linux-kernel - Re: [PATCH] hfs/hfsplus: use documented official timestamp range

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Date:   Sat, 23 Jun 2018 03:00:54 -0300
From:   Ernesto A. Fernández 
        <ernesto.mnd.fernandez@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Vyacheslav Dubeyko <slava@...eyko.com>, y2038@...ts.linaro.org,
        stable@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hfs/hfsplus: use documented official timestamp range

On Fri, Jun 22, 2018 at 04:17:32PM +0200, Arnd Bergmann wrote:
> According to the official documentation for HFS+ [1], inode timestamps
> are supposed to cover the time range from 1904 to 2040 as originally
> used in classic MacOS.
> 
> The traditional Linux usage is to convert the timestamps into an unsigned
> 32-bit number based on the Unix epoch and from there to a time_t. On
> 32-bit systems, that wraps the time from 2038 to 1902, so the last
> two years of the valid time range become garbled. On 64-bit systems,
> all times before 1970 get turned into timestamps between 2038 and 2106,
> which is more convenient but also different from the documented behavior.
> 
> The same behavior is used in Darwin and presumaby all versions of MacOS X,
> as seen in the to_hfs_time() function in [2]. It is unclear whether this
> is a bug in the file system code, or intentional but undocumented behavior.

But the to_bsd_time() function considers wrapped timestamps as invalid,
doesn't it? So it seems they simply don't care about the post-2040 (or
pre-1970) case?

> 
> This changes Linux over to the traditional MacOS (pre MacOS X)
> behavior. This means all files that are created on MacOS X or Linux
> with future timestamps between 2040 and 2106 will now show up as past
> dates. Timestamps between 2038 and 2040 will still be represented
> incorrectly on 32-bit architectures as times between 1902 and 1904,
> but that will be fixed once we have user space with 64-bit time_t.
> 
> Cc: stable@...r.kernel.org
> Link: [1] https://developer.apple.com/library/archive/technotes/tn/tn1150.html
> Link: [2] https://opensource.apple.com/source/xnu/xnu-344/bsd/hfs/MacOSStubs.c
> Suggested-by: Viacheslav Dubeyko <slava@...eyko.com>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> Note: This is the patch that Viacheslav asked for, but given how
> MacOS X behaves, I'm increasingly thinking this is a bad idea.

I agree that it made more sense before.

> ---
>  fs/hfs/hfs_fs.h         | 2 +-
>  fs/hfsplus/hfsplus_fs.h | 5 +++--
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/hfs/hfs_fs.h b/fs/hfs/hfs_fs.h
> index 6d0783e2e276..39c1f3a43ed8 100644
> --- a/fs/hfs/hfs_fs.h
> +++ b/fs/hfs/hfs_fs.h
> @@ -247,7 +247,7 @@ extern void hfs_mark_mdb_dirty(struct super_block *sb);
>   *
>   */
>  #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 d9255abafb81..57838ef4dcdc 100644
> --- a/fs/hfsplus/hfsplus_fs.h
> +++ b/fs/hfsplus/hfsplus_fs.h
> @@ -530,8 +530,9 @@ int hfsplus_submit_bio(struct super_block *sb, sector_t sector, void *buf,
>  		       void **data, int op, int op_flags);
>  int hfsplus_read_wrapper(struct super_block *sb);
>  
> -/* time macros */
> -#define __hfsp_mt2ut(t)		(be32_to_cpu(t) - 2082844800U)
> +/* time macros: convert between 1904-2040 and 1970-2106 range,
> + * pre-1970 timestamps are interpreted as post-2038 times after wrap-around */

This comment seems to be from the original series, maybe you forgot to
change it?

> +#define __hfsp_mt2ut(t)		((time64_t)be32_to_cpu(t) - 2082844800U)
>  #define __hfsp_ut2mt(t)		(cpu_to_be32(t + 2082844800U))
>  
>  /* compatibility */
> -- 
> 2.9.0
>