btrfs: fix a potential hole punching failure

In commit d7781546 ("btrfs: Avoid trucating page or punching hole
in a already existed hole."), existing holes can be skipped by calling
find_first_non_hole() to adjust start and len. However, if the given len
is invalid and large, when an EXTENT_MAP_HOLE extent is found, len will
not be set to zero because (em->start + em->len) is less than
(start + len). Then the ret will be 1 but len will not be set to 0.
The propagated non-zero ret will result in fallocate failure.

In the while-loop of btrfs_replace_file_extents(), len is not updated
every time before it calls find_first_non_hole(). That is, after
btrfs_drop_extents() successfully drops the last non-hole file extent,
it may fail with ENOSPC when attempting to drop a file extent item
representing a hole. The problem can happen. After it calls
find_first_non_hole(), the cur_offset will be adjusted to be larger
than or equal to end. However, since the len is not set to zero, the
break-loop condition (ret && !len) will not be met. After it leaves the
while-loop, fallocate will return 1, which is an unexpected return
value.

We're not able to construct a reproducible way to let
btrfs_drop_extents() fail with ENOSPC after it drops the last non-hole
file extent but with remaining holes left. However, it's quite easy to
fix. We just need to update and check the len every time before we call
find_first_non_hole(). To make the while loop more readable, we also
pull the variable updates to the bottom of loop like this:
  while (cur_offset < end) {
	  ...
	  // update cur_offset & len
	  // advance cur_offset & len in hole-punching case if needed
  }
Reported-by: Robbie Ko <robbieko@synology.com>
Fixes: d7781546 ("btrfs: Avoid trucating page or punching hole in a already existed hole.")
CC: stable@vger.kernel.org # 4.4+
Reviewed-by: Robbie Ko <robbieko@synology.com>
Reviewed-by: Chung-Chiang Cheng <cccheng@synology.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: BingJing Chang <bingjingc@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -2730,8 +2730,6 @@ int btrfs_replace_file_extents(struct btrfs_inode *inode,
 			extent_info->file_offset += replace_len;
 		}
 
-		cur_offset = drop_args.drop_end;
-
 		ret = btrfs_update_inode(trans, root, inode);
 		if (ret)
 			break;
@@ -2751,7 +2749,9 @@ int btrfs_replace_file_extents(struct btrfs_inode *inode,
 		BUG_ON(ret);	/* shouldn't happen */
 		trans->block_rsv = rsv;
 
-		if (!extent_info) {
+		cur_offset = drop_args.drop_end;
+		len = end - cur_offset;
+		if (!extent_info && len) {
 			ret = find_first_non_hole(inode, &cur_offset, &len);
 			if (unlikely(ret < 0))
 				break;