FAILED: patch "[PATCH] btrfs: fix a potential hole punching failure" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 3227788cd369d734d2d3cd94f8af7536b60fa552 Mon Sep 17 00:00:00 2001
From: BingJing Chang <[email protected]>
Date: Thu, 25 Mar 2021 09:56:22 +0800
Subject: [PATCH] btrfs: fix a potential hole punching failure

In commit d77815461f04 ("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

}

index 42634658815f..864c08d08a35 100644
@@ -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;