shim: Flush the memory region from i-cache before execution

We've seen crashes in early GRUB code on an ARM Cortex-A72-based
platform that point at seemingly harmless instructions. Flushing
the i-cache of those instructions prior to executing has been
shown to avoid the problem, which has parallels with this story:
  https://www.mail-archive.com/[email protected]/msg06203.html

Add a cache flushing utility function and provide an implementation
using a GCC intrinsic. This will need to be extended to support other
compilers. Note that this intrinsic is a no-op for x86 platforms.

This fixes issue #498.

Signed-off-by: dann frazier <[email protected]>

include/compiler.h

@@ -192,5 +192,11 @@
  */
 #define unreachable() __builtin_unreachable()
 
+#if defined(__GNUC__)
+#define cache_invalidate(begin, end)  __builtin___clear_cache(begin, end)
+#else /* __GNUC__ */
+#error shim has no cache_invalidate() implementation for this compiler
+#endif /* __GNUC__ */
+
 #endif /* !COMPILER_H_ */
 // vim:fenc=utf-8:tw=75:et

pe.c

@@ -1196,6 +1196,9 @@ handle_image (void *data, unsigned int datasize,
 
 	CopyMem(buffer, data, context.SizeOfHeaders);
 
+	/* Flush the instruction cache for the region holding the image */
+	cache_invalidate(buffer, buffer + context.ImageSize);
+
 	*entry_point = ImageAddress(buffer, context.ImageSize, context.EntryPoint);
 	if (!*entry_point) {
 		perror(L"Entry point is invalid\n");