Unfortunately, list_empty() is not usable with an entry that has been
removed from a list with list_del_rcu() as ->next must be left pointing at
the following entry so as not to break traversal under RCU.

Solve this by moving on_list_rcu() from AppArmor to linux/list.h, and
turning it into an inline function.

Also add an on_list() counterpart (functionally, this is just an antonym
for list_empty()), but the name looks less awkward when applied to a
non-head element.  We probably don't want to use on_list_rcu() generally
because it requires an extra check as ->prev is set differently in the two
cases.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
cc: John Johansen <john.johansen@canonical.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: apparmor@lists.ubuntu.com
cc: netdev@vger.kernel.org
cc: stable@kernel.org
---
 include/linux/list.h               | 26 ++++++++++++++++++++++++++
 security/apparmor/include/policy.h |  2 --
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/include/linux/list.h b/include/linux/list.h
index 00ea8e5fb88b..d224e7210d1b 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -381,6 +381,32 @@ static inline int list_empty(const struct list_head *head)
 	return READ_ONCE(head->next) == head;
 }
 
+/**
+ * on_list - Test whether an entry is on a list.
+ * @entry: The entry to check
+ *
+ * Test whether an entry is on a list.  Safe to use on an entry initialised
+ * with INIT_LIST_HEAD() or LIST_HEAD() or removed with things like
+ * list_del_init().  Not safe for use with list_del() or list_del_rcu().
+ */
+static inline bool on_list(const struct list_head *entry)
+{
+	return !list_empty(entry);
+}
+
+/**
+ * on_list_rcu - Test whether an entry is on a list (RCU-del safe).
+ * @entry: The entry to check
+ *
+ * Test whether an entry is on a list.  Safe to use on an entry initialised
+ * with INIT_LIST_HEAD() or LIST_HEAD() or removed with things like
+ * list_del_init().  Also safe for use with list_del() or list_del_rcu().
+ */
+static inline bool on_list_rcu(const struct list_head *entry)
+{
+	return !list_empty(entry) && entry->prev != LIST_POISON2;
+}
+
 /**
  * list_del_init_careful - deletes entry from list and reinitialize it.
  * @entry: the element to delete from the list.
diff --git a/security/apparmor/include/policy.h b/security/apparmor/include/policy.h
index 3895f8774a3f..c3697c23bbed 100644
--- a/security/apparmor/include/policy.h
+++ b/security/apparmor/include/policy.h
@@ -57,8 +57,6 @@ extern const char *const aa_profile_mode_names[];
 
 #define profile_is_stale(_profile) (label_is_stale(&(_profile)->label))
 
-#define on_list_rcu(X) (!list_empty(X) && (X)->prev != LIST_POISON2)
-
 /* flags in the dfa accept2 table */
 enum dfa_accept_flags {
 	ACCEPT_FLAG_OWNER = 1,