New talloc_set for auto-cleanup.

ccan/talloc/talloc.c

@@ -789,6 +789,35 @@ void *_talloc(const void *context, size_t size)
 	return __talloc(context, size);
 }
 
+static int talloc_destroy_pointer(void ***pptr)
+{
+	if ((uintptr_t)**pptr < getpagesize())
+		TALLOC_ABORT("Double free or invalid talloc_set?");
+	/* Invalidate pointer so it can't be used again. */
+	**pptr = (void *)1;
+	return 0;
+}
+
+void _talloc_set(void *ptr, const void *ctx, size_t size, const char *name)
+{
+	void ***child;
+	void **pptr = ptr;
+
+	*pptr = talloc_named_const(ctx, size, name);
+	if (unlikely(!*pptr))
+		return;
+
+	child = talloc(*pptr, void **);
+	if (unlikely(!child)) {
+		talloc_free(*pptr);
+		*pptr = NULL;
+		return;
+	}
+	*child = pptr;
+	talloc_set_name_const(child, "talloc_set destructor");
+	talloc_set_destructor(child, talloc_destroy_pointer);
+}
+
 /*
   externally callable talloc_set_name_const()
 */

ccan/talloc/talloc.h

@@ -87,6 +87,30 @@
  */
 #define talloc(ctx, type) (type *)talloc_named_const(ctx, sizeof(type), #type)
 
+/**
+ * talloc_set - allocate dynamic memory for a type, into a pointer
+ * @ptr: pointer to the pointer to assign.
+ * @ctx: context to be parent of this allocation, or NULL.
+ *
+ * talloc_set() does a talloc, but also adds a destructor which will make the
+ * pointer invalid when it is freed.  This can find many use-after-free bugs.
+ *
+ * Note that the destructor is chained off a zero-length allocation, and so
+ * is not affected by talloc_set_destructor().
+ *
+ * Example:
+ *	unsigned int *a;
+ *	a = talloc(NULL, unsigned int);
+ *	talloc_set(&b, a, unsigned int);
+ *	talloc_free(a);
+ *	*b = 1; // This will crash!
+ *
+ * See Also:
+ *	talloc.
+ */
+#define talloc_set(pptr, ctx) \
+	_talloc_set((pptr), (ctx), sizeof(&**(pptr)), __location__)
+
 /**
  * talloc_free - free talloc'ed memory and its children
  * @ptr: the talloced pointer to free
@@ -940,6 +964,7 @@ void *talloc_add_external(const void *ctx,
 
 /* The following definitions come from talloc.c  */
 void *_talloc(const void *context, size_t size);
+void _talloc_set(void *ptr, const void *ctx, size_t size, const char *name);
 void _talloc_set_destructor(const void *ptr, int (*destructor)(void *));
 size_t talloc_reference_count(const void *ptr);
 void *_talloc_reference(const void *context, const void *ptr);

ccan/talloc/test/compile_fail-talloc_set.c

+#include "talloc/talloc.c"
+
+int main(void)
+{
+	int *p;
+
+	talloc_set(
+#ifdef FAIL
+		p
+#else
+		&p
+#endif
+		, NULL);
+	return 0;
+}

ccan/talloc/test/run-talloc_set.c

+#include "talloc/talloc.c"
+#include "tap/tap.h"
+#include <assert.h>
+
+int main(void)
+{
+	char *c;
+	int *i;
+
+	plan_tests(12);
+
+	/* Set C to a valid pointer, with correct parent. */
+	talloc_set(&c, NULL);
+	ok1(c >= (char *)(intptr_t)getpagesize());
+	ok1(talloc_parent(c) == NULL);
+
+	/* Free it, should blatt c. */
+	talloc_free(c);
+	ok1(c);
+	ok1(c < (char *)(intptr_t)getpagesize());
+
+	/* Same test, indirect. */
+	talloc_set(&i, NULL);
+	talloc_set(&c, i);
+	ok1(c >= (char *)(intptr_t)getpagesize());
+	ok1(i >= (int *)(intptr_t)getpagesize());
+	ok1(talloc_parent(i) == NULL);
+	ok1(talloc_parent(c) == i);
+	talloc_free(i);
+	ok1(c);
+	ok1(c < (char *)(intptr_t)getpagesize());
+	ok1(i);
+	ok1(i < (int *)(intptr_t)getpagesize());
+
+	return exit_status();
+}