[PATCH wayland v2 2/4] wl_array: Set data to invalid address after free

[Yong Bakos]

From: Yong Bakos <ybakos at humanoriented.com>

Explicitly set the data member to an invalid memory address during
wl_array_release, such that re-using a freed wl_array without re-initializing
causes a crash. In addition, this pointer assignment makes wl_array_release
testable.

Define a constant for the invalid memory address, and add documentation about
this behavior, starting at libwayland version 1.13.

See https://lists.freedesktop.org/archives/wayland-devel/2016-September/031116.html

Signed-off-by: Yong Bakos <ybakos at humanoriented.com>
---
v2: Set data pointer to invalid memory address (pq)

 src/wayland-private.h | 3 +++
 src/wayland-util.c    | 1 +
 src/wayland-util.h    | 4 +++-
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/wayland-private.h b/src/wayland-private.h
index ac712d9..ef58ccf 100644
--- a/src/wayland-private.h
+++ b/src/wayland-private.h
@@ -36,6 +36,9 @@

 #include "wayland-util.h"

+/* Invalid memory address */
+#define WL_ARRAY_POISON_PTR (void *) 4
+
 #define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])

 #define container_of(ptr, type, member) ({				\
diff --git a/src/wayland-util.c b/src/wayland-util.c
index 639ccf8..077fec7 100644
--- a/src/wayland-util.c
+++ b/src/wayland-util.c
@@ -102,6 +102,7 @@ WL_EXPORT void
 wl_array_release(struct wl_array *array)
 {
 	free(array->data);
+	array->data = WL_ARRAY_POISON_PTR;
 }

 WL_EXPORT void *
diff --git a/src/wayland-util.h b/src/wayland-util.h
index 9b7a4b9..42b3027 100644
--- a/src/wayland-util.h
+++ b/src/wayland-util.h
@@ -381,7 +381,9 @@ wl_array_init(struct wl_array *array);
 /**
  * Releases the array data.
  *
- * \note Leaves the array in an invalid state.
+ * \note Leaves the wl_array in an invalid state.
+ *       Since libwayland version 1.13, using a released wl_array without first
+ *       re-initializing it before use will cause an intentional crash.
  *
  * \param array Array whose data is to be released
  *
--
2.7.2