Re: Extended file stat: Splitting file- and fs-specific info?

On 05/09/2012 01:19 PM, Christoph Hellwig wrote:

On Wed, May 09, 2012 at 10:21:14AM +0100, David Howells wrote:

quoted

Dave Chinner[off-list ref]  wrote:

quoted

I don't think we want to expose the inode generation numbers. It is
trivial to construct NFS file handles (usually just fsid, inode
number and generation) with that information and hence bypass
security checks to access files.

I was asked for it by Bernd Schubert for userspace NFS servers and FUSE -
maybe he can say what he wants it for.

It's entirely broken, as a generation number might be part of the file
handle (and for Linux-like filesystems normally is), but it's entirely
up to the filesystem to decide how it works.  That's why we added system
calls to do operations on opaque file handles that the file system
controls.  Exposing a completely meaningless "generation" is a bad idea.

The basic idea of generation numbers is to check if an inode was 
recycled, so only if the tuple of inode-number and generation-number 
matches we still have the same file. Kernel nfs uses that and unfs3 uses 
it via EXT2_IOC_GETVERSION, which has the overhead of an additional 
syscall. Unionfs-fuse usually keeps file open, however, it might run out 
of the maximum allowed files and I plan to add a mode to close and 
re-open files as failback mode. For that the definite knowledge if a 
file/inode is still the very same and the inode was not just recycled is 
crucial.

All of that being said, I think with open_by_handle_at() syscall we 
don't need the inode generation number any more.



Cheers,
Bernd
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