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Date:   Wed, 11 Jul 2018 19:46:26 -0300
From:   Ernesto =?utf-8?Q?A=2E_Fern=C3=A1ndez?= 
        <ernesto.mnd.fernandez@gmail.com>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Andrew Morton <akpm@linux-foundation.org>, y2038@lists.linaro.org,
        Al Viro <viro@zeniv.linux.org.uk>, stable@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Vyacheslav Dubeyko <slava@dubeyko.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] [v2] hfs/hfsplus: follow MacOS time behavior
Message-ID: <20180711224625.airwna6gzyatoowe@eaf>
References: <20180710214131.4106527-1-arnd@arndb.de>
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Hi:

On Tue, Jul 10, 2018 at 11:40:50PM +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.
> 
> Looking at the Darwin sources [2], it seems that MacOS is inconsistent in
> yet another way: all timestamps are wrapped around to a 32-bit unsigned
> number when written to the disk, but when read back, all numeric values
> lower than 2082844800U are assumed to be invalid, so we cannot represent
> the times before 1970 or the times after 2040.
> 
> While all implementations seem to agree on the interpretation of values
> between 1970 and 2038, they often differ on the exact range they support
> when reading back values outside of the common range:
> 
> MacOS (traditional):		1904-2040
> Apple Documentation:		1904-2040
> MacOS X source comments:	1970-2040
> MacOS X source code:		1970-2038
> 32-bit Linux:			1902-2038
> 64-bit Linux:			1970-2106
> hfsfuse:			1970-2040
> hfsutils (32 bit, old libc)	1902-2038
> hfsutils (32 bit, new libc)	1970-2106
> hfsutils (64 bit)		1904-2040
> hfsplus-utils			1904-2040
> hfsexplorer			1904-2040
> 7-zip				1904-2040
> 
> This changes Linux over to mostly the same behavior as described in the
> code comment in MacOS X, disallowing all times before 1970 and after
> 2040, while still allowing times between 2038 and 2040 like most other
> implementations do. Most importantly, it means we can have the same
> behavior on 32-bit and 64-bit.
> 
> Cc: stable@vger.kernel.org
> Link: [1] https://developer.apple.com/library/archive/technotes/tn/tn1150.html
> Link: [2] https://opensource.apple.com/source/hfs/hfs-407.30.1/core/MacOSStubs.c.auto.html
> Suggested-by: Viacheslav Dubeyko <slava@dubeyko.com>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> v2: treat pre-1970 dates as invalid following MacOS X behavior,
>     reword and expand changelog text
> ---
>  fs/hfs/hfs_fs.h         | 29 +++++++++++++++++++++++++----
>  fs/hfsplus/hfsplus_fs.h | 26 +++++++++++++++++++++++---
>  2 files changed, 48 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/hfs/hfs_fs.h b/fs/hfs/hfs_fs.h
> index 6d0783e2e276..1af998fb522e 100644
> --- a/fs/hfs/hfs_fs.h
> +++ b/fs/hfs/hfs_fs.h
> @@ -246,14 +246,35 @@ extern void hfs_mark_mdb_dirty(struct super_block *sb);
>   *	mac:	unsigned big-endian since 00:00 GMT, Jan. 1, 1904
>   *
>   */
> -#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)
> +static inline time64_t __hfs_m_to_utime(__be32 mt)
> +{
> +	time64_t ut = (u32)(be32_to_cpu(mt) - 2082844800U);
> +
> +	/*
> +	 * Times past 2040-02-06 06:28 are assumed to be invalid,
> +	 * matching the MacOS behavior.
> +	 */
> +	if (ut > 2082844800U + UINT_MAX)

I'm not sure what you were going for here, but this isn't right. Times
as early as 2036 will be considered invalid.

> +		ut = 0;
> +
> +	return ut + sys_tz.tz_minuteswest * 60;
> +}
>  
> +static inline __be32 __hfs_u_to_mtime(time64_t ut)
> +{
> +	ut -= - sys_tz.tz_minuteswest * 60;
	   ^^^^^
	An extra minus.

> +
> +	/*
> +	 * MacOS wraps "invalid" times after 2040 when writing back, so
> +	 * let's do the same here.
> +	 */
> +	return cpu_to_be32(lower_32_bits(ut + 2082844800U));
> +}
>  #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)
>  
> -#define hfs_m_to_utime(time)	(struct timespec){ .tv_sec = __hfs_m_to_utime(time) }
> -#define hfs_u_to_mtime(time)	__hfs_u_to_mtime((time).tv_sec)
> +#define hfs_m_to_utime(time)   (struct timespec){ .tv_sec = __hfs_m_to_utime(time) }
> +#define hfs_u_to_mtime(time)   __hfs_u_to_mtime((time).tv_sec)

Are the new spaces intentional?

>  #define hfs_mtime()		__hfs_u_to_mtime(get_seconds())
>  
>  static inline const char *hfs_mdb_name(struct super_block *sb)
> diff --git a/fs/hfsplus/hfsplus_fs.h b/fs/hfsplus/hfsplus_fs.h
> index d9255abafb81..7f0943e540a0 100644
> --- a/fs/hfsplus/hfsplus_fs.h
> +++ b/fs/hfsplus/hfsplus_fs.h
> @@ -530,9 +530,29 @@ 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)
> -#define __hfsp_ut2mt(t)		(cpu_to_be32(t + 2082844800U))
> +/* time helpers */
> +static inline time64_t __hfsp_mt2ut(__be32 mt)
> +{
> +	time64_t ut = (u32)(be32_to_cpu(mt) - 2082844800U);
> +
> +	/*
> +	 * Times past 2040-02-06 06:28 are assumed to be invalid,
> +	 * matching the MacOS behavior.
> +	 */
> +	if (ut > 2082844800U + UINT_MAX)

Same as before, 2036-2040 will be invalid.


For the record, your original solution (supporting the 1970-2106 range)
still makes more sense to me. It seems Apple is not using the 1904-1970
range for anything; if they are still supporting hfsplus by the 2030s I
assume they will deal with this in a similar way.

Thanks,
Ernest

> +		ut = 0;
> +
> +	return ut;
> +}
> +
> +static inline __be32 __hfsp_ut2mt(time64_t ut)
> +{
> +	/*
> +	 * MacOS wraps "invalid" times after 2040 when writing back, so
> +	 * let's do the same here.
> +	 */
> +	return cpu_to_be32(lower_32_bits(ut + 2082844800U));
> +}
>  
>  /* compatibility */
>  #define hfsp_mt2ut(t)		(struct timespec){ .tv_sec = __hfsp_mt2ut(t) }
> -- 
> 2.9.0
>