From 32bb9743e4bc5d7dc120ce7e8f66d1185f79db47 Mon Sep 17 00:00:00 2001
From: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
Date: Wed, 20 Jul 2022 13:51:15 +0200
Subject: [PATCH] sys/net/gnrc/pktbuf_static: make use of alignas()

Since we are now using C11, we can make use of `alignas()` provided by
`<stdalign.h>` to make the alignment code easier to read.
---
 sys/net/gnrc/pktbuf/include/pktbuf_internal.h | 13 ++++---------
 .../gnrc/pktbuf_static/gnrc_pktbuf_static.c   | 19 +++++++++----------
 .../pktbuf_static/include/pktbuf_static.h     | 12 ++++++++++++
 3 files changed, 25 insertions(+), 19 deletions(-)

diff --git a/sys/net/gnrc/pktbuf/include/pktbuf_internal.h b/sys/net/gnrc/pktbuf/include/pktbuf_internal.h
index c48db7b82d..ddc3462311 100644
--- a/sys/net/gnrc/pktbuf/include/pktbuf_internal.h
+++ b/sys/net/gnrc/pktbuf/include/pktbuf_internal.h
@@ -28,6 +28,10 @@
 
 #include "mutex.h"
 
+#if IS_USED(MODULE_GNRC_PKTBUF_STATIC)
+#include "pktbuf_static.h"
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -41,15 +45,6 @@ extern "C" {
  */
 extern mutex_t gnrc_pktbuf_mutex;
 
-#if IS_USED(MODULE_GNRC_PKTBUF_STATIC) || DOXYGEN
-/**
- * @brief   The actual static buffer used when module gnrc_pktbuf_static is used
- *
- * @warning This is an internal buffer and should not be touched by external code
- */
-extern uint8_t *gnrc_pktbuf_static_buf;
-#endif
-
 /**
  * @brief   Check if the given pointer is indeed part of the packet buffer
  *
diff --git a/sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c b/sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c
index 041d2e9a12..6e981e740b 100644
--- a/sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c
+++ b/sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c
@@ -44,12 +44,9 @@
 #endif
 
 #define CANARY 0x55
-
-/* The static buffer needs to be aligned to word size, so that its start
- * address can be casted to `_unused_t *` safely. Just allocating an array of
- * (word sized) uintptr_t is a trivial way to do this */
-static uintptr_t _pktbuf_buf[CONFIG_GNRC_PKTBUF_SIZE / sizeof(uintptr_t)];
-uint8_t *gnrc_pktbuf_static_buf = (uint8_t *)_pktbuf_buf;
+alignas(sizeof(_unused_t)) uint8_t gnrc_pktbuf_static_buf[CONFIG_GNRC_PKTBUF_SIZE];
+static_assert((CONFIG_GNRC_PKTBUF_SIZE % sizeof(_unused_t)) == 0,
+              "CONFIG_GNRC_PKTBUF_SIZE has to be a multiple of 8");
 static _unused_t *_first_unused;
 
 #ifdef DEVELHELP
@@ -91,11 +88,13 @@ void gnrc_pktbuf_init(void)
 {
     mutex_lock(&gnrc_pktbuf_mutex);
     if (CONFIG_GNRC_PKTBUF_CHECK_USE_AFTER_FREE) {
-        memset(_pktbuf_buf, CANARY, sizeof(_pktbuf_buf));
+        memset(gnrc_pktbuf_static_buf, CANARY, sizeof(gnrc_pktbuf_static_buf));
     }
-    _first_unused = (_unused_t *)_pktbuf_buf;
+    /* Silence false -Wcast-align: gnrc_pktbuf_static_buf has qualifier
+     * `alignas(_unused_t)`, so it is guaranteed to be safe */
+    _first_unused = (_unused_t *)(uintptr_t)gnrc_pktbuf_static_buf;
     _first_unused->next = NULL;
-    _first_unused->size = sizeof(_pktbuf_buf);
+    _first_unused->size = sizeof(gnrc_pktbuf_static_buf);
     mutex_unlock(&gnrc_pktbuf_mutex);
 }
 
@@ -326,7 +325,7 @@ void gnrc_pktbuf_stats(void)
 bool gnrc_pktbuf_is_empty(void)
 {
     return ((uintptr_t)_first_unused == (uintptr_t)gnrc_pktbuf_static_buf) &&
-           (_first_unused->size == sizeof(_pktbuf_buf));
+           (_first_unused->size == sizeof(gnrc_pktbuf_static_buf));
 }
 
 bool gnrc_pktbuf_is_sane(void)
diff --git a/sys/net/gnrc/pktbuf_static/include/pktbuf_static.h b/sys/net/gnrc/pktbuf_static/include/pktbuf_static.h
index dce6f8736e..7f55b040d5 100644
--- a/sys/net/gnrc/pktbuf_static/include/pktbuf_static.h
+++ b/sys/net/gnrc/pktbuf_static/include/pktbuf_static.h
@@ -22,6 +22,7 @@
 #define PKTBUF_STATIC_H
 
 #include <sys/types.h>
+#include <stdalign.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -40,6 +41,17 @@ typedef struct _unused {
     unsigned int size;      /**< the size of the unused section */
 } _unused_t;
 
+
+/**
+ * @brief   The actual static buffer used when module gnrc_pktbuf_static is used
+ *
+ * @warning This is an internal buffer and should not be touched by external
+ *          code
+ *
+ * @details This buffer is aligned to boundaries of `sizeof(_unused_t)`
+ */
+extern alignas(sizeof(_unused_t)) uint8_t gnrc_pktbuf_static_buf[CONFIG_GNRC_PKTBUF_SIZE];
+
 /**
  * @brief   Calculates the required space of a number of bytes including
  *          alignment to the size of @ref _unused_t