Re: [PATCH 08/11] xfs: widen ondisk timestamps to deal with y2038 problem

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Aug 19, 2020 at 02:43:22PM -0700, Darrick J. Wong wrote:
> On Wed, Aug 19, 2020 at 09:35:35AM +1000, Dave Chinner wrote:
> > On Mon, Aug 17, 2020 at 03:57:39PM -0700, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > > 
> > > Redesign the ondisk timestamps to be a simple unsigned 64-bit counter of
> > > nanoseconds since 14 Dec 1901 (i.e. the minimum time in the 32-bit unix
> > > time epoch).  This enables us to handle dates up to 2486, which solves
> > > the y2038 problem.
> > > 
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > ---
> > .....
> > > +/* Convert an ondisk timestamp into the 64-bit safe incore format. */
> > >  void
> > >  xfs_inode_from_disk_timestamp(
> > > +	struct xfs_dinode		*dip,
> > >  	struct timespec64		*tv,
> > >  	const union xfs_timestamp	*ts)
> > >  {
> > > +	if (dip->di_version >= 3 &&
> > > +	    (dip->di_flags2 & cpu_to_be64(XFS_DIFLAG2_BIGTIME))) {
> > > +		uint64_t		t = be64_to_cpu(ts->t_bigtime);
> > > +		uint64_t		s;
> > > +		uint32_t		n;
> > > +
> > > +		s = div_u64_rem(t, NSEC_PER_SEC, &n);
> > > +		tv->tv_sec = s - XFS_INO_BIGTIME_EPOCH;
> > > +		tv->tv_nsec = n;
> > > +		return;
> > > +	}
> > > +
> > >  	tv->tv_sec = (int)be32_to_cpu(ts->t_sec);
> > >  	tv->tv_nsec = (int)be32_to_cpu(ts->t_nsec);
> > >  }
> > 
> > Can't say I'm sold on this union. It seems cleaner to me to just
> > make the timestamp an opaque 64 bit field on disk and convert it to
> > the in-memory representation directly in the to/from disk
> > operations. e.g.:
> > 
> > void
> > xfs_inode_from_disk_timestamp(
> > 	struct xfs_dinode		*dip,
> > 	struct timespec64		*tv,
> > 	__be64				ts)
> > {
> > 
> > 	uint64_t		t = be64_to_cpu(ts);
> > 	uint64_t		s;
> > 	uint32_t		n;
> > 
> > 	if (xfs_dinode_is_bigtime(dip)) {
> > 		s = div_u64_rem(t, NSEC_PER_SEC, &n) - XFS_INO_BIGTIME_EPOCH;
> > 	} else {
> > 		s = (int)(t >> 32);
> > 		n = (int)(t & 0xffffffff);
> > 	}
> > 	tv->tv_sec = s;
> > 	tv->tv_nsec = n;
> > }
> 
> I don't like this open-coded union approach at all because now I have to
> keep the t_sec and t_nsec bits separate in my head instead of letting
> the C compiler take care of that detail.  The sample code above doesn't
> handle that correctly either:
> 
> Start with an old kernel on a little endian system; each uppercase
> letter represents a byte (A is the LSB of t_sec, D is the MSB of t_sec,
> E is the LSB of t_nsec, and H is the MSB of t_nsec):
> 
> 	sec  nsec (incore)
> 	ABCD EFGH
> 
> That gets written out as:
> 
> 	sec  nsec (ondisk)
> 	DCBA HGFE
> 
> Now reboot with a new kernel that only knows 64bit timestamps on disk:
> 
> 	64bit (ondisk)
> 	DCBAHGFE
> 
> Now it does the first be64_to_cpu conversion:
> 	64bit (incore)
> 	EFGHABCD
> 
> And then masks and shifts:
> 	sec  nsec (incore)
> 	EFGH ABCD
> 
> Oops, we just switched the values!
> 
> The correct approach (I think) is to perform the shifting and masking on
> the raw __be64 value before converting them to incore format via
> be32_to_cpu, but now I have to work out all four cases by hand instead
> of letting the compiler do the legwork for me.  I don't remember if it's
> correct to go around shifting and masking __be64 values.
> 
> I guess the good news is that at least we have generic/402 to catch
> these kinds of persistence problems, but ugh.
> 
> Anyway, what are you afraid of?  The C compiler smoking crack and not
> actually overlapping the two union elements?  We could control for
> that...

(Following up on the mailing list with something I pasted into IRC)

Ok, so I temporarily patched up my dev tree with this approximation of
how that would work, properly done:

diff --git a/fs/xfs/libxfs/xfs_inode_buf.c b/fs/xfs/libxfs/xfs_inode_buf.c
index c59ddb56bb90..7c71e4440402 100644
--- a/fs/xfs/libxfs/xfs_inode_buf.c
+++ b/fs/xfs/libxfs/xfs_inode_buf.c
@@ -176,8 +176,8 @@ xfs_inode_from_disk_timestamp(
 		return;
 	}
 
-	tv->tv_sec = (int)be32_to_cpu(ts->t_sec);
-	tv->tv_nsec = (int)be32_to_cpu(ts->t_nsec);
+	tv->tv_sec = (time64_t)be32_to_cpu((__be32)(ts->t_bigtime >> 32));
+	tv->tv_nsec = be32_to_cpu(ts->t_bigtime & 0xFFFFFFFFU);
 }
 
 int
@@ -294,8 +294,8 @@ xfs_inode_to_disk_timestamp(
 		return;
 	}
 
-	ts->t_sec = cpu_to_be32(tv->tv_sec);
-	ts->t_nsec = cpu_to_be32(tv->tv_nsec);
+	ts->t_bigtime = (__be64)cpu_to_be32(tv->tv_sec) << 32 |
+			cpu_to_be32(tv->tv_nsec);
 }
 
 void
diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
index d44e8932979b..5d36d6dea326 100644
--- a/fs/xfs/xfs_inode_item.c
+++ b/fs/xfs/xfs_inode_item.c
@@ -308,8 +308,8 @@ xfs_log_dinode_to_disk_ts(
 		return;
 	}
 
-	ts->t_sec = cpu_to_be32(its->t_sec);
-	ts->t_nsec = cpu_to_be32(its->t_nsec);
+	ts->t_bigtime = (__be64)cpu_to_be32(its->t_sec) << 32 |
+			cpu_to_be32(its->t_nsec);
 }

And immediately got a ton of smatch warnings:

xfs_inode_buf.c:179:32: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:179:32: warning: cast to restricted __be32
xfs_inode_buf.c:179:32: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:179:32: warning: cast to restricted __be32
xfs_inode_buf.c:179:32: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:179:32: warning: cast to restricted __be32
xfs_inode_buf.c:179:32: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:179:32: warning: cast to restricted __be32
xfs_inode_buf.c:179:32: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:179:32: warning: cast to restricted __be32
xfs_inode_buf.c:179:32: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:179:32: warning: cast to restricted __be32
xfs_inode_buf.c:180:23: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:180:23: warning: cast to restricted __be32
xfs_inode_buf.c:180:23: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:180:23: warning: cast to restricted __be32
xfs_inode_buf.c:180:23: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:180:23: warning: cast to restricted __be32
xfs_inode_buf.c:180:23: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:180:23: warning: cast to restricted __be32
xfs_inode_buf.c:180:23: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:180:23: warning: cast to restricted __be32
xfs_inode_buf.c:180:23: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:180:23: warning: cast to restricted __be32
xfs_inode_buf.c:297:26: warning: cast to restricted __be64
xfs_inode_buf.c:297:26: warning: cast from restricted __be32
xfs_inode_buf.c:297:26: warning: restricted __be64 degrades to integer
xfs_inode_buf.c:298:25: warning: restricted __be32 degrades to integer
xfs_inode_buf.c:297:23: warning: incorrect type in assignment (different base types)
xfs_inode_buf.c:297:23:    expected restricted __be64 [usertype] t_bigtime
xfs_inode_buf.c:297:23:    got unsigned long long

(and even more in xfs_inode_item.c)

So... while we could get rid of the union and hand-decode the timestamp
from a __be64 on legacy filesystems, I see the static checker complaints
as a second piece of evidence that this would be unnecessarily risky.

--D

> > > @@ -220,9 +234,9 @@ xfs_inode_from_disk(
> > >  	 * a time before epoch is converted to a time long after epoch
> > >  	 * on 64 bit systems.
> > >  	 */
> > > -	xfs_inode_from_disk_timestamp(&inode->i_atime, &from->di_atime);
> > > -	xfs_inode_from_disk_timestamp(&inode->i_mtime, &from->di_mtime);
> > > -	xfs_inode_from_disk_timestamp(&inode->i_ctime, &from->di_ctime);
> > > +	xfs_inode_from_disk_timestamp(from, &inode->i_atime, &from->di_atime);
> > > +	xfs_inode_from_disk_timestamp(from, &inode->i_mtime, &from->di_mtime);
> > > +	xfs_inode_from_disk_timestamp(from, &inode->i_ctime, &from->di_ctime);
> > >  
> > >  	to->di_size = be64_to_cpu(from->di_size);
> > >  	to->di_nblocks = be64_to_cpu(from->di_nblocks);
> > > @@ -235,9 +249,17 @@ xfs_inode_from_disk(
> > >  	if (xfs_sb_version_has_v3inode(&ip->i_mount->m_sb)) {
> > >  		inode_set_iversion_queried(inode,
> > >  					   be64_to_cpu(from->di_changecount));
> > > -		xfs_inode_from_disk_timestamp(&to->di_crtime, &from->di_crtime);
> > > +		xfs_inode_from_disk_timestamp(from, &to->di_crtime,
> > > +				&from->di_crtime);
> > >  		to->di_flags2 = be64_to_cpu(from->di_flags2);
> > >  		to->di_cowextsize = be32_to_cpu(from->di_cowextsize);
> > > +		/*
> > > +		 * Set the bigtime flag incore so that we automatically convert
> > > +		 * this inode's ondisk timestamps to bigtime format the next
> > > +		 * time we write the inode core to disk.
> > > +		 */
> > > +		if (xfs_sb_version_hasbigtime(&ip->i_mount->m_sb))
> > > +			to->di_flags2 |= XFS_DIFLAG2_BIGTIME;
> > >  	}
> > 
> > We do not want on-disk flags to be changed outside transactions like
> > this. Indeed, this has implications for O_DSYNC operation, in that
> > we do not trigger inode sync operations if the inode is only
> > timestamp dirty. If we've changed this flag, then the inode is more
> > than "timestamp dirty" and O_DSYNC will need to flush the entire
> > inode.... :/
> 
> I forgot about XFS_ILOG_TIMESTAMP.
> 
> > IOWs, I think we should only change this flag in a timestamp
> > transaction where the timestamps are actually being logged and hence
> > we can set inode dirty state appropriately so that everything will
> > get logged, changed and written back correctly....
> 
> Yeah, that's fair.  I'll change xfs_trans_log_inode to set the bigtime
> flag if we're logging either the timestamps or the core.
> 
> --D
> 
> > Cheers,
> > 
> > Dave.
> > -- 
> > Dave Chinner
> > david@fromorbit.com