[PATCH v5 1/2] ext4: Fix handling of extended tv_sec (bug 23732)

[David Turner <novalis@novalis.org>]

Thanks for your patient and detailed comments.
--

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.

Signed-off-by: David Turner <novalis@novalis.org>
Reported-by: Mark Harris <mh8928@yahoo.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=23732
---
 fs/ext4/ext4.h | 61 +++++++++++++++++++++++++++++++++++++---------------------
 1 file changed, 39 insertions(+), 22 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 18aa56b..ac54dac 100644
--- 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>
@@ -713,38 +714,54 @@ struct move_extent {
 	  sizeof((ext4_inode)->field))			\
 	<= (EXT4_GOOD_OLD_INODE_SIZE +			\
 	    (einode)->i_extra_isize))			\
+
 /*
- * We use the bottom 34 bits of the signed 64-bit time value, with
- * the top two of these bits in the bottom of extra.  This leads
- * to a slightly odd encoding, which works like this:
+ * We need is an encoding that preserves the times for extra epoch "00":
  *
- * extra  msb of
- * epoch  32-bit
- * bits   time    decoded 64-bit tv_sec   valid time range
- * 0 0    0    0x000000000..0x07fffffff  1970-01-01..2038-01-19
- * 0 0    1    0x080000000..0x0ffffffff  2038-01-19..2106-02-07
- * 0 1    0    0x100000000..0x17fffffff  2106-02-07..2174-02-25
- * 0 1    1    0x180000000..0x1ffffffff  2174-02-25..2242-03-16
- * 1 0    0    0x200000000..0x27fffffff  2242-03-16..2310-04-04
- * 1 0    1    0x280000000..0x2ffffffff  2310-04-04..2378-04-22
- * 1 1    0    0x300000000..0x37fffffff  2378-04-22..2446-05-10
-
- * 1 1    1    -0x80000000..-0x00000001  1901-12-13..1969-12-31
+ * 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.
+		 */
+		u64 extra_bits = le32_to_cpu(extra) & EXT4_EPOCH_MASK;
+		if (extra_bits == 3)
+			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)			       \
-- 
1.8.1.2