vm: prevent overflow of fixparam field in function.

Functions can only have 127 fixed parameters; some code
ignores this when assigning to the bitfield.

* lib.h (FIXPARAM_BITS, FIXPARAM_MAX): New preprocessor
symbols.
(struct func): Use FIXPARAM_BITS for defining fixparam and
optargs bitfields instead of hard-coded 7.

* lib.c (func_vm): Sanity check the fixparam and reqags
parameters: 0 <= reqargs <= fixparam <= FIXPARAM_MAX.

1 files changed, 5 insertions, 2 deletions

diff --git a/lib.h b/lib.h
index bd5fbfc3..35da0a43 100644
--- a/lib.h
+++ b/lib.h
@@ -147,10 +147,13 @@ struct package {
 
 typedef struct args *varg;
 
+#define FIXPARAM_BITS 7
+#define FIXPARAM_MAX  ((1U << FIXPARAM_BITS) - 1)
+
 struct func {
   obj_common;
-  unsigned fixparam : 7; /* total non-variadic parameters */
-  unsigned optargs : 7;  /* fixparam - optargs = required args */
+  unsigned fixparam : FIXPARAM_BITS; /* total non-variadic parameters */
+  unsigned optargs : FIXPARAM_BITS;  /* fixparam - optargs = required args */
   unsigned variadic : 1;
   unsigned : 1;
   unsigned functype : 16;