test: increase snprintf safety

Detected by CodeQL scanning; only in test harness, not used in main codebase.

Signed-off-by: Robin H. Johnson <[email protected]>

Author: robbat2

test/print_safe_buffer.c

@@ -15,19 +15,30 @@ void print_safe_buffer(struct safe_buffer const *sb)
 size_t snprint_safe_buffer(char *s, size_t size, struct safe_buffer const *sb)
 {
 	size_t count = 0;
+	int n;
 
-	count += snprintf((s + count), (size - count), "unsigned char expected[] = { /* sb.allocated = %ld, sb.used = %ld */", sb->allocated, sb->used);
+	n = snprintf((s + count), (size - count), "unsigned char expected[] = { /* sb.allocated = %ld, sb.used = %ld */", sb->allocated, sb->used);
+	if (n < 0 || n >= size - count) {
+		return count;
+	}
+	count += n;
 
+	char* nextline = "\n\t";
+	char* nextbyte = " ";
 	for (size_t i = 0; i < sb->used; ++i) {
-		if (i % 8 == 0) {
-			count += snprintf((s + count), (size - count), "\n\t0x%02x,", sb->buffer[i]);
-		} else {
-			count += snprintf((s + count), (size - count), " 0x%02x,", sb->buffer[i]);
+		char* nextspace = (i % 8 == 0) ? nextline : nextbyte;
+		n = snprintf((s + count), (size - count), "%s0x%02x,", nextspace, sb->buffer[i]);
+		if (n < 0 || n >= size - count) {
+			return count;
 		}
+		count += n;
 	}
 	/* Do not remove the final byte's comma. Only JSON requires the comma is
 	 * removed, and this is not JSON. */
-
-	count += snprintf((s + count), (size - count), "\n};\n");
+	n = snprintf((s + count), (size - count), "\n};\n");
+	if (n < 0 || n >= size - count) {
+		return count;
+	}
+	count += n;
 	return count;
 }