From f65846a1800ef8c48d1ae1973c30dae4c356a800 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Sat, 9 Mar 2013 07:38:41 -0800 Subject: [PATCH] list: Fix double fetch of pointer in hlist_entry_safe() The current version of hlist_entry_safe() fetches the pointer twice, once to test for NULL and the other to compute the offset back to the enclosing structure. This is OK for normal lock-based use because in that case, the pointer cannot change. However, when the pointer is protected by RCU (as in "rcu_dereference(p)"), then the pointer can change at any time. This use case can result in the following sequence of events: 1. CPU 0 invokes hlist_entry_safe(), fetches the RCU-protected pointer as sees that it is non-NULL. 2. CPU 1 invokes hlist_del_rcu(), deleting the entry that CPU 0 just fetched a pointer to. Because this is the last entry in the list, the pointer fetched by CPU 0 is now NULL. 3. CPU 0 refetches the pointer, obtains NULL, and then gets a NULL-pointer crash. This commit therefore applies gcc's "({ })" statement expression to create a temporary variable so that the specified pointer is fetched only once, avoiding the above sequence of events. Please note that it is the caller's responsibility to use rcu_dereference() as needed. This allows RCU-protected uses to work correctly without imposing any additional overhead on the non-RCU case. Many thanks to Eric Dumazet for spotting root cause! Reported-by: CAI Qian Reported-by: Eric Dumazet Signed-off-by: Paul E. McKenney Tested-by: Li Zefan --- include/linux/list.h | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/include/linux/list.h b/include/linux/list.h index d991cc147c98..6a1f8df9144b 100644 --- a/include/linux/list.h +++ b/include/linux/list.h @@ -667,7 +667,9 @@ static inline void hlist_move_list(struct hlist_head *old, pos = n) #define hlist_entry_safe(ptr, type, member) \ - (ptr) ? hlist_entry(ptr, type, member) : NULL + ({ typeof(ptr) ____ptr = (ptr); \ + ____ptr ? hlist_entry(____ptr, type, member) : NULL; \ + }) /** * hlist_for_each_entry - iterate over list of given type -- 2.20.1