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

[Al Viro]

On Mon, Jun 16, 2025 at 12:49:51PM +1000, NeilBrown wrote:

> The reality is that ->sysctl does not need rcu protection.  There is no
> concurrent update except that it can be set to NULL which is pointless.

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.