FAILED: patch "[PATCH] Btrfs: incremental send, fix file corruption when no-holes" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-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 6b1f72e5b82a5c2a4da4d1ebb8cc01913ddbea21 Mon Sep 17 00:00:00 2001
From: Filipe Manana <[email protected]>
Date: Mon, 20 May 2019 09:55:42 +0100
Subject: [PATCH] Btrfs: incremental send, fix file corruption when no-holes
feature is enabled

When using the no-holes feature, if we have a file with prealloc extents with a start offset beyond the file's eof, doing an incremental send can cause corruption of the file due to incorrect hole detection. Such case requires that the prealloc extent(s) exist in both the parent and send snapshots, and that a hole is punched into the file that covers all its extents that do not cross the eof boundary.

Example reproducer:

  $ mkfs.btrfs -f -O no-holes /dev/sdb
  $ mount /dev/sdb /mnt/sdb

  $ xfs_io -f -c "pwrite -S 0xab 0 500K" /mnt/sdb/foobar
  $ xfs_io -c "falloc -k 1200K 800K" /mnt/sdb/foobar

  $ btrfs subvolume snapshot -r /mnt/sdb /mnt/sdb/base

  $ btrfs send -f /tmp/base.snap /mnt/sdb/base

  $ xfs_io -c "fpunch 0 500K" /mnt/sdb/foobar

  $ btrfs subvolume snapshot -r /mnt/sdb /mnt/sdb/incr

  $ btrfs send -p /mnt/sdb/base -f /tmp/incr.snap /mnt/sdb/incr

  $ md5sum /mnt/sdb/incr/foobar
  816df6f64deba63b029ca19d880ee10a   /mnt/sdb/incr/foobar

  $ mkfs.btrfs -f /dev/sdc
  $ mount /dev/sdc /mnt/sdc

  $ btrfs receive -f /tmp/base.snap /mnt/sdc
  $ btrfs receive -f /tmp/incr.snap /mnt/sdc

  $ md5sum /mnt/sdc/incr/foobar
  cf2ef71f4a9e90c2f6013ba3b2257ed2   /mnt/sdc/incr/foobar

--> Different checksum, because the prealloc extent beyond the
file's eof confused the hole detection code and it assumed a hole starting at offset 0 and ending at the offset of the prealloc extent (1200Kb) instead of ending at the offset 500Kb (the file's size).

Fix this by ensuring we never cross the file's size when issuing the write operations for a hole.

index dd38dfe174df..ca30b1f06e2d 100644
@@ -4999,6 +4999,12 @@ static int send_hole(struct send_ctx *sctx, u64 end)
 	if (offset >= sctx->cur_inode_size)
 		return 0;
 
+	/*
+	 * Don't go beyond the inode's i_size due to prealloc extents that start
+	 * after the i_size.
+	 */
+	end = min_t(u64, end, sctx->cur_inode_size);
+
 	if (sctx->flags & BTRFS_SEND_FLAG_NO_FILE_DATA)
 		return send_update_extent(sctx, offset, end - offset);