[PATCH][next] container_of: add container_first() macro

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

This is like container_of() but it has an assert to ensure that it's
using the first struct member.

Co-developed-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 include/linux/container_of.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/container_of.h b/include/linux/container_of.h
index 713890c867be..928c5865c1b4 100644
--- a/include/linux/container_of.h
+++ b/include/linux/container_of.h
@@ -22,6 +22,24 @@
 		      "pointer type mismatch in container_of()");	\
 	((type *)(__mptr - offsetof(type, member))); })
 
+/**
+ * container_first - cast first member of a structure out to the containing
+ *		     structure
+ * @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.
+ *
+ * WARNING: any const qualifier of @ptr is lost.
+ */
+#define container_first(ptr, type, member) ({				\
+	void *__mptr = (void *)(ptr);                                   \
+	static_assert(__same_type(*(ptr), ((type *)0)->member) ||       \
+		      __same_type(*(ptr), void),                        \
+		      "pointer type mismatch in container_of()");       \
+	static_assert(offsetof(type, member) == 0, "not first member");	\
+	((type *)(__mptr - offsetof(type, member))); })
+
+
 /**
  * container_of_const - cast a member of a structure out to the containing
  *			structure and preserve the const-ness of the pointer
-- 
2.34.1