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Date:   Tue, 25 Oct 2022 10:26:14 +0800
From:   Baokun Li <>
To:     Jan Kara <>
CC:     <>, <>,
        <>, <>,
        <>, <>,
Subject: Re: [PATCH v2 2/2] ext4: fix bug_on in __es_tree_search caused by
 wrong boot loader inode

On 2022/10/24 22:25, Jan Kara wrote:
> On Fri 21-10-22 12:07:31, Baokun Li wrote:
>> We got a issue as fllows:
>> ...
>> In the above issue, ioctl invokes the swap_inode_boot_loader function to
>> swap inode<5> and inode<12>. However, inode<5> contain incorrect imode and
>> disordered extents, and i_nlink is set to 1. The extents check for inode in
>> the ext4_iget function can be bypassed bacause 5 is EXT4_BOOT_LOADER_INO.
>> While links_count is set to 1, the extents are not initialized in
>> swap_inode_boot_loader. After the ioctl command is executed successfully,
>> the extents are swapped to inode<12>, in this case, run the `cat` command
>> to view inode<12>. And Bug_ON is triggered due to the incorrect extents.
>> When the boot loader inode is not initialized, its imode can be one of the
>> following:
>> 1) the imode is a bad type, which is marked as bad_inode in ext4_iget and
>>     set to S_IFREG.
>> 2) the imode is good type but not S_IFREG.
>> 3) the imode is S_IFREG.
>> The BUG_ON may be triggered by bypassing the check in cases 1 and 2.
>> Therefore, when the boot loader inode is bad_inode or its imode is not
>> S_IFREG, initialize the inode to avoid triggering the BUG.
>> Signed-off-by: Baokun Li <>
> Grepping for calls to ext4_iget() in the ext4 code shows there are many
> more places that will get unhappy (and crash) when ext4_iget() returns a
> bad inode. In fact, I didn't find a place when returning bad inode would be
> useful for anything. So why don't we just return EFSCORRUPTED instead of
> returning a bad inode?
> 								Honza

Hello Honza,

In ext4_iget(), the inode is marked as bad and returned only when ino is 
equal to
EXT4_BOOT_LOADER_INO. In the error branch bad_inode, although the inode is
marked as bad, the returned value is the corresponding error number.
The boot loader inode is not initialized during mkfs. Therefore, when 
ext4_iget() is
entered for the first time, imode of the inode is bad type. However, the
swap_inode_boot_loader() needs to obtain the inode for initialization 
and swap.
Therefore, a bad_inode is returned in ext4_iget.

Generally, ext4_iget() does not get the boot loader inode. Therefore, we 
only need
to pay attention to the special inodes that can be specified.
The following figure shows the check result:

1) usr_quota_inum/grp_quota_inum/prj_quota_inum
These inodes may be faulty. In the first patch, this situation is 
At the beginning, FUZZ found that the quota inode was faulty. Later, we 
found that
the operation function swap_inode_boot_loader() related to inode 5 was 
also faulty.

2) journal_inum
In ext4_get_journal_inode(), the system checks whether the imode is 
S_IFREG. Then,
the bmap in jbd2_journal_init_inode() checks whether the inode has 
operation. The bad inode does not set the bmap operation, so there is no 

3) last_orphan
In ext4_orphan_get(), it checks if the imode is normal and if the inode 
is bad inode,
so there is no problem.

4) snapshot_inum
No place to use snapshot_inum was found in the kernel, so there is no 
kernel issue.

>> ---
>>   fs/ext4/ioctl.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
>> index ded535535b27..c41210706ea7 100644
>> --- a/fs/ext4/ioctl.c
>> +++ b/fs/ext4/ioctl.c
>> @@ -425,7 +425,7 @@ static long swap_inode_boot_loader(struct super_block *sb,
>>   	/* Protect extent tree against block allocations via delalloc */
>>   	ext4_double_down_write_data_sem(inode, inode_bl);
>> -	if (inode_bl->i_nlink == 0) {
>> +	if (is_bad_inode(inode_bl) || !S_ISREG(inode_bl->i_mode)) {
>>   		/* this inode has never been used as a BOOT_LOADER */
>>   		set_nlink(inode_bl, 1);
>>   		i_uid_write(inode_bl, 0);
>> -- 
>> 2.31.1

Thank you for your review!
With Best Regards,
Baokun Li

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