[PATCH v2] xfs: test reflux with exchange-range

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

Regression test for the XFS_IOC_EXCHRANGE flag INO1_WRITTEN clearing
reflink flags when that shouldn't happen.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
v2: add rvb tags, fix test description
---
 tests/generic/1957     |   72 ++++++++++++++++++++++++++++++++++++++++++++++++
 tests/generic/1957.out |   16 +++++++++++
 2 files changed, 88 insertions(+)
 create mode 100755 tests/generic/1957
 create mode 100644 tests/generic/1957.out

diff --git a/tests/generic/1957 b/tests/generic/1957
new file mode 100755
index 00000000000000..61a9f4677f90cd
--- /dev/null
+++ b/tests/generic/1957
@@ -0,0 +1,72 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 Oracle.  All Rights Reserved.
+#
+# FS QA Test No. 1957
+#
+# Regression test for an exchange-range bug which unintentionally results in
+# files that share data blocks but don't have the reflink flag set.  This
+# enables attackers to rewrite shared blocks to gain root privileges, similar
+# to refluxfs.
+. ./common/preamble
+_begin_fstest auto quick fiexchange
+
+_cleanup()
+{
+	cd /
+	rm -r -f $tmp.*
+	test -n "$dummydir" && rm -r -f "$dummydir"
+}
+
+. ./common/filter
+. ./common/reflink
+
+_require_scratch_reflink
+_require_xfs_io_command exchangerange
+_require_cp_reflink
+
+_fixed_by_fs_commit xfs XXXXXXXXXXXXXX \
+	"xfs: fix exchange-range reflink flag clearing issue with  INO1_WRITTEN"
+
+_scratch_mkfs >> $seqres.full
+_scratch_mount
+
+# Create file1 as a fully written file, and file2 as a sparse file with one
+# written area.
+$XFS_IO_PROG -f -c "pwrite -S 0x58 0 1m" $SCRATCH_MNT/file1 >> $seqres.full
+$XFS_IO_PROG -f -c "truncate 1m" -c "pwrite -S 0x59 64k 64k" $SCRATCH_MNT/file2 >> $seqres.full
+
+_scratch_unmount
+_scratch_xfs_db -c "path /file1" -c 'print' -c "path /file2" -c 'print' | grep reflink
+_scratch_mount
+
+# Reflink file1 so that the reflink flag gets set
+_cp_reflink $SCRATCH_MNT/file1 $SCRATCH_MNT/fileC
+
+md5sum $SCRATCH_MNT/file1 $SCRATCH_MNT/file2 $SCRATCH_MNT/fileC | _filter_scratch
+
+# Exchange the written parts of file2 with file1, but only file1 has the
+# reflink flag set.
+$XFS_IO_PROG -c "exchangerange -w $SCRATCH_MNT/file2" $SCRATCH_MNT/file1 >> $seqres.full
+
+# Check reflink flags
+_scratch_unmount
+_scratch_xfs_db -c "path /file1" -c 'print' -c "path /file2" -c 'print' | grep reflink
+_scratch_mount
+
+# Record content and layout
+md5sum $SCRATCH_MNT/file1 $SCRATCH_MNT/file2 $SCRATCH_MNT/fileC | _filter_scratch
+$XFS_IO_PROG -c 'bmap -vvvvvvvv' $SCRATCH_MNT/file[12C] >> $seqres.full
+
+# Write a single byte to file1, does that get echoed in fileC?
+$XFS_IO_PROG -c 'pwrite -S 0x59 0 1' $SCRATCH_MNT/file1 >> $seqres.full
+#
+# Check reflink flags
+_scratch_unmount
+_scratch_xfs_db -c "path /file1" -c 'print' -c "path /file2" -c 'print' | grep reflink
+_scratch_mount
+
+md5sum $SCRATCH_MNT/file1 $SCRATCH_MNT/file2 $SCRATCH_MNT/fileC | _filter_scratch
+$XFS_IO_PROG -c 'bmap -vvvvvvvv' $SCRATCH_MNT/file[12C] >> $seqres.full
+
+_exit 0
diff --git a/tests/generic/1957.out b/tests/generic/1957.out
new file mode 100644
index 00000000000000..7d64ea0e2c372d
--- /dev/null
+++ b/tests/generic/1957.out
@@ -0,0 +1,16 @@
+QA output created by 1957
+v3.reflink = 0
+v3.reflink = 0
+310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/file1
+e6ee47dca6e786a44ae5b912be870d96  SCRATCH_MNT/file2
+310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/fileC
+v3.reflink = 1
+v3.reflink = 1
+b630c7de58afd0ba5b8dfa24d701b5b8  SCRATCH_MNT/file1
+18cc9868973139a0bb558e09802a51b6  SCRATCH_MNT/file2
+310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/fileC
+v3.reflink = 1
+v3.reflink = 1
+82b27c6f1b1de8fba44a911af80d9a7a  SCRATCH_MNT/file1
+18cc9868973139a0bb558e09802a51b6  SCRATCH_MNT/file2
+310f146ce52077fcd3308dcbe7632bb2  SCRATCH_MNT/fileC