libsepol: use checked arithmetic builtin to perform safe addition

Checking whether an overflow occurred after adding two values can be
achieved using checked arithmetic builtin functions such as:

    bool __builtin_add_overflow(type1 x, type2 y, type3 *sum);

This function is available at least in clang
(at least since clang 3.8.0,
https://releases.llvm.org/3.8.0/tools/clang/docs/LanguageExtensions.html#checked-arithmetic-builtins)
and gcc
(https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html,
since gcc 5 according to https://gcc.gnu.org/gcc-5/changes.html)

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>

libsepol/src/context_record.c

@@ -267,31 +267,13 @@ int sepol_context_from_string(sepol_handle_t * handle,
 	return STATUS_ERR;
 }
 
-
-static inline int safe_sum(size_t *sum, const size_t augends[], const size_t cnt) {
-
-	size_t a, i;
-
-	*sum = 0;
-	for(i=0; i < cnt; i++) {
-		/* sum should not be smaller than the addend */
-		a = augends[i];
-		*sum += a;
-		if (*sum < a) {
-			return i;
-		}
-	}
-
-	return 0;
-}
-
 int sepol_context_to_string(sepol_handle_t * handle,
 			    const sepol_context_t * con, char **str_ptr)
 {
 
 	int rc;
 	char *str = NULL;
-	size_t total_sz, err;
+	size_t total_sz = 0, i;
 	const size_t sizes[] = {
 			strlen(con->user),                 /* user length */
 			strlen(con->role),                 /* role length */
@@ -300,10 +282,11 @@ int sepol_context_to_string(sepol_handle_t * handle,
 			((con->mls) ? 3 : 2) + 1           /* mls has extra ":" also null byte */
 	};
 
-	err = safe_sum(&total_sz, sizes, ARRAY_SIZE(sizes));
-	if (err) {
-		ERR(handle, "invalid size, overflow at position: %zu", err);
-		goto err;
+	for (i = 0; i < ARRAY_SIZE(sizes); i++) {
+		if (__builtin_add_overflow(total_sz, sizes[i], &total_sz)) {
+			ERR(handle, "invalid size, overflow at position: %zu", i);
+			goto err;
+		}
 	}
 
 	str = (char *)malloc(total_sz);

libsepol/src/module_to_cil.c

@@ -1134,16 +1134,14 @@ static int name_list_to_string(char **names, unsigned int num_names, char **stri
 	char *strpos;
 
 	for (i = 0; i < num_names; i++) {
-		len += strlen(names[i]);
-		if (len < strlen(names[i])) {
+		if (__builtin_add_overflow(len, strlen(names[i]), &len)) {
 			log_err("Overflow");
 			return -1;
 		}
 	}
 
 	// add spaces + null terminator
-	len += num_names;
-	if (len < (size_t)num_names) {
+	if (__builtin_add_overflow(len, (size_t)num_names, &len)) {
 		log_err("Overflow");
 		return -1;
 	}