[PATCH v2] xfs/842: verify CoW after exchangerange with FILE1_WRITTEN on shared extents

[Lin Jiapeng <ljp1205831794@gmail.com>]

When a full-file exchangerange is requested with FILE1_WRITTEN for a
fully sparse donor file, every mapping pair is skipped, so no extents
actually move.  However, the kernel decides to swap the reflink inode
flag based on the request geometry alone, so the target file used to
lose its reflink flag to the donor while still owning shared extents.
A subsequent write then took the non-reflink write path and modified
the shared physical blocks in place, silently corrupting the other
file sharing them:

 --- tests/xfs/842.out
 +++ results/xfs/842.out.bad
 @@ -1,4 +1,5 @@
  QA output created by 842
 +vX.reflink = 0
  vX.reflink = 1
 -vX.reflink = 1
 +orig changed: md5 e6065c4aa2ab1603008fc18410f579d4 -> 32c5189478e6bafa1cc76423f06c88f2 (write hit shared blocks in place)
  Silence is golden

This test clones a file so that both share extents, swaps the clone
against a sparse donor with FILE1_WRITTEN, and then checks the defect
twice.  First it inspects the inodes directly with xfs_db: the clone
must still be flagged reflink after the exchange (the donor carrying
the flag too is the conservative outcome of the fix and is harmless).
Then it writes to the clone and verifies after a mount cycle that the
original file's data is intact, i.e. the write was redirected through
CoW.

The kernel fix "xfs: fix exchange-range reflink flag clearing issue
with INO1_WRITTEN" refuses the reflink flag exchange whenever
FILE1_WRITTEN is requested, which makes this test pass.

Reported-by: Lin Jiapeng(TencentOS Red Team) <jiapenglin@tencent.com>
Link: https://lore.kernel.org/r/amgekiNKKoAdACnc@infradead.org
Suggested-by: Christoph Hellwig <hch@lst.de>
Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Lin Jiapeng <jiapenglin@tencent.com>
---
 tests/xfs/842     | 65 +++++++++++++++++++++++++++++++++++++++++++++++
 tests/xfs/842.out |  4 +++
 2 files changed, 69 insertions(+)
 create mode 100755 tests/xfs/842
 create mode 100644 tests/xfs/842.out

diff --git a/tests/xfs/842 b/tests/xfs/842
new file mode 100755
index 0000000..c4c2be9
--- /dev/null
+++ b/tests/xfs/842
@@ -0,0 +1,65 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0-or-later
+# Copyright (c) 2026 Tencent.  All Rights Reserved.
+#
+# FS QA Test No. 842
+#
+# Make sure that a full-file exchangerange under FILE1_WRITTEN does not strip
+# the reflink flag from a file that still owns shared extents.  When the
+# donor file is fully sparse, every mapping pair is skipped, so no extents
+# actually move; the target file must keep its reflink flag, and a later
+# write must go through CoW instead of modifying the shared blocks in place.
+
+. ./common/preamble
+_begin_fstest auto quick fiexchange
+
+# Import common functions.
+. ./common/filter
+. ./common/reflink
+
+_require_xfs_io_command exchangerange
+_require_scratch_reflink
+_require_scratch
+
+_scratch_mkfs >> $seqres.full
+_scratch_mount
+
+# Create the original file with a known pattern and clone it, so that both
+# files share the same extents.
+_pwrite_byte 0x41 0 1m $SCRATCH_MNT/orig >> $seqres.full
+_reflink $SCRATCH_MNT/orig $SCRATCH_MNT/clone >> $seqres.full
+
+# Create a fully sparse donor file of the same size.
+$XFS_IO_PROG -f -c 'truncate 1m' $SCRATCH_MNT/donor
+
+md5_before=$(md5sum $SCRATCH_MNT/orig | awk '{print $1}')
+
+# Swap the clone against the sparse donor, claiming the donor is fully
+# written (-w).  Every donor mapping is a hole, so all pairs are skipped
+# and the clone keeps its shared extents in place.
+$XFS_IO_PROG -c "exchangerange -f -w $SCRATCH_MNT/donor" $SCRATCH_MNT/clone \
+	>> $seqres.full
+
+# Directly confirm the inode flag state after the exchange: the clone
+# must still be flagged reflink.  The donor may also carry the flag, which
+# is the conservative outcome of refusing the flag exchange under
+# FILE1_WRITTEN; the extra flag is harmless and can be dropped later by
+# the regular reflink flag cleanup path.
+_scratch_unmount
+_scratch_xfs_db -c "path /clone" -c print -c "path /donor" -c print | \
+	grep reflink | sed -e 's/^v[0-9]*/vX/g'
+_scratch_mount
+
+# Overwrite part of the clone.  With the reflink flag correctly retained,
+# this must go through CoW and leave the shared blocks of orig untouched.
+_pwrite_byte 0x42 0 64k $SCRATCH_MNT/clone >> $seqres.full
+_scratch_cycle_mount
+
+md5_after=$(md5sum $SCRATCH_MNT/orig | awk '{print $1}')
+
+test "$md5_before" != "$md5_after" && \
+	echo "orig changed: md5 $md5_before -> $md5_after (write hit shared blocks in place)"
+
+echo Silence is golden
+status=0
+exit
diff --git a/tests/xfs/842.out b/tests/xfs/842.out
new file mode 100644
index 0000000..1e6345a
--- /dev/null
+++ b/tests/xfs/842.out
@@ -0,0 +1,4 @@
+QA output created by 842
+vX.reflink = 1
+vX.reflink = 1
+Silence is golden
-- 
2.50.1 (Apple Git-155)