Improve length checks in DHCP Options parsing of dhcpcd.

Bug: 26461634
Change-Id: Iefa45ba440df8ab6e2af391220fae22dcddec96b
(cherry picked from commit 3320c04abf2f5e75cf3a78ad29417753d8e3c2a2)

Author: Pierre Imai

dhcp.c

@@ -271,21 +271,34 @@ valid_length(uint8_t option, int dl, int *type)
 
 		if (type)
 			*type = opt->type;
-
+		/* The size of RFC3442 and RFC5969 options is checked at a later
+		 * stage in the code */
 		if (opt->type == 0 ||
 		    opt->type & (STRING | RFC3442 | RFC5969))
 			return 0;
-
+		/* The code does not use SINT16 / SINT32 together with ARRAY.
+		 * It is however far easier to reason about the code if all
+		 * possible array elements are included, and also does not code
+		 * any additional CPU resources. sizeof(uintXX_t) ==
+		 * sizeof(intXX_t) can be assumed. */
 		sz = 0;
-		if (opt->type & (UINT32 | IPV4))
+		if (opt->type & (UINT32 | SINT32 | IPV4))
 			sz = sizeof(uint32_t);
-		if (opt->type & UINT16)
+		else if (opt->type & (UINT16 | SINT16))
 			sz = sizeof(uint16_t);
-		if (opt->type & UINT8)
+		else if (opt->type & UINT8)
 			sz = sizeof(uint8_t);
-		if (opt->type & (IPV4 | ARRAY))
-			return dl % sz;
-		return (dl == sz ? 0 : -1);
+		if (opt->type & ARRAY) {
+			/* The result of modulo zero is undefined. There are no
+			 * options defined in this file that do not match one of
+			 * the if-clauses above, so the following is not really
+			 * necessary. However, to avoid confusion and unexpected
+			 * behavior if the defined options are ever extended,
+			 * returning false here seems sensible. */
+			if (!sz) return -1;
+			return (dl % sz == 0) ? 0 : -1;
+		}
+		return (sz == dl) ? 0 : -1;
 	}
 
 	/* unknown option, so let it pass */