llist: clang: introduce member_address_is_nonnull()

Currently llist_for_each_entry() and llist_for_each_entry_safe() iterate
until &pos->member != NULL.  But when building the kernel with Clang,
the compiler assumes &pos->member cannot be NULL if the member's offset
is greater than 0 (which would be equivalent to the object being
non-contiguous in memory).  Therefore the loop condition is always true,
and the loops become infinite.

To work around this, introduce the member_address_is_nonnull() macro,
which casts object pointer to uintptr_t, thus letting the member pointer
to be NULL.

Signed-off-by: Alexander Potapenko <glider@google.com>
Tested-by: Sodagudi Prasad <psodagud@codeaurora.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/llist.h b/include/linux/llist.h
index d11738110a7aeff53b1349f994e3e435d2f0e76b..1957635e6d5f7b677896c50921627aff79445cfb 100644 (file)
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -92,6 +92,23 @@ static inline void init_llist_head(struct llist_head *list)
 #define llist_entry(ptr, type, member)         \
        container_of(ptr, type, member)
 
+/**
+ * member_address_is_nonnull - check whether the member address is not NULL
+ * @ptr:       the object pointer (struct type * that contains the llist_node)
+ * @member:    the name of the llist_node within the struct.
+ *
+ * This macro is conceptually the same as
+ *     &ptr->member != NULL
+ * but it works around the fact that compilers can decide that taking a member
+ * address is never a NULL pointer.
+ *
+ * Real objects that start at a high address and have a member at NULL are
+ * unlikely to exist, but such pointers may be returned e.g. by the
+ * container_of() macro.
+ */
+#define member_address_is_nonnull(ptr, member) \
+       ((uintptr_t)(ptr) + offsetof(typeof(*(ptr)), member) != 0)
+
 /**
  * llist_for_each - iterate over some deleted entries of a lock-less list
  * @pos:       the &struct llist_node to use as a loop cursor
@@ -145,7 +162,7 @@ static inline void init_llist_head(struct llist_head *list)
  */
 #define llist_for_each_entry(pos, node, member)                                \
        for ((pos) = llist_entry((node), typeof(*(pos)), member);       \
-            &(pos)->member != NULL;                                    \
+            member_address_is_nonnull(pos, member);                    \
             (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))
 
 /**
@@ -167,7 +184,7 @@ static inline void init_llist_head(struct llist_head *list)
  */
 #define llist_for_each_entry_safe(pos, n, node, member)                               \
        for (pos = llist_entry((node), typeof(*pos), member);                  \
-            &pos->member != NULL &&                                           \
+            member_address_is_nonnull(pos, member) &&                         \
                (n = llist_entry(pos->member.next, typeof(*n), member), true); \
             pos = n)