Add upper bound for t in --read-data-subset=n/t (#2304)

* Add upper bound for t in --read-data-subset=n/t

* Add changelog entry

cmd/restic/cmd_check.go

@@ -67,11 +67,17 @@ func checkFlags(opts CheckOptions) error {
 		if dataSubset[0] == 0 || dataSubset[1] == 0 || dataSubset[0] > dataSubset[1] {
 			return errors.Fatalf("check flag --read-data-subset=n/t values must be positive integers, and n <= t, e.g. --read-data-subset=1/2")
 		}
+		if dataSubset[1] > totalBucketsMax {
+			return errors.Fatalf("check flag --read-data-subset=n/t t must be at most %d", totalBucketsMax)
+		}
 	}
 
 	return nil
 }
 
+// See doReadData in runCheck below for why this is 256.
+const totalBucketsMax = 256
+
 // stringToIntSlice converts string to []uint, using '/' as element separator
 func stringToIntSlice(param string) (split []uint, err error) {
 	if param == "" {
@@ -257,6 +263,8 @@ func runCheck(opts CheckOptions, gopts GlobalOptions, args []string) error {
 	doReadData := func(bucket, totalBuckets uint) {
 		packs := restic.IDSet{}
 		for pack := range chkr.GetPacks() {
+			// If we ever check more than the first byte
+			// of pack, update totalBucketsMax.
 			if (uint(pack[0]) % totalBuckets) == (bucket - 1) {
 				packs.Insert(pack)
 			}