tcon: Add an alternate way of building type canaries

The tcon module allows you to add "type canaries" to a structures, which
can be used for later typechecks.  The canaries are implemented using
a flexible array member, to avoid them taking any actual space at runtime.
That means the canaries must go at the end of your structure.

That doesn't seem like a big limitation, except that it also means the
structure containing the canaries must be at the end of any structure it
is embedded in in turn, which is a rather more serious limitation.

This patch adds a TCON_WRAP() macro which wraps a given type in a new type
which also contains type canaries, and doesn't suffer the last member
limitation.  The drawback is that if the wrapped type has smaller size than
a pointer, the type canaries will pad the wrapped type out to the size of a
pointer.

By constructing the wrappers carefully, the existing tcon macros will work
on both wrapper types constructed with TCON_WRAP and on structures
explicitly including TCON type canaries.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

ccan/tcon/_info

@@ -71,5 +71,10 @@ int main(int argc, char *argv[])
 		return 0;
 	}
 
+	if (strcmp(argv[1], "testdepends") == 0) {
+		printf("ccan/build_assert\n");
+		return 0;
+	}
+
 	return 1;
 }

ccan/tcon/tcon.h

@@ -42,6 +42,77 @@
 #define TCON(decls) struct { decls; } _tcon[1]
 #endif
 
+/**
+ * TCON_WRAP - declare a wrapper type containing a base type and type canaries
+ * @basetype: the base type to wrap
+ * @decls: the semi-colon separated list of type canaries.
+ *
+ * This expands to a new type which includes the given base type, and
+ * also type canaries, similar to those created with TCON.
+ *
+ * The embedded base type value can be accessed using tcon_unwrap().
+ *
+ * Differences from using TCON()
+ * - The wrapper type will take either the size of the base type, or
+ *   the size of a single pointer, whichever is greater (regardless of
+ *   compiler)
+ * - A TCON_WRAP type may be included in another structure, and need
+ *   not be the last element.
+ *
+ * A type of "void *" will allow tcon_check() to pass on any (pointer) type.
+ *
+ * Example:
+ *	// Simply typesafe linked list.
+ *	struct list_head {
+ *		struct list_head *prev, *next;
+ *	};
+ *
+ *	typedef TCON_WRAP(struct list_head, char *canary) string_list_t;
+ *
+ *	// More complex: mapping from one type to another.
+ *	struct map {
+ *		void *contents;
+ *	};
+ *
+ *	typedef TCON_WRAP(struct map, char *charp_canary; int int_canary)
+ *		int_to_string_map_t;
+ */
+#define TCON_WRAP(basetype, decls) \
+	union {			   \
+		basetype _base;	   \
+		struct {	   \
+			decls;	   \
+		} *_tcon;	   \
+	}
+
+/**
+ * TCON_WRAP_INIT - an initializer for a variable declared with TCON_WRAP
+ * @...: Initializer for the base type (treated as variadic so commas
+ *       can be included)
+ *
+ * Converts converts an initializer suitable for a base type into one
+ * suitable for that type wrapped with TCON_WRAP.
+ *
+ * Example:
+ *	TCON_WRAP(int, char *canary) canaried_int = TCON_WRAP_INIT(17);
+ */
+#define TCON_WRAP_INIT(...)			\
+	{ ._base = __VA_ARGS__, }
+
+/**
+ * tcon_unwrap - Access the base type of a TCON_WRAP
+ * @ptr: pointer to an object declared with TCON_WRAP
+ *
+ * tcon_unwrap() returns a pointer to the base type of the TCON_WRAP()
+ * object pointer to by @ptr.
+ *
+ * Example:
+ *	TCON_WRAP(int, char *canary) canaried_int;
+ *
+ *	*tcon_unwrap(&canaried_int) = 17;
+ */
+#define tcon_unwrap(ptr) (&((ptr)->_base))
+
 /**
  * tcon_check - typecheck a typed container
  * @x: the structure containing the TCON.

ccan/tcon/test/compile_fail-tcon_cast_wrap.c

+#include <ccan/tcon/tcon.h>
+#include <stdlib.h>
+
+struct container {
+	void *p;
+};
+
+int main(int argc, char *argv[])
+{
+	TCON_WRAP(struct container,
+		  int *tc1; char *tc2) icon;
+#ifdef FAIL
+#if !HAVE_TYPEOF
+#error We cannot detect type problems without HAVE_TYPEOF
+#endif
+	char *
+#else
+	int *
+#endif
+		x;
+
+	tcon_unwrap(&icon)->p = NULL;
+	x = tcon_cast(&icon, tc1, tcon_unwrap(&icon)->p);
+	return x != NULL ? 0 : 1;
+}

ccan/tcon/test/compile_fail-wrap.c

+#include <ccan/tcon/tcon.h>
+#include <stdlib.h>
+
+struct container {
+	void *p;
+};
+
+int main(int argc, char *argv[])
+{
+	TCON_WRAP(struct container, int *canary) icon;
+#ifdef FAIL
+	char *
+#else
+	int *
+#endif
+		x = NULL;
+
+	tcon_unwrap(tcon_check(&icon, canary, x))->p = x;
+	return 0;
+}

ccan/tcon/test/compile_ok-void.c

@@ -13,9 +13,15 @@ struct void_container {
 int main(int argc, char *argv[])
 {
 	struct void_container vcon;
+	TCON_WRAP(struct container, void *canary) vconw;
 
 	tcon_check(&vcon, canary, NULL)->raw.p = NULL;
 	tcon_check(&vcon, canary, argv[0])->raw.p = NULL;
 	tcon_check(&vcon, canary, main)->raw.p = NULL;
+
+	tcon_unwrap(tcon_check(&vconw, canary, NULL))->p = NULL;
+	tcon_unwrap(tcon_check(&vconw, canary, argv[0]))->p = NULL;
+	tcon_unwrap(tcon_check(&vconw, canary, main))->p = NULL;
+
 	return 0;
 }

ccan/tcon/test/compile_ok.c

 #include <ccan/tcon/tcon.h>
+#include <ccan/build_assert/build_assert.h>
 #include <stdlib.h>
 
 struct container {
@@ -19,9 +20,19 @@ int main(int argc, char *argv[])
 {
 	struct int_container icon;
 	struct charp_and_int_container cicon;
+	TCON_WRAP(struct container, int tc) iconw;
+	TCON_WRAP(struct container, int tc1; char *tc2) ciconw;
 
 	tcon_check(&icon, tc, 7)->raw.p = NULL;
 	tcon_check(&cicon, tc1, 7)->raw.p = argv[0];
 	tcon_check(&cicon, tc2, argv[0])->raw.p = argv[0];
+
+	tcon_unwrap(tcon_check(&iconw, tc, 7))->p = NULL;
+	tcon_unwrap(tcon_check(&ciconw, tc1, 7))->p = argv[0];
+	tcon_unwrap(tcon_check(&ciconw, tc2, argv[0]))->p = argv[0];
+
+	BUILD_ASSERT(sizeof(iconw) == sizeof(struct container));
+	BUILD_ASSERT(sizeof(ciconw) == sizeof(struct container));
+
 	return 0;
 }

ccan/tcon/test/run-wrap.c

+#include <ccan/tcon/tcon.h>
+#include <ccan/tap/tap.h>
+#include <stdlib.h>
+
+typedef TCON_WRAP(int, char *canary) canaried_int;
+
+int main(int argc, char *argv[])
+{
+	canaried_int ci = TCON_WRAP_INIT(0);
+
+	plan_tests(2);
+
+	ok1(*tcon_unwrap(&ci) == 0);
+	*tcon_unwrap(&ci) = 17;
+	ok1(*tcon_unwrap(&ci) == 17);
+
+	return exit_status();
+}