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

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Message-ID: <f2ff436b-d0de-4d68-8374-1c6bf777d6ee@kernel.org>
Date: Wed, 29 Oct 2025 18:48:03 +0800
From: Chao Yu <chao@...nel.org>
To: Xiaole He <hexiaole1994@....com>, linux-f2fs-devel@...ts.sourceforge.net
Cc: chao@...nel.org, jaegeuk@...nel.org, daehojeong@...gle.com,
 linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH v1] f2fs: fix uninitialized one_time_gc in
 victim_sel_policy

On 10/29/25 13:18, Xiaole He wrote:
> 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>

Reviewed-by: Chao Yu <chao@...nel.org>

Thanks,