what's relationship between VFS inode and ext3_inode?

[Theodore Tso]

On Tue, Jun 02, 2009 at 08:05:48PM +0800, Zhang Shukun wrote:
> thanks for your answer!
> 
> my goal is : in kernel space , get the specific disk block numbers(or sector
> number) of a file(such as /bin/ls).
> 
> because i want to use the block number in DomU, which is a VM in xen. when i
> read or write a file in DomU, front end driver will send the block number to
> back-end driver Dom0 , and then in dom0 i can check if the block could be
> write or not.

The filesystem does that already (send the block number to the
back-end driver) when it wants to read or write the block form the
filesystem.  If the goal is to do this for security purposes, you
should be calculating the inode->block mapping in dom0, *not* in domU.
After all, if you don't trust the domU to enforce the normal security
restrictions which the kernel enforces, why would you trust domU to
give you the correct inode->block mapping so that dom0 can do some
kind of security access check?

In any case, to answer your specific question, to convert from a
struct inode to a struct ext3_inode_info, *if* you know that a
particular struct inode from an ext3 filesystem (as opposed to an ext2
filesystem, ext4 filesystem, FAT filesystem, a networking socket,
etc.) you can use the EXT3_I function to get from an struct inode to
struct ext3_inode_info:

static inline struct ext3_inode_info *EXT3_I(struct inode *inode)
{
	return container_of(inode, struct ext3_inode_info, vfs_inode);
}

What happens here is that the struct inode is actually embedded inside
the ext3_inode_info structure, and what is cached is the
ext3_inode_info structure.  So container_of() basically takes a
pointer to the struct inode, and substracts the offset from the
beginning of ext3_inode_info and the vfs_inode element in
ext3_inode_info, to get a pointer to the struct ext3_inode_info.

If this isn't an ext3 inode, though, your kernel code will likely
crash from the nonsense it will get when it tries to interpret data
that isn't a struct ext3_inode_info.  In general, the *only* place
where well-written, maintainable, kernel code should try to use EXT3_I
is inside fs/ext3/*.c.

					- Ted