quote_for_pmake: fix counter underrun resulting in segfault

while (nbs--) { ... } ends with nbs == -1. Rather than a minimal fix,
introduce mempset() to make these kinds of errors less likely in the
future.

Fixes: https://bugzilla.nasm.us/show_bug.cgi?id=3392815
Reported-by: <13579and24680@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

asm/nasm.c

@@ -1,6 +1,6 @@
 /* ----------------------------------------------------------------------- *
  *
- *   Copyright 1996-2020 The NASM Authors - All Rights Reserved
+ *   Copyright 1996-2022 The NASM Authors - All Rights Reserved
  *   See the file AUTHORS included with the NASM distribution for
  *   the specific copyright holders.
  *
@@ -817,7 +817,6 @@ static char *quote_for_pmake(const char *str)
     }
 
     /* Convert N backslashes at the end of filename to 2N backslashes */
-    if (nbs)
     n += nbs;
 
     os = q = nasm_malloc(n);
@@ -827,10 +826,10 @@ static char *quote_for_pmake(const char *str)
         switch (*p) {
         case ' ':
         case '\t':
-            while (nbs--)
-                *q++ = '\\';
+            q = mempset(q, '\\', nbs);
             *q++ = '\\';
             *q++ = *p;
+            nbs = 0;
             break;
         case '$':
             *q++ = *p;
@@ -852,9 +851,8 @@ static char *quote_for_pmake(const char *str)
             break;
         }
     }
-    while (nbs--)
-        *q++ = '\\';
 
+    q = mempset(q, '\\', nbs);
     *q = '\0';
 
     return os;

configure.ac

@@ -200,6 +200,7 @@ AC_CHECK_FUNCS(strrchrnul)
 AC_CHECK_FUNCS(iscntrl)
 AC_CHECK_FUNCS(isascii)
 AC_CHECK_FUNCS(mempcpy)
+AC_CHECK_FUNCS(mempset)
 
 AC_CHECK_FUNCS(getuid)
 AC_CHECK_FUNCS(getgid)

include/compiler.h

@@ -252,6 +252,13 @@ static inline void *mempcpy(void *dst, const void *src, size_t n)
 }
 #endif
 
+#ifndef HAVE_MEMPSET
+static inline void *mempset(void *dst, int c, size_t n)
+{
+    return (char *)memset(dst, c, n) + n;
+}
+#endif
+
 /*
  * Hack to support external-linkage inline functions
  */