Message-ID: <4E12ECBF.2060307@rs.jp.nec.com>
Date:	Tue, 05 Jul 2011 19:51:43 +0900
From:	Akira Fujita <a-fujita@...jp.nec.com>
To:	Andreas Dilger <adilger@...ger.ca>, Mark Harris <mhlk@....us>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [BUG] ext4 timestamps corruption

Hi, Andreas and Mark,
Thank you for looking at this issue.

(2011/06/27 18:04), Andreas Dilger wrote:
> On 2011-06-24, at 11:12 PM, Mark Harris wrote:
>> On Fri, Jun 24, 2011 at 15:46, Andreas Dilger wrote:
>>> The problem with this encoding is that it requires existing 32-bit
>>> timestamps before 1970 to have encoded "11" in the extra epoch bits,
>>> which is not the case.  Current pre-1970 timestamps would be encoded
>>> with "00" there, which would (according to your table) bump them past
>>> 2038 incorrectly.
>>
>> I was under the impression that the encoding code stored bits
>> 33&  32 of tv_sec there, which would be 1,1 for a negative value
>> like -1.  Certainly the decoding would be simpler if the extra
>> value was only non-zero for large timestamps.
> 
> One problem with a symmetrical encoding is that it wastes half of the
> dynamic range for values that nobody will ever use.  Even values before
> 1970 seem so unlikely that I question whether we should support them
> at all.
> 
>> On closer inspection of ext4_encode_extra_time, it looks like for
>> tv_sec = -1, a 64-bit kernel will store 1,1 in the extra bits and
>> a 32-bit kernel will store 0,0 in the extra bits.  That is a problem
>> if both of these need to be decoded as -1 on a 64-bit system.
> 
> That is definitely a problem.
> 
>>> What we need is an encoding that preserves the times for extra epoch "00":
>>>
>>>   2    msb of                         adjustment needed to convert
>>> extra  32-bit                         sign-extended 32-bit tv_sec
>>>   bits   time   decoded 64-bit tv_sec   to decoded 64-bit tv_sec
>>>   0 0     1    -0x80000000..-1           0
>>>   0 0     0    0x000000000..0x07fffffff  0
>>>   0 1     1    0x080000000..0x0ffffffff  0x100000000
>>>   0 1     0    0x100000000..0x17fffffff  0x100000000
>>>   1 0     1    0x180000000..0x1ffffffff  0x200000000
>>>   1 0     0    0x200000000..0x27fffffff  0x200000000
>>>   1 1     1    0x280000000..0x2ffffffff  0x300000000
>>>   1 1     0    0x300000000..0x37fffffff  0x300000000
>>>
>>> So, looking at the above desired encoding, it looks like the error in
>>> the existing code is that it is doing a boolean operation on decode
>>> instead of a mathematical one, and it was incorrectly trying to extend
>>> the time to (1ULL<<34).  The below should fix this:
>>>
>>> static inline void ext4_decode_extra_time(struct timespec *time, __le32 extra)
>>> {
>>>         if (unlikely(sizeof(time->tv_sec)>  4&&
>>>                      (extra&  cpu_to_le32(EXT4_EPOCH_MASK)))
>>>                 time->tv_sec += (u64)(le32_to_cpu(extra)&  EXT4_EPOCH_MASK)<<  32;
>>>
>>>         time->tv_nsec = (le32_to_cpu(extra)&  EXT4_NSEC_MASK)>>  EXT4_EPOCH_BITS;
>>> }
>>
>> That is not compatible with the existing ext4_encode_extra_time.
>> For example, 2038-01-31 (0x80101500) is encoded with extra bits
>> equal to bits 33&  32, i.e. 0,0.  But this code would decode it
>> as 1901-12-25 (i.e. it would leave the sign-extended 32-bit value
>> unchanged).
> 
> Part of the problem is that the encoding/decoding of timestamps beyond
> 2038 is already broken today, so I don't think anyone has been using
> them so far.  This gives us some leeway for fixing this problem I think.
> 
>> Possible solutions:
>>
>> (a) Define the current 64-bit encoding as the correct encoding since
>>      the 2 extra bits are not even decoded on 32-bit kernels, so its
>>      encoding doesn't matter much.  However, if anyone with existing
>>      pre-1970 timestamps written using a 32-bit kernel wants to use
>>      their ext4 filesystem with a 64-bit kernel, the pre-1970
>>      timestamps would be wrong unless they re-write them with a
>>      fixed kernel.
>>
>>      Change ext4_decode_extra_time "if" body to something like:
>>         time->tv_sec += ((__u32)time->tv_sec +
>>                 ((__u64)le32_to_cpu(extra)<<  32) +
>>                 0x80000000LL)&  0x300000000LL;
>>
>>      Change ext4_encode_extra_time ": 0" to something like:
>>         time->tv_sec<  0 ? EXT4_EPOCH_MASK : 0
> 
> The real-world problem isn't with 32-bit systems, where it doesn't
> really matter at all how time is encoded, nor with files on 64-bit systems
> with timestamps 26 years in the future, but rather 256-byte inodes that
> were previously written with ext3 that will break if they are mounted
> with ext4 on a 64-bit system.
> 
>> (b) Change the encoding of the extra bits to be those in your new
>>      table.  This is compatible with the 32-bit kernel encoding
>>      (which does not decode these bits) but incompatible with the
>>      64-bit kernel encoding.  Existing pre-1970 timestamps written
>>      with a 64-bit kernel would be decoded as dates far in the future.
>>
>>      Requires your change to ext4_decode_extra_time.
>>      Also requires a change to ext4_encode_extra_time, changing
>>      (time->tv_sec>>  32) to something like:
>>         ((time->tv_sec - (signed int)time->tv_sec)>>  32)
> 
> I think this is a reasonable solution, but I dislike that it breaks
> pre-1970 timestamps on 64-bit systems.

I agree with this solution.
I guess that no one has pre-1970 timestamps on ext4, actually.

Mark, are you working on this right now?
If you have a patch to fix this issue, please send it to the list.

Regards,
Akira Fujita
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