From a7179b89a68c7cb6f8623cedcce02b98d9e072a1 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Fri, 28 Jun 2013 10:34:48 -0700 Subject: [PATCH] rculist: list_first_or_null_rcu() should use list_entry_rcu() commit c34ac00caefbe49d40058ae7200bd58725cebb45 upstream. list_first_or_null() should test whether the list is empty and return pointer to the first entry if not in a RCU safe manner. It's broken in several ways. * It compares __kernel @__ptr with __rcu @__next triggering the following sparse warning. net/core/dev.c:4331:17: error: incompatible types in comparison expression (different address spaces) * It doesn't perform rcu_dereference*() and computes the entry address using container_of() directly from the __rcu pointer which is inconsitent with other rculist interface. As a result, all three in-kernel users - net/core/dev.c, macvlan, cgroup - are buggy. They dereference the pointer w/o going through read barrier. * While ->next dereference passes through list_next_rcu(), the compiler is still free to fetch ->next more than once and thus nullify the "__ptr != __next" condition check. Fix it by making list_first_or_null_rcu() dereference ->next directly using ACCESS_ONCE() and then use list_entry_rcu() on it like other rculist accessors. v2: Paul pointed out that the compiler may fetch the pointer more than once nullifying the condition check. ACCESS_ONCE() added on ->next dereference. v3: Restored () around macro param which was accidentally removed. Spotted by Paul. Signed-off-by: Tejun Heo Reported-by: Fengguang Wu Cc: Dipankar Sarma Cc: "Paul E. McKenney" Cc: "David S. Miller" Cc: Li Zefan Cc: Patrick McHardy Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett Signed-off-by: Greg Kroah-Hartman --- include/linux/rculist.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/include/linux/rculist.h b/include/linux/rculist.h index 6f95e24..3863352 100644 --- a/include/linux/rculist.h +++ b/include/linux/rculist.h @@ -254,8 +254,9 @@ static inline void list_splice_init_rcu(struct list_head *list, */ #define list_first_or_null_rcu(ptr, type, member) \ ({struct list_head *__ptr = (ptr); \ - struct list_head __rcu *__next = list_next_rcu(__ptr); \ - likely(__ptr != __next) ? container_of(__next, type, member) : NULL; \ + struct list_head *__next = ACCESS_ONCE(__ptr->next); \ + likely(__ptr != __next) ? \ + list_entry_rcu(__next, type, member) : NULL; \ }) /** -- 2.7.4