[build] Fix build failures with older versions of gcc

Some versions of gcc (observed with gcc 4.8.5 in CentOS 7) will report
spurious build_assert() failures for some assertions about structure
layouts.  There is no clear pattern as to what causes these spurious
failures, and the build assertion does succeed in that no unresolvable
symbol reference is generated in the compiled code.

Adjust the assertions to work around these apparent compiler issues.

Signed-off-by: Michael Brown <[email protected]>

src/crypto/gcm.c

@@ -469,13 +469,15 @@ int gcm_setkey ( struct gcm_context *context, const void *key, size_t keylen,
  * @v ivlen		Initialisation vector length
  */
 void gcm_setiv ( struct gcm_context *context, const void *iv, size_t ivlen ) {
-	union gcm_block *check = ( ( void * ) context );
 
-	/* Sanity checks */
-	build_assert ( &context->hash == check );
-	build_assert ( &context->len == check + 1 );
-	build_assert ( &context->ctr == check + 2 );
-	build_assert ( &context->key == check + 3 );
+	/* Sanity check: ensure that memset()s will clear expected state */
+	build_assert ( &context->hash < &context->ctr );
+	build_assert ( &context->len < &context->ctr );
+	build_assert ( &context->ctr < &context->key );
+	build_assert ( ( ( void * ) &context->raw_cipher ) >
+		       ( ( void * ) &context->key ) );
+	build_assert ( ( ( void * ) context->raw_ctx ) >
+		       ( ( void * ) &context->key ) );
 
 	/* Reset non-key state */
 	memset ( context, 0, offsetof ( typeof ( *context ), key ) );

src/include/ipxe/gcm.h

@@ -89,7 +89,8 @@ static int _gcm_name ## _setkey ( void *ctx, const void *key,		\
 				  size_t keylen ) {			\
 	struct _gcm_name ## _context *context = ctx;			\
 	build_assert ( _blocksize == sizeof ( context->gcm.key ) );	\
-	build_assert ( ( ( void * ) &context->gcm ) == ctx );		\
+	build_assert ( ( ( void * ) &context->gcm ) ==			\
+		       ( ( void * ) context ) );			\
 	build_assert ( ( ( void * ) &context->raw ) ==			\
 		       ( ( void * ) context->gcm.raw_ctx ) );		\
 	return gcm_setkey ( &context->gcm, key, keylen, &_raw_cipher );	\