utils: Add malloc/free wrappers returning aligned data

While we could use posix_memalign(3), that is not fully portable. Further, it
might be difficult on some platforms to properly catch it in leak-detective,
which results in invalid free()s when releasing such memory.

We instead use a simple wrapper, which allocates larger data, and saves the
padding size in the allocated header. This requires that memory is released
using a dedicated function.

To reduce the risk of invalid free() when working on corrupted data, we fill up
all the padding with the padding length, and verify it during free_align().

src/libstrongswan/utils/utils.h

@@ -566,6 +566,24 @@ typedef struct timespec timespec_t;
  */
 typedef struct sockaddr sockaddr_t;
 
+/**
+ * malloc(), but returns aligned memory.
+ *
+ * The returned pointer must be freed using free_align(), not free().
+ *
+ * @param size			size of allocated data
+ * @param align			alignment, up to 255 bytes, usually a power of 2
+ * @return				allocated hunk, aligned to align bytes
+ */
+void* malloc_align(size_t size, u_int8_t align);
+
+/**
+ * Free a hunk allocated by malloc_align().
+ *
+ * @param ptr			hunk to free
+ */
+void free_align(void *ptr);
+
 /**
  * Same as memcpy, but XORs src into dst instead of copy
  */