[Help] How to Replace File Operations in File System?

[Abhijit Chandrakant Pawar]

Hi,

On Thursday 13 February 2014 07:40 AM, freeman wrote:
> Hi list,
> 
> I am a newbie in linux kernel programming. Recently I got stuck in a
> problem when doing
> practice in file system programming. I hope this list is the right place
> I can turn to.
> 
> I want to replace some file operations of files in a certain
> directory,so that data can be
> decrypted/encrypted through read/write system call. So I:
> 
> #1: Find the directory inode, save its original inode operation
> table,then replace
> the table:
> 
> kern_path(pathname, LOOKUP_FOLLOW, &target_dir_path);
> lower_iops = target_dir_path.dentry->d_inode->i_op;
> target_dir_path.dentry->d_inode->i_op = &my_iops;
> 
I assume that you are writing your own stackable filesystem.
Take a look at WRAPFS[1] and ecryptfs[2]. As a matter of fact, ecryptfs
does what you are expecting.

To do this, you need to set your superblock operations for the lower
directory inode so the VFS use your filesystem instead of the original
filesystem.
Important steps to look are:
1. get lower superblock from the lower directory inode
2. assign this superblock as an overlay for your own superblock.
3. Set your own superblock operations for the new superblock
4. get a root inode for your superblock using the lower directory inode
5. While you are getting the inode, you can set the file operations on
this inode which will help you achieve your case.

The point to note that you need to interpose the inodes with VFS so that
everything would be routed to your filesystem.

--
Abhijit.
[1]http://wrapfs.filesystems.org/
[2]http://ecryptfs.org/


> #2: In my_iops, I mainly changed ".lookup" function like this to achive
> my goal －－
> replace the file operation table of all files in the directory.
> 
> static struct dentry *my_inode_lookup(struct inode *dir, struct dentry
> *dentry,
> struct nameidata *nd)
> {
> struct dentry *ret_dentry;
> 
> ret_dentry = lower_iops->lookup(dir,dentry,nd);
> if (!ret_dentry)
> goto out;
> ret_dentry->d_inode->i_fop = &my_fops;
> out:
> return ret_dentry;
> }
> 
> Things turns out that replacement of inode operation table of directory
> is successful
> but the changes in file operations are not functional: system works as
> it used to,
> totally ignore my_fops!
> 
> I have no idea how to fix it. Can anybody help?
> Thanks for your attention!
> Regards
> 
> Freeman Zhang
> 
> 
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