linux-kernel - Re: [PATCH v5 7/8] fat (exportfs): rebuild directory-inode if fat

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Date:	Fri, 21 Dec 2012 19:36:33 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Namjae Jeon <linkinjeon@...il.com>
Cc:	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Namjae Jeon <namjae.jeon@...sung.com>,
	Ravishankar N <ravi.n1@...sung.com>,
	Amit Sahrawat <a.sahrawat@...sung.com>
Subject: Re: [PATCH v5 7/8] fat (exportfs): rebuild directory-inode if fat_dget() fails

OGAWA Hirofumi <hirofumi@...l.parknet.co.jp> writes:

> Namjae Jeon <linkinjeon@...il.com> writes:
>
>>> Yes, we can't use fat_search_long() as is, of course. However, we can
>>> share the basic algorithm and code.
>>>
>>> The both are doing,
>>>
>>> 1) traverse the blocks chained by ->i_start.
>>> 2) get the record (dirent) from blocks.
>>> 3) check the detail of record
>>>
>>> The difference is only (3), right? I know, the code has many differences
>>> though. The actual logic are almost same.
>>>
>>> And see, e.g., fat_get_cluster() is checking several unexpected
>>> state. We have to care about corrupting data. It is not only
>>> "infinite-loop" case.  And why I'm saying it is better to share code.
>>
>> Regarding unexpected conditions,
>> When we compare the unexpected conditions in fat_get_cluster:
>> We get these as conditions which can arise:
>> fat_end_read() returns:
>>  1) < 0
>>  2) FAT_ENT_FREE
>>  3) FAT_ENT_EOF
>> And the last is
>>       4) Preventing the infinite looping of cluster chain.
>>
>> Our patch is already covering the cases:
>>  case 1) , 2) =>  if (search_clus < 0 || search_clus == FAT_ENT_FREE)
>>  about  case 3) => while (search_clus != FAT_ENT_EOF);
>>
>> while for Case 4:
>> We can make changes like this:
>>
>> @@ -179,8 +179,9 @@ struct dentry *fat_get_parent(struct dentry *child_dir)
>>         struct inode *parent_inode = NULL;
>>         struct msdos_sb_info *sbi = MSDOS_SB(sb);
>>         int parent_logstart;
>> -       int search_clus, clus_to_match;
>> +       int search_clus, clus_to_match, clus_count = 0;
>>         sector_t blknr;
>> +       const int limit = sb->s_maxbytes >> MSDOS_SB(sb)->cluster_bits;
>>
>>         if (!fat_get_dotdot_entry(child_dir->d_inode, &dotdot_bh, &de)) {
>>                 parent_logstart = fat_get_start(sbi, de);
>> @@ -223,6 +224,14 @@ struct dentry *fat_get_parent(struct dentry *child_dir)
>>                                                 search_clus, clus_to_match);
>>                                 if (IS_ERR(parent_inode) || parent_inode)
>>                                         break;
>> +                               if (++clus_count > limit) {
>> +                                       fat_fs_error_ratelimit(sb,
>> +                                       "%s: detected the cluster chain loop"
>> +                                       " while reading directory entries from"
>> +                                       " cluster %d", __func__, search_clus);
>> +                                       parent_inode = ERR_PTR(-EIO);
>> +                                       break;
>> +                               }
>>                                 fatent_init(&fatent);
>>                                 search_clus = fat_ent_read(sb, &fatent,
>>                                                            search_clus);
>
> and it is copy of fat_get_cluster(), right? It would be sane as start
> though, it is true to increase maintain cost, and it makes more similar
> to fat_search_long() path. It is why I said, copy at first, and
> refactoring after that.

BTW, fat_search_long() was wrong as similar function. Actually it would
be fat_scan(), because we don't care longname entry.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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