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...
