[PATCH 1/2] f2fs: fix to avoid return invalid mtime from f2fs_get_section_mtime()

[PATCH 1/2] f2fs: fix to avoid return invalid mtime from f2fs_get_section_mtime()

[Chao Yu]

syzbot reported a f2fs bug as below:

------------[ cut here ]------------
kernel BUG at fs/f2fs/gc.c:373!
CPU: 0 UID: 0 PID: 5316 Comm: syz.0.0 Not tainted 6.13.0-rc3-syzkaller-00044-gaef25be35d23 #0
RIP: 0010:get_cb_cost fs/f2fs/gc.c:373 [inline]
RIP: 0010:get_gc_cost fs/f2fs/gc.c:406 [inline]
RIP: 0010:f2fs_get_victim+0x68b1/0x6aa0 fs/f2fs/gc.c:912
Call Trace:
 <TASK>
 __get_victim fs/f2fs/gc.c:1707 [inline]
 f2fs_gc+0xc89/0x2f60 fs/f2fs/gc.c:1915
 f2fs_ioc_gc fs/f2fs/file.c:2624 [inline]
 __f2fs_ioctl+0x4cc9/0xb8b0 fs/f2fs/file.c:4482
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

w/ below testcase, it can reproduce directly:
- dd if=/dev/zero of=/tmp/file bs=1M count=64
- mkfs.f2fs /tmp/file
- mount -t f2fs -o loop,mode=fragment:block /tmp/file /mnt/f2fs
- echo 0 >  /sys/fs/f2fs/loop0/min_ssr_sections
- dd if=/dev/zero of=/mnt/f2fs/file bs=1M count=5
- umount /mnt/f2fs
- for((i=4096;i<16384;i+=512)) do inject.f2fs --sit 0 --blk $i --mb mtime --val -1 /tmp/file; done
- mount -o loop /tmp/file /mnt/f2fs
- f2fs_io gc 0 /mnt/f2fs/file

static unsigned int get_cb_cost()
{
	...
	mtime = f2fs_get_section_mtime(sbi, segno);
	f2fs_bug_on(sbi, mtime == INVALID_MTIME);
	...
}

The root cause is: mtime in f2fs_sit_entry can be fuzzed to INVALID_MTIME,
then it will trigger BUG_ON in get_cb_cost() during GC.

Let's change behavior of f2fs_get_section_mtime() as below for fix:
- return INVALID_MTIME only if total valid blocks is zero.
- return INVALID_MTIME - 1 if average mtime calculated is
INVALID_MTIME.

Fixes: b19ee7272208 ("f2fs: introduce f2fs_get_section_mtime")
Reported-by: syzbot+b9972806adbe20a910eb@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-f2fs-devel/6768c82e.050a0220.226966.0035.GAE@google.com
Cc: liuderong <liuderong@oppo.com>
Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/segment.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 813254dcc00e..b3a82a8cdc5f 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -5549,8 +5549,10 @@ unsigned long long f2fs_get_section_mtime(struct f2fs_sb_info *sbi,
 	secno = GET_SEC_FROM_SEG(sbi, segno);
 	start = GET_SEG_FROM_SEC(sbi, secno);
 
-	if (!__is_large_section(sbi))
-		return get_seg_entry(sbi, start + i)->mtime;
+	if (!__is_large_section(sbi)) {
+		mtime = get_seg_entry(sbi, start + i)->mtime;
+		goto out;
+	}
 
 	for (i = 0; i < usable_segs_per_sec; i++) {
 		/* for large section, only check the mtime of valid segments */
@@ -5563,7 +5565,11 @@ unsigned long long f2fs_get_section_mtime(struct f2fs_sb_info *sbi,
 	if (total_valid_blocks == 0)
 		return INVALID_MTIME;
 
-	return div_u64(mtime, total_valid_blocks);
+	mtime = div_u64(mtime, total_valid_blocks);
+out:
+	if (unlikely(mtime == INVALID_MTIME))
+		mtime -= 1;
+	return mtime;
 }
 
 /*
-- 
2.48.1.262.g85cc9f2d1e-goog

[PATCH 2/2] f2fs: procfs: show mtime in segment_bits

[Chao Yu]

Show mtime in segment_bits for debug.

cat /proc/fs//f2fs/loop0/segment_bits
format: segment_type|valid_blocks|bitmaps|mtime
segment_type(0:HD, 1:WD, 2:CD, 3:HN, 4:WN, 5:CN)
0         3|1  | 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00| ffffffffffffffff
1         4|3  | 00 d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00| ffffffffffffffff
2         5|0  | 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00| ffffffffffffffff
3         0|1  | 40 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00| ffffffffffffffff

Signed-off-by: Chao Yu <chao@kernel.org>
---
 fs/f2fs/sysfs.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
index f81190fabdd3..b01c5984cf22 100644
--- a/fs/f2fs/sysfs.c
+++ b/fs/f2fs/sysfs.c
@@ -1480,7 +1480,7 @@ static int __maybe_unused segment_bits_seq_show(struct seq_file *seq,
 			le32_to_cpu(sbi->raw_super->segment_count_main);
 	int i, j;
 
-	seq_puts(seq, "format: segment_type|valid_blocks|bitmaps\n"
+	seq_puts(seq, "format: segment_type|valid_blocks|bitmaps|mtime\n"
 		"segment_type(0:HD, 1:WD, 2:CD, 3:HN, 4:WN, 5:CN)\n");
 
 	for (i = 0; i < total_segs; i++) {
@@ -1490,6 +1490,7 @@ static int __maybe_unused segment_bits_seq_show(struct seq_file *seq,
 		seq_printf(seq, "%d|%-3u|", se->type, se->valid_blocks);
 		for (j = 0; j < SIT_VBLOCK_MAP_SIZE; j++)
 			seq_printf(seq, " %.2x", se->cur_valid_map[j]);
+		seq_printf(seq, "| %llx", se->mtime);
 		seq_putc(seq, '\n');
 	}
 	return 0;
-- 
2.48.1.262.g85cc9f2d1e-goog

Re: [f2fs-dev] [PATCH 1/2] f2fs: fix to avoid return invalid mtime from f2fs_get_section_mtime()

[patchwork-bot+f2fs]

Hello:

This series was applied to jaegeuk/f2fs.git (dev)
by Jaegeuk Kim <jaegeuk@kernel.org>:

On Mon, 20 Jan 2025 19:19:40 +0800 you wrote:
> syzbot reported a f2fs bug as below:
> 
> ------------[ cut here ]------------
> kernel BUG at fs/f2fs/gc.c:373!
> CPU: 0 UID: 0 PID: 5316 Comm: syz.0.0 Not tainted 6.13.0-rc3-syzkaller-00044-gaef25be35d23 #0
> RIP: 0010:get_cb_cost fs/f2fs/gc.c:373 [inline]
> RIP: 0010:get_gc_cost fs/f2fs/gc.c:406 [inline]
> RIP: 0010:f2fs_get_victim+0x68b1/0x6aa0 fs/f2fs/gc.c:912
> Call Trace:
>  <TASK>
>  __get_victim fs/f2fs/gc.c:1707 [inline]
>  f2fs_gc+0xc89/0x2f60 fs/f2fs/gc.c:1915
>  f2fs_ioc_gc fs/f2fs/file.c:2624 [inline]
>  __f2fs_ioctl+0x4cc9/0xb8b0 fs/f2fs/file.c:4482
>  vfs_ioctl fs/ioctl.c:51 [inline]
>  __do_sys_ioctl fs/ioctl.c:906 [inline]
>  __se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> [...]

Here is the summary with links:
  - [f2fs-dev,1/2] f2fs: fix to avoid return invalid mtime from f2fs_get_section_mtime()
    https://git.kernel.org/jaegeuk/f2fs/c/75fe3d484897
  - [f2fs-dev,2/2] f2fs: procfs: show mtime in segment_bits
    https://git.kernel.org/jaegeuk/f2fs/c/46bbac0db377

You are awesome, thank you!
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