Fix AFL crashes in C++ demangler

American Fuzzy Lop didn't need to try very hard, to crash our privileged
__demangle() implementation. This change helps ensure our barebones impl
will fail rather than crash when given adversarial input data.

libc/intrin/demangle.c

@@ -380,13 +380,16 @@ demangle_free(struct demangle_data *h, void *ptr)
 }
 
 static privileged returnspointerwithnoaliases returnsnonnull void *
-demangle_malloc(struct demangle_data *h, int a, int n)
+demangle_malloc(struct demangle_data *h, long a, long n)
 {
-	int rem;
+	long rem;
 	uintptr_t ptr;
 	index_t next, next2;
 	index_t *link, *link2;
-	int b = sizeof(index_t);
+	long b = sizeof(index_t);
+
+	if (n < 0 || n >= 32768)
+		__builtin_longjmp(h->jmpbuf, 1);
 
 	/* Roundup size. */
 	n += a - 1;
@@ -2098,10 +2101,11 @@ demangle_read_tmpl_param(struct demangle_data *ddata)
 		/* T_ is first */
 		++nth;
 
-		while (*ddata->cur != '_')
+		while (*ddata->cur && *ddata->cur != '_')
 			++ddata->cur;
 
-		ASSERT(nth > 0);
+		if (nth <= 0)
+			return 0;
 
 		return demangle_get_tmpl_param(ddata, nth);
 	}
@@ -2752,7 +2756,7 @@ demangle_read_offset_number(struct demangle_data *ddata)
 		start = ddata->cur;
 	}
 
-	while (*ddata->cur != '_')
+	while (*ddata->cur && *ddata->cur != '_')
 		++ddata->cur;
 
 	if (negative && !DEM_PUSH_STR(ddata, "-"))
@@ -2859,13 +2863,12 @@ demangle_read_number(struct demangle_data *ddata, long *rtn)
 		return 0;
 
 	len = demangle_strtol(ddata->cur, 10);
+	if (len < 0)
+		__builtin_longjmp(ddata->jmpbuf, 1);
 
 	while (ELFTC_ISDIGIT(*ddata->cur))
 		++ddata->cur;
 
-	ASSERT(len >= 0);
-	ASSERT(negative_factor == 1 || negative_factor == -1);
-
 	*rtn = len * negative_factor;
 
 	return 1;
@@ -3419,6 +3422,7 @@ demangle_read_pointer_to_member(struct demangle_data *ddata,
 static privileged int
 demangle_read_sname(struct demangle_data *ddata)
 {
+	size_t lim;
 	long len;
 	int err;
 
@@ -3438,6 +3442,9 @@ demangle_read_sname(struct demangle_data *ddata)
 		ddata->last_sname = VEC_STR(ddata, ddata->cur_output,
 		    ddata->cur_output->size - 1);
 
+	lim = demangle_strlen(ddata->cur);
+	if (len > lim)
+		len = lim;
 	ddata->cur += len;
 
 	return 1;
@@ -3647,10 +3654,11 @@ demangle_read_subst(struct demangle_data *ddata)
 		/* first was '_', so increase one */
 		++nth;
 
-		while (*ddata->cur != '_')
+		while (*ddata->cur && *ddata->cur != '_')
 			++ddata->cur;
 
-		ASSERT(nth > 0);
+		if (nth <= 0)
+			return 0;
 
 		return demangle_get_subst(ddata, nth);
 	}
@@ -3881,7 +3889,7 @@ demangle_read_type(struct demangle_data *ddata, struct type_delimit *td)
 
 	case 'E':
 		/* unexpected end (except some things) */
-		if (ddata->is_guard_variable)
+		if (td && ddata->is_guard_variable)
 			td->paren = false;
 		if (ddata->is_guard_variable ||
 		    (ddata->ref_qualifier && ddata->is_functype)) {
@@ -4102,6 +4110,8 @@ demangle_read_type(struct demangle_data *ddata, struct type_delimit *td)
 		if (!demangle_vector_str_push(ddata, &v.ext_name, ddata->cur,
 			len))
 			return 0;
+		if (len > demangle_strlen(ddata->cur))
+			len = demangle_strlen(ddata->cur);
 		ddata->cur += len;
 		if (!demangle_vector_type_qualifier_push(ddata, &v, TYPE_EXT))
 			return 0;