[PATCH] ext4: Fix reading of extended tv_sec (bug 23732)

[David Turner <novalis@novalis.org>]

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.  The representation (which this patch does not alter) is a bit
hackish, in that the most-significant bit is no longer (alone)
sufficient to indicate the sign.  That's because we're representing an
asymmetric range, with seven times as many positive values as
negative.

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).

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 | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index af815ea..7b73c26 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -722,10 +722,15 @@ static inline __le32 ext4_encode_extra_time(struct timespec *time)
 
 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 (sizeof(time->tv_sec) > 4) {
+		u64 extra_bits = (__u64)(le32_to_cpu(extra) & EXT4_EPOCH_MASK);
+		if (time->tv_sec > 0 || extra_bits != EXT4_EPOCH_MASK) {
+			time->tv_sec &= 0xFFFFFFFF;
+			time->tv_sec |= extra_bits << 32;
+		}
+		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