Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcumacro

[Roman Gushchin]

> If other rcu accessors have the same problem, a more complete patch is
> needed.

[PATCH] rcu: fix a race in rcu lists traverse macros

Some network functions (udp4_lib_lookup2(), for instance) use the
rcu lists traverse macros (hlist_nulls_for_each_entry_rcu, for instance)
in a way that assumes restarting of a loop. In this case, it is strictly
necessary to reread the head->first value from the memory before each scan.
Without additional hints, gcc caches this value in a register. In this case,
if a cached node is moved to another chain during the scan, we can loop
forever getting wrong nulls values and restarting the loop uninterruptedly.

Signed-off-by: Roman Gushchin <klamm@xxxxxxxxxxxxxx>
Reported-by: Boris Zhmurov <zhmurov@xxxxxxxxxxxxxx>
---
 include/linux/compiler.h      | 6 ++++++
 include/linux/rculist.h       | 6 ++++--
 include/linux/rculist_nulls.h | 3 ++-
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 92669cd..0e05d7c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -351,6 +351,12 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
  */
 #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))

+/*
+ * Prevent the compiler from optimizing accesses to structure member.
+ */
+#define GET_STRUCT_FIELD_VOLATILE(struct_ptr, field)		\
+	(((volatile typeof(*struct_ptr) *)struct_ptr)->field)
+
 /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
 #ifdef CONFIG_KPROBES
 # define __kprobes	__attribute__((__section__(".kprobes.text")))
diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 8089e35..6c3eea7 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -282,7 +282,8 @@ static inline void list_splice_init_rcu(struct list_head *list,
  * as long as the traversal is guarded by rcu_read_lock().
  */
 #define list_for_each_entry_rcu(pos, head, member) \
-	for (pos = list_entry_rcu((head)->next, typeof(*pos), member); \
+	for (pos = list_entry_rcu(GET_STRUCT_FIELD_VOLATILE(head, next), \
+				  typeof(*pos), member); \
 		&pos->member != (head); \
 		pos = list_entry_rcu(pos->member.next, typeof(*pos), member))

@@ -348,7 +349,8 @@ static inline void hlist_replace_rcu(struct hlist_node *old,
 /*
  * return the first or the next element in an RCU protected hlist
  */
-#define hlist_first_rcu(head)	(*((struct hlist_node __rcu **)(&(head)->first)))
+#define hlist_first_rcu(head)	(*((struct hlist_node __rcu **)		\
+				   (&GET_STRUCT_FIELD_VOLATILE(head, first))))
 #define hlist_next_rcu(node)	(*((struct hlist_node __rcu **)(&(node)->next)))
 #define hlist_pprev_rcu(node)	(*((struct hlist_node __rcu **)((node)->pprev)))

diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index 2ae1371..b04fd0a 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -38,7 +38,8 @@ static inline void hlist_nulls_del_init_rcu(struct hlist_nulls_node *n)
 }

 #define hlist_nulls_first_rcu(head) \
-	(*((struct hlist_nulls_node __rcu __force **)&(head)->first))
+	(*((struct hlist_nulls_node __rcu __force **)	\
+	   &(GET_STRUCT_FIELD_VOLATILE(head, first))))

 #define hlist_nulls_next_rcu(node) \
 	(*((struct hlist_nulls_node __rcu __force **)&(node)->next))
--
1.8.1.2

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