tail: do not abort on a -c +N offset past the end of the file

src/uu/tail/src/tail.rs

@@ -480,7 +480,11 @@ fn bounded_tail(file: &mut File, settings: &Settings) -> UResult<()> {
         FilterMode::Bytes(Signum::Positive(count)) if count > &1 => {
             // GNU `tail` seems to index bytes and lines starting at 1, not
             // at 0. It seems to treat `+0` and `+1` as the same thing.
-            file.seek(SeekFrom::Start(*count - 1)).unwrap();
+            // A start offset past the end of the file may not be seekable (the
+            // seek fails instead of landing past EOF); GNU prints nothing then,
+            // so fall back to the end of the file rather than aborting.
+            file.seek(SeekFrom::Start(*count - 1))
+                .or_else(|_| file.seek(SeekFrom::End(0)))?;
         }
         _ => {}
     }

tests/by-util/test_tail.rs

@@ -1118,6 +1118,19 @@ fn test_positive_zero_bytes() {
         .no_output();
 }
 
+/// A start byte offset past the end of a regular file must not abort. The file
+/// is larger than the block size so the seekable (`bounded_tail`) path is used,
+/// and the offset exceeds `i64::MAX` so the seek itself fails; GNU prints
+/// nothing in that case instead of crashing.
+#[test]
+fn test_positive_bytes_past_end_of_file_does_not_panic() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.write("f", &"a".repeat(1_000_000));
+    ucmd.args(&["-c", "+18446744073709551615", "f"])
+        .succeeds()
+        .no_output();
+}
+
 /// Test for reading all but the first NUM lines: `tail -n +3`.
 #[test]
 fn test_positive_lines() {