From ba7554682925e76753fd71fe81965526de52e7de Mon Sep 17 00:00:00 2001
From: Jason Gunthorpe <jgg@mellanox.com>
Date: Tue, 27 Mar 2018 21:39:10 -0600
Subject: [PATCH] container_of: Fail compilation when void * is used as the
 member_ptr

This is actually a check_types_match problem, as
   check_types_match(*(void *)0, (int)0)

Will succeed because C allows 'void *' to compare against any other
pointer. The solution is the same as typecheck.h uses in the kernel:
add another pointer '*' to the type.

However, this means
  check_types_match(*(const int *)0, (int)0)

Will start failing as the implicit elimination of const is gone. This
is actually used by a few container_of users in ccan, and seems desirable.

Use a simple solution to constify both sides before adding the extra
pointer '*'.

Add tests to show that all of these above scenarios work.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
---
 ccan/check_type/check_type.h                  |  6 ++++-
 .../test/compile_fail-bad-void-type.c         | 26 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 ccan/container_of/test/compile_fail-bad-void-type.c

diff --git a/ccan/check_type/check_type.h b/ccan/check_type/check_type.h
index 837aef7b1..9df25a970 100644
--- a/ccan/check_type/check_type.h
+++ b/ccan/check_type/check_type.h
@@ -49,8 +49,12 @@
 #define check_type(expr, type)			\
 	((typeof(expr) *)0 != (type *)0)
 
-#define check_types_match(expr1, expr2)		\
+#define _check_types_match(expr1, expr2)		\
 	((typeof(expr1) *)0 != (typeof(expr2) *)0)
+
+#define check_types_match(expr1, expr2)                         \
+	_check_types_match((const typeof(expr1) *)0,            \
+			   (const typeof(expr2) *)0)
 #else
 #include <ccan/build_assert/build_assert.h>
 /* Without typeof, we can only test the sizes. */
diff --git a/ccan/container_of/test/compile_fail-bad-void-type.c b/ccan/container_of/test/compile_fail-bad-void-type.c
new file mode 100644
index 000000000..1faa8955d
--- /dev/null
+++ b/ccan/container_of/test/compile_fail-bad-void-type.c
@@ -0,0 +1,26 @@
+#include <ccan/container_of/container_of.h>
+#include <stdlib.h>
+
+struct foo {
+	int *a;
+	char b;
+};
+
+int main(void)
+{
+	struct foo foo = { .a = NULL, .b = 2 };
+	void *voidp = &foo.a;
+	const char *ccharp = &foo.b;
+	char *charp = &foo.b;
+	struct foo *p;
+
+#ifdef FAIL
+	/* voidp is a void * but b is an int */
+	p = container_of(voidp, struct foo, a);
+#else
+	p = voidp;
+	p = container_of(ccharp, struct foo, b);
+	p = container_of(charp, struct foo, b);
+#endif
+	return p == NULL;
+}