Btrfs: make sure not to return overlapping extents to fiemap

The btrfs fiemap code was incorrectly returning duplicate or overlapping
extents in some cases. cp was blindly trusting this result and we would
end up with a destination file that was bigger than the original because
some bytes were copied twice.

The fix here adjusts our offsets to make sure we're always moving
forward in the fiemap results.

Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/extent_io.c

···
 	}
 
 	while (!end) {
-		off = extent_map_end(em);
-		if (off >= max)
-			end = 1;
+		u64 offset_in_extent;
 
-		em_start = em->start;
-		em_len = em->len;
+		/* break if the extent we found is outside the range */
+		if (em->start >= max || extent_map_end(em) < off)
+			break;
+
+		/*
+		 * get_extent may return an extent that starts before our
+		 * requested range.  We have to make sure the ranges
+		 * we return to fiemap always move forward and don't
+		 * overlap, so adjust the offsets here
+		 */
+		em_start = max(em->start, off);
+
+		/*
+		 * record the offset from the start of the extent
+		 * for adjusting the disk offset below
+		 */
+		offset_in_extent = em_start - em->start;
 		em_end = extent_map_end(em);
+		em_len = em_end - em_start;
 		emflags = em->flags;
 		disko = 0;
 		flags = 0;
+
+		/*
+		 * bump off for our next call to get_extent
+		 */
+		off = extent_map_end(em);
+		if (off >= max)
+			end = 1;
 
 		if (em->block_start == EXTENT_MAP_LAST_BYTE) {
 			end = 1;
···
 			flags |= (FIEMAP_EXTENT_DELALLOC |
 				  FIEMAP_EXTENT_UNKNOWN);
 		} else {
-			disko = em->block_start;
+			disko = em->block_start + offset_in_extent;
 		}
 		if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags))
 			flags |= FIEMAP_EXTENT_ENCODED;