[PATCH] vfs: Fix possible NULL pointer dereference in inode_permission()

[Ben Myers]

Christoph,

On Fri, Jan 10, 2014 at 01:31:48AM -0800, Christoph Hellwig wrote:
> On Fri, Jan 10, 2014 at 12:06:42AM +0000, Al Viro wrote:
> > Check what XFS is doing ;-/  That's where those call_rcu() have come from.
> > Sure, we can separate the simple "just do call_rcu(...->free_inode)" case
> > and hit it whenever full ->free_inode is there and ->destroy_inode isn't.
> > Not too pretty, but removal of tons of boilerplate might be worth doing
> > that anyway.  But ->destroy_inode() is still needed for cases where fs
> > has its own idea of inode lifetime rules.  Again, check what XFS is doing
> > in that area...
> 
> Btw, I'd really love to get rid of the XFS ->destroy_inode abuse, it's
> been a long time thorn in the flesh.

I believe this behavior is related to freeing of an inode cluster.

> What's really needed there to make XFS behave more similar to everyone
> else is a way for the filesystem to say: "I can't actually free this
> inode right now, but I'll come back to you later".

This test might read something like:  "If my link count has gone to zero, and I
am the last inode in my cluster to be freed, and there are other inodes from my
cluster incore, I cannot be freed."

Should be doable.  Maybe there are other reasons.

-Ben