On Mon, Feb 15, 2016 at 4:01 AM, Deepa Dinamani deepa.kernel@gmail.com wrote:
long/ kernel_time_t is 32 bit on a 32 bit system and 64 bit on a 64 bit system.
ceph_encode_timespec() encodes only the lower 32 bits on a 64 bit system and encodes all of 32 bits on a 32bit system.
ceph_decode_timespec() decodes 32 bit tv_sec and tv_nsec into kernel_time_t/ long.
The encode and decode functions do not match when the values are negative:
Consider the following scenario on a 32 bit system: When a negative number is cast to u32 as encode does, the value is positive and is greater than INT_MAX. Decode reads back this value. And, this value cannot be represented by long on 32 bit systems. So by section 6.3.1.3 of the C99 standard, the result is implementation defined.
Consider the following scenario on a 64 bit system: When a negative number is cast to u32 as encode does, the value is positive. This value is later assigned by decode function by a cast to long. Since this value can be represented in long data type, this becomes a positive value greater than INT_MAX. But, the value encoded was negative, so the encode and decode functions do not match.
Change the decode function as follows to overcome the above bug: The decode should first cast the value to a s64 this will be positive value greater than INT_MAX(in case of a negative encoded value)and then cast this value again as s32, which drops the higher order 32 bits. On 32 bit systems, this is the right value in kernel_time_t/ long. On 64 bit systems, assignment to kernel_time_t/ long will sign extend this value to reflect the signed bit encoded.
Assume ceph timestamp ranges permitted are 1902..2038.
Suggested-by: Arnd Bergmann arnd@arndb.de Signed-off-by: Deepa Dinamani deepa.kernel@gmail.com
include/linux/ceph/decode.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/linux/ceph/decode.h b/include/linux/ceph/decode.h index a6ef9cc..e777e99 100644 --- a/include/linux/ceph/decode.h +++ b/include/linux/ceph/decode.h @@ -137,8 +137,8 @@ bad: static inline void ceph_decode_timespec(struct timespec *ts, const struct ceph_timespec *tv) {
ts->tv_sec = (__kernel_time_t)le32_to_cpu(tv->tv_sec);ts->tv_nsec = (long)le32_to_cpu(tv->tv_nsec);
ts->tv_sec = (s32)(s64)le32_to_cpu(tv->tv_sec);ts->tv_nsec = (s32)(s64)le32_to_cpu(tv->tv_nsec);} static inline void ceph_encode_timespec(struct ceph_timespec *tv, const struct timespec *ts)
I think you are forgetting that this is a wire protocol. Changing how values are interpreted on one side without looking at the other side is generally not what you want to do.
There aren't any casts on the server side: __u32 is simply assigned to time_t, meaning no sign-extension is happening - see src/include/utime.h in ceph.git. The current __kernel_time_t and long casts on the kernel side are meaningless - they don't change the bit pattern, so everything is interpreted the same way. Your patch changes that.
If there is anything to be done here, it is documenting the existing behavior. This isn't to say that there aren't any quirks or bugs in our time handling, it's that any changes concerning the protocol or how the actual bits are interpreted should follow or align with the rest of ceph and have a note in the changelog on that.
(As Greg and I mentioned before, we do have a semi-concrete plan on how to deal with y2038 in ceph, just no timetable yet.)
Thanks,
Ilya