[PATCH 1/1] ntdll: Treat read faults on readable pages as write faults on macOS.

[Tim Clem]

From: Tim Clem

Working around a Rosetta bug on Apple Silicon - faults for certain
instructions (e.g. lock cmpxchg8b) are misreported, which can break
VPROT_WRITEWATCH handling.
---
 dlls/ntdll/unix/virtual.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/dlls/ntdll/unix/virtual.c b/dlls/ntdll/unix/virtual.c
index 9a42db3c77a..8621b2ac6aa 100644
--- a/dlls/ntdll/unix/virtual.c
+++ b/dlls/ntdll/unix/virtual.c
@@ -3692,6 +3692,16 @@ NTSTATUS virtual_handle_fault( void *addr, DWORD err, void *stack )

mutex_lock( &virtual_mutex );  /* no need for signal masking inside signal handler */
     vprot = get_page_vprot( page );
+
+#ifdef __APPLE__
+    /* Rosetta on Apple Silicon misreports certain write faults as read faults. */
+    if (err == EXCEPTION_READ_FAULT && (get_unix_prot( vprot ) & PROT_READ))
+    {
+        WARN( "treating read fault in a readable page as a write fault, addr %p\n", addr );
+        err = EXCEPTION_WRITE_FAULT;
+    }
+#endif
+
     if (!is_inside_signal_stack( stack ) && (vprot & VPROT_GUARD))
     {
         struct thread_stack_info stack_info;