dd: use seek for stdin skip when possible (#9821)

* dd: use seek for stdin skip when possible

* Add Rust tests for skip with seekable stdin

* Address review comments: fix return value, add comment, refactor tests

* dd: use ibs-sized buffer for ESPIPE fallback in skip

src/uu/dd/src/dd.rs

@@ -272,14 +272,40 @@ impl Source {
                         return Ok(len);
                     }
                 }
-                let m = read_and_discard(f, n, ibs)?;
-                if m < n {
-                    show_error!(
-                        "{}",
-                        translate!("dd-error-cannot-skip-offset", "file" => "standard input")
-                    );
+                // Get file length before seeking to avoid race condition
+                let file_len = f.metadata().map(|m| m.len()).unwrap_or(u64::MAX);
+                // Try seek first; fall back to read if not seekable
+                match n.try_into().ok().map(|n| f.seek(SeekFrom::Current(n))) {
+                    Some(Ok(pos)) => {
+                        if pos > file_len {
+                            show_error!(
+                                "{}",
+                                translate!("dd-error-cannot-skip-offset", "file" => "standard input")
+                            );
+                        }
+                        Ok(n)
+                    }
+                    // ESPIPE means the file descriptor is not seekable (e.g., a pipe),
+                    // so fall back to reading and discarding bytes using ibs-sized buffer
+                    Some(Err(e)) if e.raw_os_error() == Some(libc::ESPIPE) => {
+                        let m = read_and_discard(f, n, ibs)?;
+                        if m < n {
+                            show_error!(
+                                "{}",
+                                translate!("dd-error-cannot-skip-offset", "file" => "standard input")
+                            );
+                        }
+                        Ok(m)
+                    }
+                    _ => {
+                        show_error!(
+                            "{}",
+                            translate!("dd-error-cannot-skip-invalid", "file" => "standard input")
+                        );
+                        set_exit_code(1);
+                        Ok(0)
+                    }
                 }
-                Ok(m)
             }
             Self::File(f) => f.seek(SeekFrom::Current(n.try_into().unwrap())),
             #[cfg(unix)]

tests/by-util/test_dd.rs

@@ -669,6 +669,39 @@ fn test_skip_beyond_file() {
         );
 }
 
+#[test]
+#[cfg(unix)]
+fn test_skip_beyond_file_seekable_stdin() {
+    // When stdin is a seekable file, dd should use seek to skip bytes.
+    // This tests that skipping beyond the file size issues a warning.
+    use std::process::Stdio;
+
+    // Test cases: (bs, skip) pairs that skip beyond a 4-byte file
+    let test_cases = [
+        ("bs=1", "skip=5"), // skip 5 bytes
+        ("bs=3", "skip=2"), // skip 6 bytes
+    ];
+
+    for (bs, skip) in test_cases {
+        let (at, mut ucmd) = at_and_ucmd!();
+        at.write("in", "abcd");
+
+        let stdin = OwnedFileDescriptorOrHandle::open_file(
+            OpenOptions::new().read(true),
+            at.plus("in").as_path(),
+        )
+        .unwrap();
+
+        ucmd.args(&[bs, skip, "count=0", "status=noxfer"])
+            .set_stdin(Stdio::from(stdin))
+            .succeeds()
+            .no_stdout()
+            .stderr_contains(
+                "'standard input': cannot skip to specified offset\n0+0 records in\n0+0 records out\n",
+            );
+    }
+}
+
 #[test]
 fn test_seek_do_not_overwrite() {
     let (at, mut ucmd) = at_and_ucmd!();