linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: fix ctx->pos in f2fs_read_inline

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Date:   Sat, 17 Jul 2021 22:30:42 +0800
From:   Chao Yu <chao@...nel.org>
To:     李扬韬 <frank.li@...o.com>
Cc:     jaegeuk@...nel.org, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix ctx->pos in f2fs_read_inline_dir()

On 2021/7/17 21:25, 李扬韬 wrote:
> HI Chao,
> 
> From: Chao Yu <chao@...nel.org>
> Date: 2021-07-17 16:56:01
> To:  Yangtao Li <frank.li@...o.com>,jaegeuk@...nel.org
> Cc:  linux-kernel@...r.kernel.org,linux-f2fs-devel@...ts.sourceforge.net
> Subject: Re: [f2fs-dev] [PATCH] f2fs: fix ctx->pos in f2fs_read_inline_dir()>On 2021/7/17 11:49, Yangtao Li wrote:
>>> I recently found a case where de->name_len is 0 in f2fs_fill_dentries() easily reproduced,
>>> and finally set the fsck flag.
>>>
>>> Thread A					Thread B
>>>
>>> f2fs_readdir
>>> 	f2fs_read_inline_dir
>>> 		ctx->pos = d.max
>>> 						f2fs_add_dentry
>>> 							f2fs_add_inline_entry
>>> 								do_convert_inline_dir
>>> 							f2fs_add_regular_entry
>>> f2fs_readdir
>>> 	f2fs_fill_dentries
>>> 		set_sbi_flag(sbi, SBI_NEED_FSCK)
>>>
>>> Process A opens the folder, and has been reading without closing it. During this period,
>>> Process B created a file under the folder (occupying multiple f2fs_dir_entry, exceeding
>>> the d.max of the inline dir). After creation, process A uses the d.max of inline dir to
>>> read it again, and it will read that de->name_len is 0.
>>
>> Nice catch!
>>
>>>
>>> And returning early in f2fs_read_inline_dir will cause the process to be unable to see
>>> the changes before reopening the file.
>>>
>>> So don't return early and remove the modification of ctx->pos in f2fs_read_inline_dir().
>>>
>>> Signed-off-by: Yangtao Li <frank.li@...o.com>
>>> ---
>>>    fs/f2fs/inline.c | 5 -----
>>>    1 file changed, 5 deletions(-)
>>>
>>> diff --git a/fs/f2fs/inline.c b/fs/f2fs/inline.c
>>> index 56a20d5c15da..fc6551139a3d 100644
>>> --- a/fs/f2fs/inline.c
>>> +++ b/fs/f2fs/inline.c
>>> @@ -729,9 +729,6 @@ int f2fs_read_inline_dir(struct file *file, struct dir_context *ctx,
>>>    
>>>    	make_dentry_ptr_inline(inode, &d, inline_dentry);
>>>    
>>> -	if (ctx->pos == d.max)
>>> -		return 0;
>>> -
>>>    	ipage = f2fs_get_node_page(F2FS_I_SB(inode), inode->i_ino);
>>>    	if (IS_ERR(ipage))
>>>    		return PTR_ERR(ipage);
>>> @@ -747,8 +744,6 @@ int f2fs_read_inline_dir(struct file *file, struct dir_context *ctx,
>>>    	make_dentry_ptr_inline(inode, &d, inline_dentry);
>>>    
>>>    	err = f2fs_fill_dentries(ctx, &d, 0, fstr);
>>
>> After this function, ctx->pos will point to start position of first free slot after
>> last dir_entry, we can't guarantee that the free slot won't be used in above race
>> condition, right?
>>
>> Moreover, w/o inline conversion, the race condition still can happen as below, right?
>>
>> dir_entry1: A
>> dir_entry2: B
>> dir_entry3: C
>> free slot: _
>>
>> Before:
>> AAAABBBB___
>>          ^
>> Thread B delete dir_entry2, and create dir_entry3.
>>
>> After:
>> AAAACCCCC__
>>          ^
> 
> Taking into account the above situations, I think this case where de->name_len is 0 in f2fs_fill_dentries()
> should be normal and there is no way to avoid it. Maybe we shouldn't set fsck flag at this time?
> Because the file system is not damaged.

Yangtao,

IMO, it should bypass tagging FSCK flag only if:
a) bit_pos (:= ctx->pos % d->max) is non-zero & b) before bit_pos moves to first
valid dir_entry.

Thanks,

> 
> MBR / Yangtao
> 
>