From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Tue, 20 Apr 2021 20:07:54 +0000 (-0400)
Subject: Fix: hlist iteration relies on undefined behavior
X-Git-Tag: v0.12.3~13
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=decfadf67412c383a6bdd674bea4da7ff4d9c1b4;p=userspace-rcu.git

Fix: hlist iteration relies on undefined behavior

Comparing an offset from an object with NULL is undefined behavior
and the compiler may assume that this is never true.

This is indeed what is observed with gcc-10 miscompiling
cds_hlist_for_each_entry_rcu_2().

Fix this by introducing cds_hlist_entry_safe() rather than open-coding
the NULL check comparisons, and move cds_hlist_for_each_entry_2()
and cds_hlist_for_each_entry_safe_2() to this scheme as well.

Fixes: #1308
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ief3531cf04e54b6dae05eb28a6822adfa141fdeb
---

diff --git a/include/urcu/hlist.h b/include/urcu/hlist.h
index 3444811..5e3c27d 100644
--- a/include/urcu/hlist.h
+++ b/include/urcu/hlist.h
@@ -17,6 +17,7 @@
  */
 
 #include <stddef.h>
+#include <urcu/compiler.h>
 
 struct cds_hlist_head {
 	struct cds_hlist_node *next;
@@ -40,8 +41,14 @@ void CDS_INIT_HLIST_HEAD(struct cds_hlist_head *ptr)
 	{ .next = NULL }
 
 /* Get typed element from list at a given position. */
-#define cds_hlist_entry(ptr, type, member) \
-	((type *) ((char *) (ptr) - (unsigned long) (&((type *) 0)->member)))
+#define cds_hlist_entry(ptr, type, member)	caa_container_of(ptr, type, member)
+
+/* Get typed element from list at a given position, keeping NULL pointers. */
+#define cds_hlist_entry_safe(ptr, type, member)					\
+	({									\
+		__typeof__(ptr) ____ret = ptr;					\
+		____ret ? cds_hlist_entry(____ret, type, member) : NULL;	\
+	})
 
 /* Add new element at the head of the list. */
 static inline
@@ -93,17 +100,13 @@ void cds_hlist_del(struct cds_hlist_node *elem)
 			entry = cds_hlist_entry(pos, __typeof__(*entry), member))
 
 #define cds_hlist_for_each_entry_2(entry, head, member) \
-	for (entry = ((head)->next == NULL ? NULL \
-			: cds_hlist_entry((head)->next, __typeof__(*entry), member)); \
+	for (entry = cds_hlist_entry_safe((head)->next, __typeof__(*entry), member); \
 		entry != NULL; \
-		entry = (entry->member.next == NULL ? NULL \
-			: cds_hlist_entry(entry->member.next, __typeof__(*entry), member)))
+		entry = cds_hlist_entry_safe(entry->member.next, __typeof__(*entry), member))
 
 #define cds_hlist_for_each_entry_safe_2(entry, e, head, member) \
-	for (entry = ((head)->next == NULL ? NULL \
-			: cds_hlist_entry((head)->next, __typeof__(*entry), member)); \
-		(entry != NULL) && (e = (entry->member.next == NULL ? NULL \
-					: cds_hlist_entry(entry->member.next, \
+	for (entry = cds_hlist_entry_safe((head)->next, __typeof__(*entry), member); \
+		(entry != NULL) && (e = (cds_hlist_entry_safe(entry->member.next, \
 						__typeof__(*entry), member)), 1); \
 		entry = e)
 
diff --git a/include/urcu/rcuhlist.h b/include/urcu/rcuhlist.h
index 69c4d31..ca1da06 100644
--- a/include/urcu/rcuhlist.h
+++ b/include/urcu/rcuhlist.h
@@ -72,10 +72,10 @@ void cds_hlist_del_rcu(struct cds_hlist_node *elem)
 			entry = cds_hlist_entry(pos, __typeof__(*entry), member))
 
 #define cds_hlist_for_each_entry_rcu_2(entry, head, member) \
-	for (entry = cds_hlist_entry(rcu_dereference((head)->next), \
+	for (entry = cds_hlist_entry_safe(rcu_dereference((head)->next), \
 			__typeof__(*entry), member); \
-		&entry->member != NULL; \
-		entry = cds_hlist_entry(rcu_dereference(entry->member.next), \
+		entry != NULL; \
+		entry = cds_hlist_entry_safe(rcu_dereference(entry->member.next), \
 			__typeof__(*entry), member))
 
 #endif	/* _URCU_RCUHLIST_H */