[[ Bug ]] Fix handler function pointers on newer Android versions

runrevmark · runrevmark · commit 9ec704015da1 · 2019-05-15T13:59:57.000+01:00
This patch fixes an issue with the generation of handler function
pointers on newer versions of Android (starting with 9).

The issue is that libffi is failing to generate closure trampolines
in memory which has the executable permission bit set. This has been
worked around by explicitly changing the memory protection flags
to be read/write/exec *after* libffi has created the closure.
diff --git a/libfoundation/src/foundation-handler.cpp b/libfoundation/src/foundation-handler.cpp
@@ -301,6 +301,40 @@ static void __exec_closure(ffi_cif *cif, void *ret, void **args, void *user_data
         MCValueRelease(t_value_args[i]);
 }
 
+#if defined(__ANDROID__)
+
+/* For some reason the current version / config of libffi on newer versions of
+ * android fails to generate closure trampolines in memory with the exec bit
+ * set. To work-around this we set the exec bit of the memory page containing
+ * the function ptr directly. */
+
+#include <sys/mman.h>
+
+static bool ensure_block_is_executable(void *p_block, size_t p_size)
+{
+    /* Round the start of the block pointer down to the nearest page. */
+    byte_t *t_start_page = (byte_t *)(((uintptr_t)p_block) & ~4095);
+    
+    /* Round the end of the block pointer up to the nearest page. */
+    byte_t *t_finish_page = (byte_t *)((((uintptr_t)p_block + p_size) + 4095) & ~4095);
+    
+    /* Change the protection flags of the page range to read/write/exec. */
+    return mprotect(t_start_page,
+                    t_finish_page - t_start_page,
+                    PROT_READ|PROT_EXEC|PROT_WRITE) == 0;
+}
+
+#else
+
+/* Other platforms do not need any explicit action as libffi works correctly. */
+
+static bool ensure_block_is_executable(void *p_block, size_t p_size)
+{
+    return true;
+}
+
+#endif
+
 MC_DLLEXPORT_DEF
 bool MCHandlerGetFunctionPtr(MCHandlerRef self, void*& r_function_ptr)
 {
@@ -326,6 +360,13 @@ bool MCHandlerGetFunctionPtr(MCHandlerRef self, void*& r_function_ptr)
         self -> closure = nil;
         return MCErrorThrowGeneric(MCSTR("unexpected libffi failure"));
     }
+
+    /* Change the protection flags of the page range to read/write/exec. */
+    if (!ensure_block_is_executable(self->closure,
+                                    sizeof(ffi_closure)))
+    {
+        return MCErrorThrowGeneric(MCSTR("unable to generate executable closure trampoline"));
+    }
     
     r_function_ptr = self -> function_ptr;
     return true;
