fix proc_sys_compare() handling of in-lookup dentries

There's one case where ->d_compare() can be called for an in-lookup
dentry; usually that's nothing special from ->d_compare() point of
view, but... proc_sys_compare() is weird.

The thing is, /proc/sys subdirectories can look differently for
different processes. Up to and including having the same name
resolve to different dentries - all of them hashed.

The way it's done is ->d_compare() refusing to admit a match unless
this dentry is supposed to be visible to this caller. The information
needed to discriminate between them is stored in inode; it is set
during proc_sys_lookup() and until it's done d_splice_alias() we really
can't tell who should that dentry be visible for.

Normally there's no negative dentries in /proc/sys; we can run into
a dying dentry in RCU dcache lookup, but those can be safely rejected.

However, ->d_compare() is also called for in-lookup dentries, before
they get positive - or hashed, for that matter. In case of match
we will wait until dentry leaves in-lookup state and repeat ->d_compare()
afterwards. In other words, the right behaviour is to treat the
name match as sufficient for in-lookup dentries; if dentry is not
for us, we'll see that when we recheck once proc_sys_lookup() is
done with it.

While we are at it, fix the misspelled READ_ONCE and WRITE_ONCE there.

Fixes: d9171b934526 ("parallel lookups machinery, part 4 (and last)")
Reported-by: NeilBrown <[e-mail]>
Reviewed-by: Christian Brauner <[e-mail]>
Reviewed-by: NeilBrown <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/proc/inode.c

···
 
 	head = ei->sysctl;
 	if (head) {
-		RCU_INIT_POINTER(ei->sysctl, NULL);
+		WRITE_ONCE(ei->sysctl, NULL);
 		proc_sys_evict_inode(inode, head);
 	}
 }

fs/proc/proc_sysctl.c

···
 	struct ctl_table_header *head;
 	struct inode *inode;
 
-	/* Although proc doesn't have negative dentries, rcu-walk means
-	 * that inode here can be NULL */
-	/* AV: can it, indeed? */
-	inode = d_inode_rcu(dentry);
-	if (!inode)
-		return 1;
 	if (name->len != len)
 		return 1;
 	if (memcmp(name->name, str, len))
 		return 1;
-	head = rcu_dereference(PROC_I(inode)->sysctl);
+
+	// 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 = READ_ONCE(PROC_I(inode)->sysctl);
 	return !head || !sysctl_is_seen(head);
 }