fstests: add test coverage for cloned filesystem ids

Introduce _mkfs_scratch_clone() to mkfs the scratch device and clone it to the next device in SCRATCH_DEV_POOL.

Signed-off-by Anand Jain <[email protected]>

---
 common/rc | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

common/rc +0 -0

index 9db8b3e88996..2253438ef0f6 100644
@@ -1503,6 +1503,38 @@ _scratch_resvblks()
 	esac
 }
 

common/rc +11 -0

+		_notrun "Test requires exactly 2 devices"
+	fi
+
+	case "$FSTYP" in
+	"btrfs")
+		_scratch_mkfs_sized $size
+		_scratch_mount
+		$BTRFS_UTIL_PROG subvolume create $SCRATCH_MNT/sv1
+		_scratch_unmount
+		;;
+	"xfs"|"ext4")

common/rc +1 -0

+}
 
 # Repair scratch filesystem.  Returns 0 if the FS is good to go (either no
 # errors found or errors were fixed) and nonzero otherwise; also spits out

Signature · 1 line

2.43.0

What about if ${#devs[@]} > 2 ?

Quoted 15 lines

common/rc +15 -0

+	fi
+
+	case "$FSTYP" in
+	"btrfs")
+		_scratch_mkfs_sized $size
+		_scratch_mount
+		$BTRFS_UTIL_PROG subvolume create $SCRATCH_MNT/sv1
+		_scratch_unmount
+		;;
+	"xfs"|"ext4")
+		_scratch_mkfs_sized $size
+		;;
+	*)
+		_notrun "fstests clone op unsupported for FS $FSTYP"
+		;;

common/rc _scratch_resvblks() · line 1536

Quoted 5 lines

common/rc +5 -0

+	esac
+
+	# clone SCRATCH_DEV devs[0] to devs[1].
+	dd if=$SCRATCH_DEV of=${devs[1]} bs=$size status=none count=1 || \
+							_fail "Clone failed"

I'm wondering if we absolutely need to use SCRATCH_DEV_POOL for this test. Could we clone SCRATCH_DEV to an image file instead? Or would it be feasible to simply run the test using two image files?

Quoted 2 lines

What about if ${#devs[@]} > 2 ?

On purpose- so tests using _scratch_mkfs_sized_clone() don't pick a random higher number. If a test needs more than two devices, opens an opportunity to critically review this function.

Quoted 15 lines

Quoted 15 lines

common/rc +15 -0

+	fi
+
+	case "$FSTYP" in
+	"btrfs")
+		_scratch_mkfs_sized $size
+		_scratch_mount
+		$BTRFS_UTIL_PROG subvolume create $SCRATCH_MNT/sv1
+		_scratch_unmount
+		;;
+	"xfs"|"ext4")
+		_scratch_mkfs_sized $size
+		;;
+	*)
+		_notrun "fstests clone op unsupported for FS $FSTYP"
+		;;

Quoted 8 lines

common/rc _scratch_resvblks() · line 1536

Quoted 5 lines

common/rc +5 -0

+	esac
+
+	# clone SCRATCH_DEV devs[0] to devs[1].
+	dd if=$SCRATCH_DEV of=${devs[1]} bs=$size status=none count=1 || \
+							_fail "Clone failed"

I'm wondering if we absolutely need to use SCRATCH_DEV_POOL for this test. Could we clone SCRATCH_DEV to an image file instead? Or would it be feasible to simply run the test using two image files?

Using a config file and testing directly on block devices lets us cover a wider range of devices.

When I wrote this test case, I was also considering whether a serial-number–based mount without extra mount option, would be better in the kernel.

The downside is that xfs/ext4 testers now need to use SCRATCH_DEV_POOL. Let me know what you prefer.

I'm concerned that testers from other filesystems might not be familiar with SCRATCH_DEV_POOL, as it's typically not a requirement for their testing. I'm not sure if they would be willing to configure and switch to SCRATCH_DEV_POOL, just for these few test cases. If we can avoid making the pool to be a hard requirement, it might be more user-friendly for other fs list. Welcome feedback and review points from other filesystem mailing lists :)

Quoted 7 lines · Zorro Lang

I'm concerned that testers from other filesystems might not be familiar with SCRATCH_DEV_POOL, as it's typically not a requirement for their testing. I'm not sure if they would be willing to configure and switch to SCRATCH_DEV_POOL, just for these few test cases. If we can avoid making the pool to be a hard requirement, it might be more user-friendly for other fs list. Welcome feedback and review points from other filesystem mailing lists :)

Yes, doing this using loop devices or ramdisk is going to get us much better coverage.

Add a new test, to verify that the kernel correctly differentiates between two block devices sharing the same FSID/UUID.

Signed-off-by Anand Jain <[email protected]>

---
 common/config         |  1 +
 tests/generic/790     | 78 +++++++++++++++++++++++++++++++++++++++++++
 tests/generic/790.out |  7 ++++
 3 files changed, 86 insertions(+)
 create mode 100644 tests/generic/790
 create mode 100644 tests/generic/790.out

common/config +1 -0

index 1420e35ddfee..c08f828575a2 100644
@@ -228,6 +228,7 @@ export BTRFS_MAP_LOGICAL_PROG=$(type -P btrfs-map-logical)
 export PARTED_PROG="$(type -P parted)"
 export XFS_PROPERTY_PROG="$(type -P xfs_property)"
 export FSCRYPTCTL_PROG="$(type -P fscryptctl)"
+export INOTIFYWAIT_PROG="$(type -P inotifywait)"
 
 # udev wait functions.
 #

tests/generic/790 new file +49 -0

new file mode 100644
index 000000000000..3809fced622d
@@ -0,0 +1,78 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 Anand Jain <[email protected]>.  All Rights Reserved.
+#
+# FS QA Test 790
+#
+# Verify if the kernel or userspace becomes confused when two block devices
+# share the same fid/fsid/uuid. Create inotify on both original and cloned
+# filesystem. Monitor the notification in the respective logs.
+
+. ./common/preamble
+
+_begin_fstest auto quick mount clone
+
+_require_test
+_require_scratch_dev_pool 2
+_require_command "$INOTIFYWAIT_PROG" inotifywait
+
+_cleanup()
+{
+	cd /
+	rm -r -f $tmp.*
+	umount $mnt1 $mnt2 2>/dev/null
+	_scratch_dev_pool_put
+}
+
+_scratch_dev_pool_get 2
+_scratch_mkfs_sized_clone >$seqres.full 2>&1
+devs=($SCRATCH_DEV_POOL)
+mnt2=$TEST_DIR/mnt2
+mkdir -p $mnt2
+
+_scratch_mount $(_clone_mount_option)
+_mount $(_common_dev_mount_options) $(_clone_mount_option) ${devs[1]} $mnt2 || \
+						_fail "Failed to mount dev2"
+
+log1=$tmp.inotify1
+log2=$tmp.inotify2
+
+echo "Setup inotify watchers on both SCRATCH_MNT and mnt2"
+$INOTIFYWAIT_PROG -m -e create --format '%f' $SCRATCH_MNT > $log1 2>&1 &
+pid1=$!
+$INOTIFYWAIT_PROG -m -e create --format '%f' $mnt2 > $log2 2>&1 &
+pid2=$!
+sleep 2
+
+echo "Trigger file creation on SCRATCH_MNT"
+touch $SCRATCH_MNT/file_on_scratch_mnt
+sync

tests/generic/790 new file +19 -0

+
+if grep -q "file_on_scratch_mnt" $log1 && ! grep -q "file_on_mnt2" $log1; then
+	echo "SUCCESS: SCRATCH_MNT events isolated."
+else
+	echo "FAIL: SCRATCH_MNT inotify confusion!"
+	[ ! -s $log1 ] && echo "  - SCRATCH_MNT received no events."
+	grep -q "file_on_mnt2" $log1 && echo "  - SCRATCH_MNT received event from mnt2."
+fi
+
+if grep -q "file_on_mnt2" $log2 && ! grep -q "file_on_scratch_mnt" $log2; then
+	echo "SUCCESS: mnt2 events isolated."
+else
+	echo "FAIL: mnt2 inotify confusion!"
+	[ ! -s $log2 ] && echo "  - mnt2 received no events."
+	grep -q "file_on_scratch_mnt" $log2 && echo "  - mnt2 received event from SCRATCH_MNT."
+fi
+
+status=0
+exit

tests/generic/790.out new file +7 -0

new file mode 100644
index 000000000000..3c92c34ffbda
@@ -0,0 +1,7 @@
+QA output created by 790
+Setup inotify watchers on both SCRATCH_MNT and mnt2
+Trigger file creation on SCRATCH_MNT
+Trigger file creation on mnt2
+Verify inotify isolation
+SUCCESS: SCRATCH_MNT events isolated.
+SUCCESS: mnt2 events isolated.

Signature · 1 line

2.43.0

Quoted 37 lines

tests/generic/790 new file +36 -0

+_cleanup()
+{
+	cd /
+	rm -r -f $tmp.*
+	umount $mnt1 $mnt2 2>/dev/null
+	_scratch_dev_pool_put
+}
+
+_scratch_dev_pool_get 2
+_scratch_mkfs_sized_clone >$seqres.full 2>&1
+devs=($SCRATCH_DEV_POOL)
+mnt2=$TEST_DIR/mnt2
+mkdir -p $mnt2
+
+_scratch_mount $(_clone_mount_option)
+_mount $(_common_dev_mount_options) $(_clone_mount_option) ${devs[1]} $mnt2 || \
+						_fail "Failed to mount dev2"
+
+log1=$tmp.inotify1
+log2=$tmp.inotify2
+
+echo "Setup inotify watchers on both SCRATCH_MNT and mnt2"
+$INOTIFYWAIT_PROG -m -e create --format '%f' $SCRATCH_MNT > $log1 2>&1 &
+pid1=$!
+$INOTIFYWAIT_PROG -m -e create --format '%f' $mnt2 > $log2 2>&1 &
+pid2=$!
+sleep 2
+
+echo "Trigger file creation on SCRATCH_MNT"
+touch $SCRATCH_MNT/file_on_scratch_mnt
+sync
+sleep 1
+
+echo "Trigger file creation on mnt2"
+touch $mnt2/file_on_mnt2
+sync

tests/generic/790 line 59

Quoted 5 lines

tests/generic/790 new file +5 -0

+sleep 1
+
+echo "Verify inotify isolation"
+kill $pid1 $pid2
+wait $pid1 $pid2 2>/dev/null

I think you also need to take care of killing the bg process in _cleanup() so that the test could be cleanly aborted.

Verify that fanotify events are correctly routed to the appropriate watcher when cloned filesystems are mounted.
Helps verify kernel's event notification distinguishes between devices sharing the same FSID/UUID.

Signed-off-by Anand Jain <[email protected]>

---
 .gitignore            |  1 +
 src/Makefile          |  2 +-
 src/fanotify.c        | 66 +++++++++++++++++++++++++++++++++
 tests/generic/791     | 86 +++++++++++++++++++++++++++++++++++++++++++
 tests/generic/791.out |  7 ++++
 5 files changed, 161 insertions(+), 1 deletion(-)
 create mode 100644 src/fanotify.c
 create mode 100644 tests/generic/791
 create mode 100644 tests/generic/791.out

.gitignore +1 -0

index 82c57f415301..7f91310ce58b 100644
@@ -212,6 +212,7 @@ tags
 /src/dio-writeback-race
 /src/unlink-fsync
 /src/file_attr
+/src/fanotify
 
 # Symlinked files
 /tests/generic/035.out

src/Makefile +0 -0

index d0a4106e6be8..ff71cde936a7 100644

src/Makefile +0 -0

 EXTRA_EXECS = dmerror fill2attr fill2fs fill2fs_check scaleread.sh \
 	      btrfs_crc32c_forged_name.py popdir.pl popattr.py \

src/fanotify.c new file +66 -0

new file mode 100644
index 000000000000..e30c48dc0e52
@@ -0,0 +1,66 @@
+/*
+ * SPDX-License-Identifier: GPL-2.0
+ * Copyright (c) 2026 Anand Jain <[email protected]>.  All Rights Reserved.
+ *
+ * Simple fanotify monitor to verify mount-point event isolation.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <stdint.h>
+#include <sys/fanotify.h>
+
+int main(int argc, char *argv[])
+{
+	int fd;
+	char buf[4096] __attribute__((aligned(8)));
+	setlinebuf(stdout);
+
+	if (argc < 2) {
+		fprintf(stderr, "Usage: %s <path>\n", argv[0]);
+		return 1;
+	}
+
+	// Initialize with FID reporting
+	fd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_FID, O_RDONLY);
+	if (fd < 0) {
+		perror("fanotify_init");
+		return 1;
+	}
+
+	if (fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_FILESYSTEM,
+			  FAN_CREATE, AT_FDCWD, argv[1]) < 0) {
+		perror("fanotify_mark");
+		return 1;
+	}
+
+	printf("Listening for events on %s...\n", argv[1]);
+	while (1) {
+		struct fanotify_event_metadata *metadata = (struct fanotify_event_metadata *)buf;
+		ssize_t len = read(fd, buf, sizeof(buf));
+
+		if (len <= 0) break;
+
+		while (FAN_EVENT_OK(metadata, len)) {
+			// metadata_len is the offset to the first info record
+			if (metadata->event_len > metadata->metadata_len) {
+				struct fanotify_event_info_header *hdr =
+(struct fanotify_event_info_header *)((char *)metadata + metadata->metadata_len);
+
+				if (hdr->info_type == FAN_EVENT_INFO_TYPE_FID) {
+					struct fanotify_event_info_fid *fid = (struct fanotify_event_info_fid *)hdr;
+					printf("FSID: %08x%08x\n",
+						fid->fsid.val[0], fid->fsid.val[1]);
+				}
+			}
+			metadata = FAN_EVENT_NEXT(metadata, len);
+		}
+	}
+
+	fflush(stdout);
+	close(fd);
+	return 0;
+}

tests/generic/791 new file +86 -0

new file mode 100644
index 000000000000..fe8109083732
@@ -0,0 +1,86 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 Anand Jain <[email protected]>.  All Rights Reserved.
+#
+# FS QA Test 791
+# Verify fanotify FID functionality on cloned filesystems by setting up
+# watchers and making sure notifications are in the correct logs files.
+
+. ./common/preamble
+
+_begin_fstest auto quick mount clone
+
+_require_test
+_require_scratch_dev_pool 2
+
+[ "$FSTYP" = "ext4" ] && _fixed_by_kernel_commit xxxxxxxxxxxx \
+	"ext4: derive f_fsid from block device to avoid collisions"
+
+_cleanup()
+{
+	cd /
+	rm -r -f $tmp.*
+	umount $mnt1 $mnt2 2>/dev/null
+	_scratch_dev_pool_put
+}
+
+_scratch_dev_pool_get 2
+_scratch_mkfs_sized_clone >$seqres.full 2>&1
+devs=($SCRATCH_DEV_POOL)
+mnt2=$TEST_DIR/mnt2
+mkdir -p $mnt2
+
+_scratch_mount $(_clone_mount_option)
+_mount $(_common_dev_mount_options) $(_clone_mount_option) ${devs[1]} $mnt2 || \
+						_fail "Failed to mount dev2"
+
+fsid1=$(stat -f -c "%i" $SCRATCH_MNT)
+fsid2=$(stat -f -c "%i" $mnt2)
+
+[[ "$fsid1" == "$fsid2" ]] && \
+	_notrun "Require clone filesystem with unique f_fsid"
+
+log1=$tmp.fanotify1
+log2=$tmp.fanotify2
+
+echo "Setup FID fanotify watchers on both SCRATCH_MNT and mnt2"
+$here/src/fanotify $SCRATCH_MNT > $log1 2>&1 &
+pid1=$!
+$here/src/fanotify $mnt2 > $log2 2>&1 &
+pid2=$!
+sleep 2
+
+echo "Trigger file creation on SCRATCH_MNT"
+touch $SCRATCH_MNT/file_on_scratch_mnt
+sync
+sleep 1
+
+echo "Trigger file creation on mnt2"
+touch $mnt2/file_on_mnt2
+sync
+sleep 1
+
+echo "Verify fsid in the fanotify"
+kill $pid1 $pid2
+wait $pid1 $pid2 2>/dev/null
+
+echo fsid1=$fsid1 fsid2=$fsid2 >> $seqres.full
+cat $log1 >> $seqres.full
+cat $log2 >> $seqres.full
+
+if grep -q "${fsid1}" $log1 && ! grep -q "${fsid2}" $log1; then
+	echo "SUCCESS: SCRATCH_MNT events found"
+else
+	[ ! -s $log1 ] && echo "  - SCRATCH_MNT received no events."
+	grep -q "${fsid2}" $log1 && echo "  - SCRATCH_MNT received event from mnt2."
+fi
+
+if grep -q "${fsid2}" $log2 && ! grep -q "${fsid1}" $log2; then
+	echo "SUCCESS: mnt2 events found"
+else
+	[ ! -s $log2 ] && echo "  - mnt2 received no events."
+	grep -q "${fsid1}" $log2 && echo "  - mnt2 received event from SCRATCH_MNT."
+fi
+
+status=0
+exit

tests/generic/791.out new file +7 -0

new file mode 100644
index 000000000000..9725c99bcb4b
@@ -0,0 +1,7 @@
+QA output created by 791
+Setup FID fanotify watchers on both SCRATCH_MNT and mnt2
+Trigger file creation on SCRATCH_MNT
+Trigger file creation on mnt2
+Verify fsid in the fanotify
+SUCCESS: SCRATCH_MNT events found
+SUCCESS: mnt2 events found

Signature · 1 line

2.43.0

src/Makefile LINUX_TARGETS = xfsctl bstat t_mtab getdevicesize preallo_rw_pattern_reader \ · line 39

Quoted 11 lines

src/Makefile +1 -1

index d0a4106e6be8..ff71cde936a7 100644
@@ -36,7 +36,7 @@ LINUX_TARGETS = xfsctl bstat t_mtab getdevicesize preallo_rw_pattern_reader \
 	fscrypt-crypt-util bulkstat_null_ocount splice-test chprojid_fail \
 	detached_mounts_propagation ext4_resize t_readdir_3 splice2pipe \
 	uuid_ioctl t_snapshot_deleted_subvolume fiemap-fault min_dio_alignment \
-	rw_hint
+	rw_hint fanotify

Check if you already have fsnotifywait installed on your system most likely you do. It was added to inotify-tools quite some time ago. Could save you from adding a custom prog.
Not 100% sure about fsnotifywait, but quite sure that fsnotifywatch --verbose prints the FSID of events.

Thanks,
Amir.