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

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

On Mon, Jun 16, 2025 at 09:00:39AM +1000, NeilBrown wrote:
> 
> The rcu_dereference() call in proc_sys_compare() is problematic as
> ->d_compare is not guaranteed to be called with rcu_read_lock() held and
> rcu_dereference() can cause a warning when used without that lock.
> 
> Specifically d_alloc_parallel() will call ->d_compare() without
> rcu_read_lock(), but with ->d_lock to ensure stability.  In this case
> ->d_inode is usually NULL so the rcu_dereference() will normally not be
> reached, but it is possible that ->d_inode was set while waiting for
> ->d_lock which could lead to the warning.

Huh?

There are two call sites of d_same_name() in d_alloc_parallel() - one
in the loop (under rcu_read_lock()) and another after the thing we
are comparing has ceased to be in-lookup.  The latter is under ->d_lock,
stabilizing everything (and it really can't run into NULL ->d_inode
for /proc/sys/ stuff).

->d_compare() instances are guaranteed dentry->d_lock or rcu_read_lock();
in the latter case we'll either recheck or validate on previously sampled
->d_seq.  And the second call in d_alloc_parallel() is just that - recheck
under ->d_lock.

Just use rcu_dereference_check(...., spin_is_locked(&dentry->d_lock)) and
be done with that...

The part where we have a somewhat wrong behaviour is not the second call
in d_alloc_parallel() - it's the first one.  Something like this

static int proc_sys_compare(const struct dentry *dentry,
		unsigned int len, const char *str, const struct qstr *name)
{
	struct ctl_table_header *head;
	struct inode *inode;

	if (name->len != len)
		return 1;
	if (memcmp(name->name, str, len))
		return 1;

	// false positive is fine here - we'll recheck anyway
	if (d_in_lookup(dentry))
		return 0;

	inode = d_inode_rcu(dentry);
	// we just might have run into dentry in the middle of __dentry_kill()
	if (!inode)
		return 1;
	head = rcu_dereference_check(PROC_I(inode)->sysctl,
				     spin_is_locked(&dentry->d_lock));
	return !head || !sysctl_is_seen(head);
}