[PATCH v2] fstests: btrfs: a new test case to verify scrub and rescue=idatacsums

[Qu Wenruo <wqu@suse.com>]

There is a kernel bug report that scrub will trigger a NULL pointer
dereference when rescue=idatacsums mount option is provided.

Add a test case for such situation, to verify kernel can gracefully
reject scrub when  there is no csum tree.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
Changelog:
v2:
- Strictly require the scrub to fail
  Suggested by Filipe
---
 tests/btrfs/336     | 35 +++++++++++++++++++++++++++++++++++
 tests/btrfs/336.out |  2 ++
 2 files changed, 37 insertions(+)
 create mode 100755 tests/btrfs/336
 create mode 100644 tests/btrfs/336.out

diff --git a/tests/btrfs/336 b/tests/btrfs/336
new file mode 100755
index 00000000..f6691bae
--- /dev/null
+++ b/tests/btrfs/336
@@ -0,0 +1,35 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2025 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test 336
+#
+# Make sure read-only scrub won't cause NULL pointer dereference with
+# rescue=idatacsums mount option
+#
+. ./common/preamble
+_begin_fstest auto scrub quick
+
+_fixed_by_kernel_commit 6aecd91a5c5b \
+	"btrfs: avoid NULL pointer dereference if no valid extent tree"
+
+_require_scratch
+_scratch_mkfs >> $seqres.full
+
+_try_scratch_mount "-o ro,rescue=ignoredatacsums" > /dev/null 2>&1 ||
+	_notrun "rescue=ignoredatacsums mount option not supported"
+
+# For unpatched kernel this will cause NULL pointer dereference and crash the kernel.
+$BTRFS_UTIL_PROG scrub start -Br $SCRATCH_MNT >> $seqres.full 2>&1
+# For patched kernel scrub will be gracefully rejected.
+if [ $? -eq 0 ]; then
+	echo "read-only scrub should fail but didn't"
+fi
+
+_scratch_unmount
+
+echo "Silence is golden"
+
+# success, all done
+status=0
+exit
diff --git a/tests/btrfs/336.out b/tests/btrfs/336.out
new file mode 100644
index 00000000..9263628e
--- /dev/null
+++ b/tests/btrfs/336.out
@@ -0,0 +1,2 @@
+QA output created by 336
+Silence is golden
-- 
2.47.1