From 12c5f3231e480988c47a8c912ba2ea2ca0a95a0d Mon Sep 17 00:00:00 2001
From: Nick Briggs <nicholas.h.briggs@gmail.com>
Date: Fri, 1 Jan 2021 14:08:41 -0800
Subject: [PATCH] Correct handling of return value of N_OP_unwind()

The return value of N_OP_unwind is really of type LispPTR *, however it was
declared as UNSIGNED (effectively uintptr_t) so that the ERROR_EXIT macro
could be used to return an error indication (-1, =UINT_MAX). The call site
checked for the error condition with (int)result < 0, not accounting for the
case where a native pointer may have the high-order bit set.

We correct this problem by declaring the return type as LispPTR *,
and expand the ERROR_EXIT macro in place substituting (LispPTR *)(-1)
as the error return value, and at the single call site check for equality
with that same value.

The test case was executing the opcode tester on a Raspberry Pi or
a BeagleBone Black/Debian 10.3 where the non-error result was > 0xB0000000

  	modified:   inc/inlineC.h
        modified:   inc/unwinddefs.h
        modified:   src/unwind.c
---
 inc/inlineC.h    | 3 +--
 inc/unwinddefs.h | 2 +-
 src/unwind.c     | 9 ++++++---
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/inc/inlineC.h b/inc/inlineC.h
index 5950829..9d09aaf 100755
--- a/inc/inlineC.h
+++ b/inc/inlineC.h
@@ -852,8 +852,7 @@ nextop2;								\
 
 #define UNWIND(n, m)							\
 		{	 						\
-		 if ((INT)(CSTKPTRL = (LispPTR *)			\
-		      N_OP_unwind(CSTKPTR, TOPOFSTACK, n, m)) < 0) 	\
+                    if ((CSTKPTRL = N_OP_unwind(CSTKPTR, TOPOFSTACK, n, m)) == (LispPTR *)-1) \
 			goto unwind_err;				\
 		 POP;							\
 		 nextop3;						\
diff --git a/inc/unwinddefs.h b/inc/unwinddefs.h
index 07789aa..cf8caaf 100644
--- a/inc/unwinddefs.h
+++ b/inc/unwinddefs.h
@@ -1,4 +1,4 @@
 #ifndef UNWINDDEFS_H
 #define UNWINDDEFS_H 1
-UNSIGNED N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int keep);
+LispPTR *N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int keep);
 #endif
diff --git a/src/unwind.c b/src/unwind.c
index f9d02af..b32b189 100644
--- a/src/unwind.c
+++ b/src/unwind.c
@@ -28,7 +28,7 @@
 
 #include "unwinddefs.h"
 
-UNSIGNED N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int keep) {
+LispPTR *N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int keep) {
   register int num;           /* number of UNBOUND slot */
   register LispPTR *endptr;   /* unwind limit */
   register LispPTR *lastpvar; /* points PVar slot that is unbounded. */
@@ -64,7 +64,10 @@ UNSIGNED N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int
 
   if (endptr > cstkptr) {
     CurrentStackPTR = (DLword *)cstkptr;
-    ERROR_EXIT(tos);
+    /* this would be ERROR_EXIT(tos); but for having to return a pointer */
+    TopOfStack = tos;
+    Error_Exit = 1;
+    return (LispPTR *)(-1);
   }
   *cstkptr++ = tos;
 
@@ -85,7 +88,7 @@ UNSIGNED N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int
   /* endptr = cstkptr */
 
   if (keep) { *(cstkptr++) = tos; }
-  return ((UNSIGNED)cstkptr);
+  return (cstkptr);
 
 } /* N_OP_unwind */