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.c

@@ -58,6 +58,50 @@ ENUM(status_names, SUCCESS, NEED_MORE,
 	"NEED_MORE",
 );
 
+/**
+ * Described in header.
+ */
+void* malloc_align(size_t size, u_int8_t align)
+{
+	u_int8_t pad;
+	void *ptr;
+
+	if (align == 0)
+	{
+		align = 1;
+	}
+	ptr = malloc(align + sizeof(pad) + size);
+	if (!ptr)
+	{
+		return NULL;
+	}
+	/* store padding length just before data, down to the allocation boundary
+	 * to do some verification during free_align() */
+	pad = align - ((uintptr_t)ptr % align);
+	memset(ptr, pad, pad);
+	return ptr + pad;
+}
+
+/**
+ * Described in header.
+ */
+void free_align(void *ptr)
+{
+	u_int8_t pad, *pos;
+
+	pos = ptr - 1;
+	/* verify padding to check any corruption */
+	for (pad = *pos; (void*)pos >= ptr - pad; pos--)
+	{
+		if (*pos != pad)
+		{
+			DBG1(DBG_LIB, "!!!! invalid free_align() !!!!");
+			return;
+		}
+	}
+	free(ptr - pad);
+}
+
 /**
  * Described in header.
  */