Use emergency TEB selector to avoid debugger crashes when stepping

through relay stubs (or when FS is invalid otherwise).

include/sig_context.h

@@ -219,8 +219,10 @@ typedef struct _CONTEXT		/* Note 1 */
 #ifdef FS_sig
 #include "syslevel.h"
 #define HANDLER_INIT() \
-    SET_FS(IS_SELECTOR_SYSTEM(CS_sig(HANDLER_CONTEXT)) ? \
-           FS_sig(HANDLER_CONTEXT) : SYSLEVEL_Win16CurrentTeb)
+    do { int fs = IS_SELECTOR_SYSTEM(CS_sig(HANDLER_CONTEXT)) ?       \
+                  FS_sig(HANDLER_CONTEXT) : SYSLEVEL_Win16CurrentTeb; \
+         if (!fs) fs = SYSLEVEL_EmergencyTeb;                         \
+         SET_FS(fs);                           } while (0)
 #else
 #define HANDLER_INIT() /* nothing */
 #endif

include/syslevel.h

@@ -11,6 +11,7 @@
 #include "winbase.h"
 
 extern WORD SYSLEVEL_Win16CurrentTeb;
+extern WORD SYSLEVEL_EmergencyTeb;
 
 void SYSLEVEL_Init(void);
 VOID SYSLEVEL_EnterWin16Lock(VOID);

scheduler/process.c

@@ -15,6 +15,7 @@
 #include "heap.h"
 #include "task.h"
 #include "ldt.h"
+#include "syslevel.h"
 #include "thread.h"
 #include "winerror.h"
 #include "pe_image.h"
@@ -237,6 +238,9 @@ BOOL32 PROCESS_Init(void)
     if (!(thdb = THREAD_Create( pdb, 0, FALSE, NULL, NULL, NULL, NULL ))) return FALSE;
     thdb->unix_pid = getpid();
 
+    /* Remember TEB selector of initial process for emergency use */
+    SYSLEVEL_EmergencyTeb = thdb->teb_sel;
+
     /* Create the environment DB of the first process */
     if (!PROCESS_BuildEnvDB( pdb )) return FALSE;
 

scheduler/syslevel.c

@@ -16,6 +16,9 @@ static SEGPTR segpWin16Mutex;
 /* Global variable to save current TEB while in 16-bit code */
 WORD SYSLEVEL_Win16CurrentTeb = 0;
 
+/* TEB of initial process for emergency use */
+WORD SYSLEVEL_EmergencyTeb = 0;
+
 
 /************************************************************************
  *           SYSLEVEL_Init