linux-kernel - [PATCH v1] f2fs: fix uninitialized one_time_gc in victim_sel

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Message-Id: <20251029051807.3804-1-hexiaole1994@126.com>
Date: Wed, 29 Oct 2025 13:18:07 +0800
From: Xiaole He <hexiaole1994@....com>
To: linux-f2fs-devel@...ts.sourceforge.net
Cc: jaegeuk@...nel.org,
	chao@...nel.org,
	daehojeong@...gle.com,
	linux-kernel@...r.kernel.org,
	Xiaole He <hexiaole1994@....com>,
	stable@...nel.org
Subject: [PATCH v1] f2fs: fix uninitialized one_time_gc in victim_sel_policy

The one_time_gc field in struct victim_sel_policy is conditionally
initialized but unconditionally read, leading to undefined behavior
that triggers UBSAN warnings.

In f2fs_get_victim() at fs/f2fs/gc.c:774, the victim_sel_policy
structure is declared without initialization:

    struct victim_sel_policy p;

The field p.one_time_gc is only assigned when the 'one_time' parameter
is true (line 789):

    if (one_time) {
        p.one_time_gc = one_time;
        ...
    }

However, this field is unconditionally read in subsequent get_gc_cost()
at line 395:

    if (p->one_time_gc && (valid_thresh_ratio < 100) && ...)

When one_time is false, p.one_time_gc contains uninitialized stack
memory. Hence p.one_time_gc is an invalid bool value.

UBSAN detects this invalid bool value:

    UBSAN: invalid-load in fs/f2fs/gc.c:395:7
    load of value 77 is not a valid value for type '_Bool'
    CPU: 3 UID: 0 PID: 1297 Comm: f2fs_gc-252:16 Not tainted 6.18.0-rc3
    #5 PREEMPT(voluntary)
    Hardware name: OpenStack Foundation OpenStack Nova,
    BIOS 1.13.0-1ubuntu1.1 04/01/2014
    Call Trace:
     <TASK>
     dump_stack_lvl+0x70/0x90
     dump_stack+0x14/0x20
     __ubsan_handle_load_invalid_value+0xb3/0xf0
     ? dl_server_update+0x2e/0x40
     ? update_curr+0x147/0x170
     f2fs_get_victim.cold+0x66/0x134 [f2fs]
     ? sched_balance_newidle+0x2ca/0x470
     ? finish_task_switch.isra.0+0x8d/0x2a0
     f2fs_gc+0x2ba/0x8e0 [f2fs]
     ? _raw_spin_unlock_irqrestore+0x12/0x40
     ? __timer_delete_sync+0x80/0xe0
     ? timer_delete_sync+0x14/0x20
     ? schedule_timeout+0x82/0x100
     gc_thread_func+0x38b/0x860 [f2fs]
     ? gc_thread_func+0x38b/0x860 [f2fs]
     ? __pfx_autoremove_wake_function+0x10/0x10
     kthread+0x10b/0x220
     ? __pfx_gc_thread_func+0x10/0x10 [f2fs]
     ? _raw_spin_unlock_irq+0x12/0x40
     ? __pfx_kthread+0x10/0x10
     ret_from_fork+0x11a/0x160
     ? __pfx_kthread+0x10/0x10
     ret_from_fork_asm+0x1a/0x30
     </TASK>

This issue is reliably reproducible with the following steps on a
100GB SSD /dev/vdb:

    mkfs.f2fs -f /dev/vdb
    mount /dev/vdb /mnt/f2fs_test
    fio --name=gc --directory=/mnt/f2fs_test --rw=randwrite \
        --bs=4k --size=8G --numjobs=12 --fsync=4 --runtime=10 \
        --time_based
    echo 1 > /sys/fs/f2fs/vdb/gc_urgent

The uninitialized value causes incorrect GC victim selection, leading
to unpredictable garbage collection behavior.

Fix by zero-initializing the entire victim_sel_policy structure to
ensure all fields have defined values.

Fixes: e791d00bd06c ("f2fs: add valid block ratio not to do excessive GC for one time GC")
Cc: stable@...nel.org
Signed-off-by: Xiaole He <hexiaole1994@....com>
---
 fs/f2fs/gc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index a7708cf80c04..56e372c23a78 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -771,7 +771,7 @@ int f2fs_get_victim(struct f2fs_sb_info *sbi, unsigned int *result,
 {
 	struct dirty_seglist_info *dirty_i = DIRTY_I(sbi);
 	struct sit_info *sm = SIT_I(sbi);
-	struct victim_sel_policy p;
+	struct victim_sel_policy p = {0};
 	unsigned int secno, last_victim;
 	unsigned int last_segment;
 	unsigned int nsearched;
-- 
2.34.1