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[PATCH 1/2] include/linux: provide a safe version of container_of()

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Some code relies on "negative" (i.e. too big) pointer values being
returned by container_of() when its first argument is NULL. But doing
so breaks the compiler's assumptions that pointer arithmetic never
overflows.

container_of_safe() checks its arguments and returns NULL in the case
the member offset within the container is greater than the pointer to
the member, otherwise it returns the result of container_of().

Signed-off-by: Alexander Potapenko <glider@google.com>
---
Note that checkpatch.pl reports whitespace problems in this patch, but
the diff is consistent with the rest of the file.
---
 include/linux/kernel.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index d96a611..820b134 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -831,6 +831,21 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
 	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
 	(type *)( (char *)__mptr - offsetof(type,member) );})
 
+/**
+ * container_of_safe - safe version of container_of
+ * @ptr:	the pointer to the member.
+ * @type:	the type of the container struct this is embedded in.
+ * @member:	the name of the member within the struct.
+ *
+ * In the case the value of @ptr is smaller than the offset of @member within
+ * @type, return 0.
+ */
+#define container_of_safe(ptr, type, member) ({			\
+	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
+        (size_t)__mptr >= offsetof(type,member) ?		\
+	(type *)( (char *)__mptr - offsetof(type,member) ) : (type *)0 ;})
+
+
 /* Rebuild everything on CONFIG_FTRACE_MCOUNT_RECORD */
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
 # define REBUILD_DUE_TO_FTRACE_MCOUNT_RECORD
-- 
2.8.0.rc3.226.g39d4020

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[PATCH 0/2] Clang: avoid undefined behavior in llist iterators Alexander Potapenko <glider@google.com> - 2016-09-23 15:30 +0200
  [PATCH 2/2] llist: introduce llist_entry_safe() Alexander Potapenko <glider@google.com> - 2016-09-23 15:30 +0200
  [PATCH 1/2] include/linux: provide a safe version of container_of() Alexander Potapenko <glider@google.com> - 2016-09-23 15:30 +0200

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