vn_fullpath() again

[Robert Watson]

On Mon, 5 Sep 2005, Sergey Uvarov wrote:

> all knows that vn_fullpath() is unreliable. However I really need to get 
> a filename for a given vnode. To simplify the task, I do not care of 
> synthetic file systems or hardlinks.
>
> I have looked through archives in hope to find a better solution. It 
> seems that linux_getcwd() approach could help. However to make that code 
> work for me, I need to know a directory vnode where the file resides. 
> vnode->v_dd field looks promising. But as I understand it did not help 
> if file name is not in a name cache.
>
> So the question: is it ever possible to get directory vnode for a given 
> file vnode?

One way to look at the problem is from the perspective of how you might 
derive that information from an on-disk inode.  If you look at the UFS 
layout on-disk, you'll see that there is no pointer to a directory back 
from a leaf inode; in kernel, you can have a reference to a vnode with no 
back pointer to a directory vnode.  In order to find the parent, you 
potentially have to iterate through all directories on the hard disk 
looking for the parent, which is a potentially long-running activity. 
It's also not at all theoretical: vnodes are often accessed without any 
path lookup at all.  For example, background fsck may pull inodes off disk 
without a name lookup, and the NFS server can receive file handle 
references following a reboot from a live client that maintains cached 
references -- it will service them without performing a lookup.

So unfortunately, the answer is complex: (a) you may have to search the 
disk for a name, and (b) you may not even find one, since there can be 
files without any name at all (i.e., a temporary file that has been 
unlinked).

On non-UFS style file systems, such as HFS+, it is possible to generate a 
path from the file system root without extensive disk I/O.  However, all 
common UNIX-like file systems don't have this property -- Sun's version of 
UFS, ext2fs/ext3fs, and so on.

If the child vnode is a directory, you can just follow it's '..' link or 
covered vnode, of course...

Robert N M Watson