split: fix add-overflow in suffix-length calculation

src/uu/split/src/filenames.rs

@@ -199,7 +199,8 @@ impl Suffix {
         // Auto pre-calculate new suffix length (auto-width) if necessary
         if let Strategy::Number(number_type) = strategy {
             let chunks = number_type.num_chunks();
-            let required_length = ((start as u64 + chunks) as f64)
+            // u128 keeps the sum from overflowing when start is near usize::MAX.
+            let required_length = ((start as u128 + chunks as u128) as f64)
                 .log(stype.radix() as f64)
                 .ceil() as usize;
 

tests/by-util/test_split.rs

@@ -942,6 +942,47 @@ fn test_suffix_length_req() {
         .stderr_only("split: the suffix length needs to be at least 2\n");
 }
 
+#[test]
+fn test_numeric_suffix_huge_start_requires_suffix_length() {
+    new_ucmd!()
+        .args(&[
+            "-n",
+            "5",
+            "--numeric-suffixes=18446744073709551615",
+            "asciilowercase.txt",
+        ])
+        .fails()
+        .stderr_only("split: the suffix length needs to be at least 20\n");
+}
+
+#[test]
+fn test_hex_suffix_huge_start_requires_suffix_length() {
+    new_ucmd!()
+        .args(&[
+            "-n",
+            "5",
+            "--hex-suffixes=ffffffffffffffff",
+            "asciilowercase.txt",
+        ])
+        .fails()
+        .stderr_only("split: the suffix length needs to be at least 16\n");
+}
+
+#[test]
+fn test_numeric_suffix_huge_chunk_count_requires_suffix_length() {
+    new_ucmd!()
+        .args(&[
+            "-n",
+            "18446744073709551615",
+            "--numeric-suffixes=5",
+            "-a",
+            "2",
+            "asciilowercase.txt",
+        ])
+        .fails()
+        .stderr_only("split: the suffix length needs to be at least 20\n");
+}
+
 #[test]
 fn test_large_suffix_length_is_rejected() {
     new_ucmd!()