lib/parser.c: avoid overflow in match_number()

The result of converting an integer value to another signed integer type
that's unable to represent the original value is implementation defined.
(See notes in section 6.3.1.3 of the C standard.)

In match_number(), the result of simple_strtol() (which returns type long)
is assigned to a value of type int.

Instead, handle the result of simple_strtol() in a well-defined way, and
return -ERANGE if the result won't fit in the int variable used to hold
the parsed result.

No current callers pay attention to the particular error value returned,
so this additional return code shouldn't do any harm.

[[e-mail]: coding-style tweaks]
Signed-off-by: Alex Elder <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/parser.c

···
  *
  * Description: Given a &substring_t and a base, attempts to parse the substring
  * as a number in that base. On success, sets @result to the integer represented
- * by the string and returns 0. Returns either -ENOMEM or -EINVAL on failure.
+ * by the string and returns 0. Returns -ENOMEM, -EINVAL, or -ERANGE on failure.
  */
 static int match_number(substring_t *s, int *result, int base)
 {
 	char *endp;
 	char *buf;
 	int ret;
+	long val;
 	size_t len = s->to - s->from;
 
 	buf = kmalloc(len + 1, GFP_KERNEL);
···
 		return -ENOMEM;
 	memcpy(buf, s->from, len);
 	buf[len] = '\0';
-	*result = simple_strtol(buf, &endp, base);
+
 	ret = 0;
+	val = simple_strtol(buf, &endp, base);
 	if (endp == buf)
 		ret = -EINVAL;
+	else if (val < (long)INT_MIN || val > (long)INT_MAX)
+		ret = -ERANGE;
+	else
+		*result = (int) val;
 	kfree(buf);
 	return ret;
 }