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

From: OGAWA Hirofumi
Date: Fri Dec 21 2012 - 03:19:25 EST

Namjae Jeon <linkinjeon@xxxxxxxxx> writes:

>>> Hm, start with copy of fat_search_long() and refactoring it. With it, we
>>> will be able to avoid the fixed bugs.
>>> After that, we might be able to merge those somehow. Well, I'm not
>>> pretty sure without doing it actually though.
> When we checked to merge it with fat_search_long, we really did not
> find any possibility of code reusing for fat_traverse_cluster.
> It will not be proper. In case of fat_search_long()-> it is performing
> a name based lookup in a particular directory.
> While for reconnection with parent from NFS, we do not have the name
> of the parent directory. We are relying on ‘i_pos’ on disk position of
> directory entry.
> So, on first request for lookup for “..” (i.e if we call
> fat_search_long(child_dir->d_inode, MSDOS_DOTDOT,2,slot_info) it will
> return the directory entry for “..” itself. From this entry we can
> read the “log start” which is the starting cluster of the parent
> directory, but instead we need the “directory entry” where this is
> stored.
> So, from this level we need to go further one level up and read the
> parent ->parent-> cluster. After reading that cluster, we need to do a
> lookup of the “required ipos” in this directory block.
> i.e., if the path is A/B/C and we call the get_parent() from ‘C’. We
> need to read the directory block contents of ‘A’ and from those
> ‘directory entries' compare the log_start with the log_start value of
> B, which was obtained by doing a lookup “..” in C.
> So, Instead of it, we suggest we fix the “infinite-loop” condition in
> fat_traverse_logic and retain the code.
> of course, we will test it with corrupted FATfs.
> Please share your thoughts on this.

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.

OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
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