Re: [PATCH v3?] proc_sysctl: remove rcu_dereference() for accessing ->sysctl

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Jul 04, 2025 at 11:39:52AM +1000, NeilBrown wrote:
> On Fri, 04 Jul 2025, Al Viro wrote:
> > On Fri, Jul 04, 2025 at 12:43:13AM +0100, Al Viro wrote:
> > 
> > > I would rather *not* leave a dangling pointer there, and yes, it can
> > > end up being dangling.  kfree_rcu() from inside the ->evict_inode()
> > > may very well happen earlier than (also RCU-delayed) freeing of struct
> > > inode itself.
> > > 
> > > What we can do is WRITE_ONCE() to set it to NULL on the evict_inode
> > > side and READ_ONCE() in the proc_sys_compare().
> > > 
> > > The reason why the latter is memory-safe is that ->d_compare() for
> > > non-in-lookup dentries is called either under rcu_read_lock() (in which
> > > case observing non-NULL means that kfree_rcu() couldn't have gotten to
> > > freeing the sucker) *or* under ->d_lock, in which case the inode can't
> > > reach ->evict_inode() until we are done.
> > > 
> > > So this predicate is very much relevant.  Have that fucker called with
> > > neither rcu_read_lock() nor ->d_lock, and you might very well end up
> > > with dereferencing an already freed ctl_table_header.
> > 
> > IOW, I would prefer to do this:
> 
> Looks good - thanks,
> NeilBrown

See viro/vfs.git #fixes...