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

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Message-ID: <2d651236-40de-2e7b-d6c6-9a18ce9a25ff@kernel.org>
Date:   Sat, 17 Jul 2021 16:56:01 +0800
From:   Chao Yu <chao@...nel.org>
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__
         ^
Thanks,

> -	if (!err)
> -		ctx->pos = d.max;
>   
>   	f2fs_put_page(ipage, 0);
>   	return err < 0 ? err : 0;
>