Re: [PATCH 3.2 33/77] ext4: Fix handling of extended tv_sec

[David Turner <novalis@novalis.org>]

LGTM.

On Thu, 2015-12-24 at 15:37 +0000, Ben Hutchings wrote:
> 3.2.75-rc1 review patch.  If anyone has any objections, please let me
> know.
> 
> ------------------
> 
> From: David Turner <novalis@novalis.org>
> 
> commit a4dad1ae24f850410c4e60f22823cba1289b8d52 upstream.
> 
> In ext4, the bottom two bits of {a,c,m}time_extra are used to extend
> the {a,c,m}time fields, deferring the year 2038 problem to the year
> 2446.
> 
> When decoding these extended fields, for times whose bottom 32 bits
> would represent a negative number, sign extension causes the 64-bit
> extended timestamp to be negative as well, which is not what's
> intended.  This patch corrects that issue, so that the only negative
> {a,c,m}times are those between 1901 and 1970 (as per 32-bit signed
> timestamps).
> 
> Some older kernels might have written pre-1970 dates with 1,1 in the
> extra bits.  This patch treats those incorrectly-encoded dates as
> pre-1970, instead of post-2311, until kernel 4.20 is released.
> Hopefully by then e2fsck will have fixed up the bad data.
> 
> Also add a comment explaining the encoding of ext4's extra
> {a,c,m}time
> bits.
> 
> Signed-off-by: David Turner <novalis@novalis.org>
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
> Reported-by: Mark Harris <mh8928@yahoo.com>
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=23732
> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> ---
>  fs/ext4/ext4.h | 51 ++++++++++++++++++++++++++++++++++++++++++++----
> ---
>  1 file changed, 44 insertions(+), 7 deletions(-)
> 
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -26,6 +26,7 @@
>  #include <linux/seqlock.h>
>  #include <linux/mutex.h>
>  #include <linux/timer.h>
> +#include <linux/version.h>
>  #include <linux/wait.h>
>  #include <linux/blockgroup_lock.h>
>  #include <linux/percpu_counter.h>
> @@ -698,19 +699,55 @@ struct move_extent {
>  	<= (EXT4_GOOD_OLD_INODE_SIZE +			\
>  	    (einode)->i_extra_isize))			\
>  
> +/*
> + * We use an encoding that preserves the times for extra epoch "00":
> + *
> + * extra  msb of                         adjust for signed
> + * epoch  32-bit                         32-bit tv_sec to
> + * bits   time    decoded 64-bit tv_sec  64-bit tv_sec      valid
> time range
> + * 0 0    1    -0x80000000..-0x00000001  0x000000000 1901-12
> -13..1969-12-31
> + * 0 0    0    0x000000000..0x07fffffff  0x000000000 1970-01
> -01..2038-01-19
> + * 0 1    1    0x080000000..0x0ffffffff  0x100000000 2038-01
> -19..2106-02-07
> + * 0 1    0    0x100000000..0x17fffffff  0x100000000 2106-02
> -07..2174-02-25
> + * 1 0    1    0x180000000..0x1ffffffff  0x200000000 2174-02
> -25..2242-03-16
> + * 1 0    0    0x200000000..0x27fffffff  0x200000000 2242-03
> -16..2310-04-04
> + * 1 1    1    0x280000000..0x2ffffffff  0x300000000 2310-04
> -04..2378-04-22
> + * 1 1    0    0x300000000..0x37fffffff  0x300000000 2378-04
> -22..2446-05-10
> + *
> + * Note that previous versions of the kernel on 64-bit systems would
> + * incorrectly use extra epoch bits 1,1 for dates between 1901 and
> + * 1970.  e2fsck will correct this, assuming that it is run on the
> + * affected filesystem before 2242.
> + */
> +
>  static inline __le32 ext4_encode_extra_time(struct timespec *time)
>  {
> -       return cpu_to_le32((sizeof(time->tv_sec) > 4 ?
> -			   (time->tv_sec >> 32) & EXT4_EPOCH_MASK :
> 0) |
> -                          ((time->tv_nsec << EXT4_EPOCH_BITS) &
> EXT4_NSEC_MASK));
> +	u32 extra = sizeof(time->tv_sec) > 4 ?
> +		((time->tv_sec - (s32)time->tv_sec) >> 32) &
> EXT4_EPOCH_MASK : 0;
> +	return cpu_to_le32(extra | (time->tv_nsec <<
> EXT4_EPOCH_BITS));
>  }
>  
>  static inline void ext4_decode_extra_time(struct timespec *time,
> __le32 extra)
>  {
> -       if (sizeof(time->tv_sec) > 4)
> -	       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;
> +	if (unlikely(sizeof(time->tv_sec) > 4 &&
> +			(extra & cpu_to_le32(EXT4_EPOCH_MASK)))) {
> +#if LINUX_VERSION_CODE < KERNEL_VERSION(4,20,0)
> +		/* Handle legacy encoding of pre-1970 dates with
> epoch
> +		 * bits 1,1.  We assume that by kernel version 4.20,
> +		 * everyone will have run fsck over the affected
> +		 * filesystems to correct the problem.  (This
> +		 * backwards compatibility may be removed before
> this
> +		 * time, at the discretion of the ext4 developers.)
> +		 */
> +		u64 extra_bits = le32_to_cpu(extra) &
> EXT4_EPOCH_MASK;
> +		if (extra_bits == 3 && ((time->tv_sec) & 0x80000000)
> != 0)
> +			extra_bits = 0;
> +		time->tv_sec += extra_bits << 32;
> +#else
> +		time->tv_sec += (u64)(le32_to_cpu(extra) &
> EXT4_EPOCH_MASK) << 32;
> +#endif
> +	}
> +	time->tv_nsec = (le32_to_cpu(extra) & EXT4_NSEC_MASK) >>
> EXT4_EPOCH_BITS;
>  }
>  
>  #define EXT4_INODE_SET_XTIME(xtime, inode, raw_inode)		
> 	       \
>