Re: About the conflict between XFS inode recycle and VFS rcu-walk

[Brian Foster <bfoster@redhat.com>]

On Mon, May 27, 2024 at 01:18:23AM +0100, Al Viro wrote:
> On Mon, May 27, 2024 at 07:51:39AM +0800, Ian Kent wrote:
> 
> > Indeed, that's what I found when I had a quick look.
> > 
> > 
> > Maybe a dentry (since that's part of the subject of the path walk and inode
> > is readily
> > 
> > accessible) flag could be used since there's opportunity to set it in vfs
> > callbacks that
> > 
> > are done as a matter of course.
> 
> You might recheck ->d_seq after fetching ->get_link there; with XFS
> ->get_link() unconditionlly failing in RCU mode that would prevent
> this particular problem.  But it would obviously have to be done
> in pick_link() itself (and I refuse to touch that area in 5.4 -
> carrying those changes across the e.g. 5.6 changes in pathwalk
> machinery is too much).
> 

Ian sent a patch along those lines a couple years or so ago:

https://lore.kernel.org/linux-fsdevel/164180589176.86426.501271559065590169.stgit@mickey.themaw.net/

I'm still not quite sure why we didn't merge this, at least as a bandaid
fix for the symlink variant of this particular problem..?

Brian

> And it's really just the tip of the iceberg - e.g. I'd expect a massive
> headache in ACL-related part of permission checks, etc.
>