tail: don't panic on '-c -N' with a huge N

src/uu/tail/src/tail.rs

@@ -469,7 +469,11 @@
             file.seek(SeekFrom::End(0)).unwrap();
         }
         FilterMode::Bytes(Signum::Negative(count)) => {
-            if file.seek(SeekFrom::End(-(*count as i64))).is_err() {
+            // A count above `i64::MAX` has no negative counterpart to seek by,
+            // and reaches further back than any file can be long, so treat it
+            // like any other offset landing before the start of the file.
+            let offset = i64::try_from(*count).ok();
+            if offset.is_none_or(|offset| file.seek(SeekFrom::End(-offset)).is_err()) {
                 file.seek(SeekFrom::Start(0)).unwrap();
             }
             limit = Some(*count);
@@ -478,7 +482,7 @@
             // 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.
             // A start offset past the largest seekable position makes the
             // underlying `lseek` fail with `EINVAL`; treat that like a start
             // beyond the end of the file and produce no output.
             file.seek(SeekFrom::Start(*count - 1))
                 .or_else(|_| file.seek(SeekFrom::End(0)))

tests/by-util/test_tail.rs

@@ -1377,6 +1377,20 @@ fn test_positive_bytes_file_offset_past_seek_limit() {
         .no_stdout();
 }
 
+// `tail -c -N` on a seekable file larger than the block size seeks back `N`
+// bytes from the end. An `N` that does not fit in an `i64` must not abort the
+// process; it should print the whole file, like any other too-large count.
+#[test]
+fn test_negative_bytes_file_count_past_seek_limit() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    // Larger than tail's `sane_blksize` (~4 KiB) so the seek path is taken.
+    let data = "a".repeat(8192);
+    at.write("big", &data);
+    ucmd.args(&["-c", "-9223372036854775808", "big"])
+        .succeeds()
+        .stdout_is(&data);
+}
+
 #[test]
 fn test_num_with_undocumented_sign_bytes() {
     // tail: '-' is not documented (8.32 man pages)