When 256-byte inodes are used, ext4 attempts to extend the tv_sec
portion of file timestamps from 32 to 34 bits on 64-bit systems,
presumably to defer the Y2038 problem.  However the code to do
this still fails in 2038.  Nevertheless, some timestamps beyond
the year 2038 work correctly.

Using 2.6.35/x86_64 on ext4 with 256-byte inodes:

markh@zacc:~$ touch -d 2038-01-31 /test/2038
markh@zacc:~$ touch -d 2106-02-22 /test/2106
markh@zacc:~$ ls -l /test/2038 /test/2106
-rw-r--r-- 1 markh markh 0 2038-01-31 00:00 /test/2038
-rw-r--r-- 1 markh markh 0 2106-02-22 00:00 /test/2106
markh@zacc:~$ sudo umount /test
markh@zacc:~$ sudo mount /dev/sda2 /test
markh@zacc:~$ ls -l /test/2038 /test/2106
-rw-r--r-- 1 markh markh 0 1901-12-25 17:31 /test/2038
-rw-r--r-- 1 markh markh 0 2106-02-22 00:00 /test/2106
markh@zacc:~$ 

The problem is in fs/ext4/ext.h.  The macros EXT4_INODE_GET_XTIME
and EXT4_EINODE_GET_XTIME sign-extend the low 32 bits of tv_sec,
and then ext4_decode_extra_time uses "|=" to tack on the 2
additional bits.  However if bit 31 is 1, bits 32..63 will always
be 1 due to the sign extension regardless of the 2 extra 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;
}

...

#define EXT4_INODE_GET_XTIME(xtime, inode, raw_inode)			       \
do {									       \
	(inode)->xtime.tv_sec = (signed)le32_to_cpu((raw_inode)->xtime);       \
	if (EXT4_FITS_IN_INODE(raw_inode, EXT4_I(inode), xtime ## _extra))     \
		ext4_decode_extra_time(&(inode)->xtime,			       \
				       raw_inode->xtime ## _extra);	       \
} while (0)


It is not clear what time range was intended to be handled,
but the range should be continuous.  Preferably it would be the
range -0x80000000 (1901-12-13) to 0x37fffffff (2446-05-10),
in order to handle all legacy 32-bit timestamps and as many
consecutive future dates as possible.