As Han Qi reported in bugzilla [1], commit 5c26d2f1d3f5 ("unicode: Don't
special case ignorable code points") changes logic to handle ignorable
code points, when we handle filename which has special unicode charactor
like red heart ❤️, utf8_casefold() may return different length than in the
past, result in that f2fs cacluates hash code incorreclty, eventually,
filename w/ special unicode charactor will not be found w/ the wrong hash
code.

f2fs has introduces a linear lookup fallback w/ commit 91b587ba79e1
("f2fs: Introduce linear search for dentries"), once hash-based lookup
fails, it tries linear lookup as a fallback.

This patch can help to check whether linear lookup fallback works well
or not.

Firstly, we use below testcase to check how hash code changes:

mkfs.f2fs -f -O casefold -C utf8 /dev/vda
mount /dev/vda /mnt/f2fs
mkdir /mnt/f2fs/dir/
f2fs_io setflags casefold /mnt/f2fs/dir
touch /mnt/f2fs/dir/$'\u2764\ufe0f'
umount /mnt/f2fs
dump.f2fs -i 4 -d 3 /dev/vda
dump.f2fs -b $blkaddr_of_dir -d 3 /dev/vda

w/o ("unicode: Don't special case ignorable code points")
[dump_dirent: 991] bitmap pos[0x2] name[❤️] len[0x6] hash[0x19dd7132] ino[0x5] 
type[0x1]
[dump_dirent: 998] name(hex)[0xe2 0x9d 0xa4 0xef 0xb8 0x8f 0x0]

w/ ("unicode: Don't special case ignorable code points")
[dump_dirent: 991] bitmap pos[0x2] name[❤️] len[0x6] hash[0x9a2ea068] ino[0x5] 
type[0x1]
[dump_dirent: 998] name(hex)[0xe2 0x9d 0xa4 0xef 0xb8 0x8f 0x0]

Correct hash code:      0x19dd7132
Wrong hash code:        0x9a2ea068

So, we can test as below:

1. create file w/ red heart as its filename
2. inject wrong hash code to the file
3. disable linear lookup, expect lookup failure
4. enable linear lookup, expect lookup succeed

[1] https://bugzilla.kernel.org/show_bug.cgi?id=219586

Cc: Jaegeuk Kim <jaeg...@kernel.org>
Cc: Daniel Lee <chul...@google.com>
Signed-off-by: Chao Yu <c...@kernel.org>
---
v2:
- tune linear_lookup feature before fault injection
- check status after tuning linear_lookup feature
- clean up codes
 tests/f2fs/012     | 73 ++++++++++++++++++++++++++++++++++++++++++++++
 tests/f2fs/012.out |  2 ++
 2 files changed, 75 insertions(+)
 create mode 100755 tests/f2fs/012
 create mode 100644 tests/f2fs/012.out

diff --git a/tests/f2fs/012 b/tests/f2fs/012
new file mode 100755
index 00000000..38a63f2e
--- /dev/null
+++ b/tests/f2fs/012
@@ -0,0 +1,73 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2025 Chao Yu.  All Rights Reserved.
+#
+# FS QA Test No. f2fs/012
+#
+# This testcase checks whether linear lookup fallback works well
+# or not as below:
+# 1.create file w/ red heart as its filename
+# 2.inject wrong hash code to the file
+# 3.disable linear lookup, expect lookup failure
+# 4.enable linear lookup, expect lookup succeed
+#
+. ./common/preamble
+_begin_fstest auto quick casefold
+
+_fixed_by_kernel_commit 91b587ba79e1 \
+       "f2fs: Introduce linear search for dentries"
+
+export LC_ALL=C.UTF-8
+_require_scratch
+_require_command "$F2FS_IO_PROG" f2fs_io
+_require_command "$F2FS_INJECT_PROG" inject.f2fs
+
+check_lookup()
+{
+       local nolinear_lookup=$1
+       local dir=$SCRATCH_MNT/dir
+       # red heart charactor in unicode format
+       local redheart=$dir/$'\u2764\ufe0f'
+
+       _scratch_mkfs -O casefold -C utf8 >> $seqres.full
+       _scratch_mount
+
+       mkdir $dir
+       $F2FS_IO_PROG setflags casefold $dir >> $seqres.full
+       touch $redheart
+       ino=`stat -c '%i' $redheart`
+       _scratch_unmount
+
+       # should tune nolinear_lookup feature before fault injection
+       $F2FS_FSCK_PROG --nolinear-lookup=$nolinear_lookup $SCRATCH_DEV >> 
$seqres.full
+
+       # check whether linear_lookup is changed as expected
+       if [ $nolinear_lookup == "1" ]; then
+               $F2FS_FSCK_PROG $SCRATCH_DEV | grep -q "linear_lookup 
\[disable\]" ||   \
+                                       _fail "linear_lookup is not disabled"
+       else
+               $F2FS_FSCK_PROG $SCRATCH_DEV | grep -q "linear_lookup 
\[enable\]" ||    \
+                                       _fail "linear_lookup is not enabled"
+       fi
+
+       $F2FS_INJECT_PROG --dent --mb d_hash --nid $ino --val 0x9a2ea068 
$SCRATCH_DEV >> $seqres.full
+
+       _scratch_mount
+       if [ $nolinear_lookup == "1" ]; then
+               [ -f $redheart ] && _fail "red heart file should not exist"
+       else
+               [ -f $redheart ] || _fail "red heart file should exist"
+       fi
+       _scratch_unmount
+
+       # to avoid _check_filesystems failure in the end of test
+       _repair_scratch_fs >> $seqres.full
+}
+
+check_lookup 1
+check_lookup 0
+
+echo "Silence is golden"
+
+status=0
+exit
diff --git a/tests/f2fs/012.out b/tests/f2fs/012.out
new file mode 100644
index 00000000..3afeca2f
--- /dev/null
+++ b/tests/f2fs/012.out
@@ -0,0 +1,2 @@
+QA output created by 012
+Silence is golden
-- 
2.49.0



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