Patch from Bastian Blank:

The updated patch adds a config option to explicitely enable 64 bit
    arithmetic.

    Also it removes the arith prototype from libbb.h as it is not used
    outside of ash.

    Bastian

this patch has been slightly modified by Erik for cleanliness.

include/libbb.h

@@ -289,8 +289,6 @@ char *concat_path_file(const char *path, const char *filename);
 char *concat_subpath_file(const char *path, const char *filename);
 char *last_char_is(const char *s, int c);
 
-extern long arith (const char *startbuf, int *errcode);
-
 int read_package_field(const char *package_buffer, char **field_name, char **field_value);
 //#warning yuk!
 char *fgets_str(FILE *file, const char *terminating_string);

shell/Config.in

@@ -67,6 +67,15 @@ config CONFIG_ASH_MATH_SUPPORT
 	help
 	  Enable math support in the ash shell.
 
+config CONFIG_ASH_MATH_SUPPORT_64
+	bool "    Extend Posix math support to 64 bit"
+	default n
+	depends on CONFIG_ASH_MATH_SUPPORT
+	help
+	  Enable 64-bit math support in the ash shell.  This will make
+	  the shell slightly larger, but will allow computation with very
+	  large numbers.
+
 config CONFIG_ASH_GETOPTS
 	bool "  Enable getopt builtin to parse positional parameters"
 	default n

shell/ash.c

@@ -1446,7 +1446,13 @@ static void defun(char *, union node *);
 static void unsetfunc(const char *);
 
 #ifdef CONFIG_ASH_MATH_SUPPORT
-static long dash_arith(const char *);
+#ifdef CONFIG_ASH_MATH_SUPPORT_64
+typedef int64_t arith_t;
+#else
+typedef long arith_t;
+#endif
+static arith_t dash_arith(const char *);
+static arith_t arith(const char *expr, int *perrcode);
 #endif
 
 #ifdef CONFIG_ASH_RANDOM_SUPPORT
@@ -4531,7 +4537,7 @@ static void ifsfree(void);
 static void expandmeta(struct strlist *, int);
 static int patmatch(char *, const char *);
 
-static int cvtnum(long);
+static int cvtnum(arith_t);
 static size_t esclen(const char *, const char *);
 static char *scanleft(char *, char *, char *, char *, int, int);
 static char *scanright(char *, char *, char *, char *, int, int);
@@ -5902,12 +5908,16 @@ casematch(union node *pattern, char *val)
  */
 
 static int
-cvtnum(long num)
+cvtnum(arith_t num)
 {
 	int len;
 
 	expdest = makestrspace(32, expdest);
+#ifdef CONFIG_ASH_MATH_SUPPORT_64
+	len = fmtstr(expdest, 32, "%lld", (long long) num);
+#else
 	len = fmtstr(expdest, 32, "%ld", num);
+#endif
 	STADJUST(len, expdest);
 	return len;
 }
@@ -12488,10 +12498,10 @@ static int timescmd(int ac, char **av)
 }
 
 #ifdef CONFIG_ASH_MATH_SUPPORT
-static long
+static arith_t
 dash_arith(const char *s)
 {
-	long result;
+	arith_t result;
 	int errcode = 0;
 
 	INTOFF;
@@ -12523,7 +12533,7 @@ static int
 letcmd(int argc, char **argv)
 {
 	char **ap;
-	long i;
+	arith_t i;
 
 	ap = argv + 1;
 	if(!*ap)
@@ -13094,8 +13104,8 @@ static inline int is_right_associativity(operator prec)
 
 
 typedef struct ARITCH_VAR_NUM {
-	long val;
+	arith_t val;
-	long contidional_second_val;
+	arith_t contidional_second_val;
 	char contidional_second_val_initialized;
 	char *var;      /* if NULL then is regular number,
 			   else is variable name */
@@ -13152,9 +13162,9 @@ static int arith_lookup_val(v_n_t *t)
 static inline int
 arith_apply(operator op, v_n_t *numstack, v_n_t **numstackptr)
 {
-	long numptr_val;
+	int64_t numptr_val;
 	v_n_t *numptr_m1;
-	long rez;
+	int64_t rez;
 	int ret_arith_lookup_val;
 
 	if (NUMPTR == numstack) goto err; /* There is no operator that can work
@@ -13280,7 +13290,7 @@ arith_apply(operator op, v_n_t *numstack, v_n_t **numstackptr)
 			goto err;
 		}
 		/* save to shell variable */
-		sprintf(buf, "%ld", rez);
+		sprintf(buf, "%lld", (long long) rez);
 		setvar(numptr_m1->var, buf, 0);
 		/* after saving, make previous value for v++ or v-- */
 		if(op == TOK_POST_INC)
@@ -13343,7 +13353,7 @@ static const char op_tokens[] = {
 #define endexpression &op_tokens[sizeof(op_tokens)-7]
 
 
-extern long arith (const char *expr, int *perrcode)
+static arith_t arith (const char *expr, int *perrcode)
 {
     register char arithval; /* Current character under analysis */
     operator lasttok, op;