [1] declares
> > this:
> > 
> > "HFS Plus stores dates in several data structures, including the
> > volume
> > header and catalog records. These dates are stored in unsigned 32-
> > bit
> > integers (UInt32) containing the number of seconds since midnight,
> > January 1, 1904, GMT. This is slightly different from HFS, where
> > the
> > value represents local time. The maximum representable date is
> > February
> > 6, 2040 at 06:28:15 GMT."
> > 
> > So, I am not sure that we are able to support later dates because
> > such
> > timestamps cannot be stored on HFS/HFS+ volumes and will be
> > incompatible with Mac OS X.
> We never followed that interpretation in Linux though. As I wrote,
> on 64-bit machines, these two calculations (hfs and hfs+,
> respectively)
> 
> #define __hfs_m_to_utime(sec)   (be32_to_cpu(sec) - 2082844800U  +
> sys_tz.tz_minuteswest * 60)
> #define __hfsp_mt2ut(t)                (be32_to_cpu(t) - 2082844800U)
> 
> just wrap around when reading the timestamps before 1970 from
> disk. On 32-bit machines they get wrapped another time when
> we assign them to a signed 32-bit time_t.
> 

The whole patchset looks reasonable for me. I simply guess what the
correct behaviour of HFS/HFS+ file system driver could look like for
the case of achieving 2040 year. So, maybe the good way could be to
mount in the READ-ONLY mode. What do you think? 

> > 
> > Also, I am not sure that anybody will use HFS/HFS+ after 2040.
> I'm trying to fix all file systems to be unambiguous regarding
> inode timestamps. This means it should behave the same way
> on 32-bit and 64-bit kernels, and if possible in a sane way.
> 
> Even if you don't care about running HFS in the future, you
> can trivially create files with arbitrary timestamps, just try
> 
> touch -d "Jan 1 1901" 1901
> touch -d "Jan 1 1905" 1905
> touch -d "Jan 1 1969" 1969
> touch -d "Jan 1 2038" 2038
> touch -d "Jan 1 2040" 2040
> touch -d "Jan 1 2106" 2106
> touch -d "Jan 1 2107" 2107
> 
> on HFS and do an 'ls -l' after an unmount/remount.
> 
> If you think it's important that we change the current behavior
> to be compatible with MacOS and represent the 1904..2040
> time range rather than 1970..2106, we can definitely do that
> as well, using this patch:
> 
> diff --git a/fs/hfs/hfs_fs.h b/fs/hfs/hfs_fs.h
> index ff432931a5b1..2c7366342656 100644
> --- a/fs/hfs/hfs_fs.h
> +++ b/fs/hfs/hfs_fs.h
> @@ -249,7 +249,7 @@ extern void hfs_mark_mdb_dirty(struct super_block
> *sb);
>   * actually works until year 2106
>   */
>  #define __hfs_u_to_mtime(sec)  cpu_to_be32(sec + 2082844800U -
> sys_tz.tz_minuteswest * 60)
> -#define __hfs_m_to_utime(sec)  (be32_to_cpu(sec) - 2082844800U  +
> sys_tz.tz_minuteswest * 60)
> +#define __hfs_m_to_utime(sec)  ((time64_t)be32_to_cpu(sec) -
> 2082844800U  + sys_tz.tz_minuteswest * 60)
> 
>  #define HFS_I(inode)   (container_of(inode, struct hfs_inode_info,
> vfs_inode))
>  #define HFS_SB(sb)     ((struct hfs_sb_info *)(sb)->s_fs_info)
> diff --git a/fs/hfsplus/hfsplus_fs.h b/fs/hfsplus/hfsplus_fs.h
> index 1a6b469f8d22..4eaee8bdfcb2 100644
> --- a/fs/hfsplus/hfsplus_fs.h
> +++ b/fs/hfsplus/hfsplus_fs.h
> @@ -534,7 +534,7 @@ int hfsplus_read_wrapper(struct super_block *sb);
> 
>  /* time macros: convert between 1904-2040 and 1970-2106 range,
>   * pre-1970 timestamps are interpreted as post-2038 times after
> wrap-around */
> -#define __hfsp_mt2ut(t)                (be32_to_cpu(t) -
> 2082844800U)
> +#define __hfsp_mt2ut(t)                ((time64_t)be32_to_cpu(t) -
> 2082844800U)
>  #define __hfsp_ut2mt(t)                (cpu_to_be32(t +
> 2082844800U))
> 
>  /* compatibility */
> 
> I can submit that separately so that it can get backported into
> stable kernels if you like, with the type changes as a follow-up
> on top.
> 

Sounds good.

Thanks,
Vyacheslav Dubeyko.