head: add TOCTOU regression tests and fix comment typo (#12637)

Follow-up to the metadata TOCTOU fix (#11972, PR #12439):

- Add a syscall-level regression guard in util/check-safe-traversal.sh:
  under strace, head must read metadata from the open descriptor
  (fstat/statx on the fd) and must not stat the path before opening it.
  Verified this fails on the pre-fix code and passes after.
- Add an integration test asserting that an unreadable file produces an
  error but no "==> name <==" header, matching GNU (the header is only
  printed after a successful open).
- Fix a stray paren in the Windows-branch comment.

src/uu/head/src/head.rs

@@ -471,7 +471,7 @@ fn uu_head(options: &HeadOptions) -> UResult<()> {
                 Ok(f) => f,
                 Err(err) => {
                     #[cfg(windows)]
-                    // On Windows, `File::open` on a directory fails with "Permission denied").
+                    // On Windows, `File::open` on a directory fails with "Permission denied".
                     if err.kind() == io::ErrorKind::PermissionDenied {
                         if let Ok(m) = Path::new(file).metadata() {
                             if m.is_dir() {

tests/by-util/test_head.rs

@@ -980,3 +980,28 @@ fn test_directory_header_with_multiple_files_zero_output() {
         .stdout_is("==> d <==\n\n==> f <==\n")
         .no_stderr();
 }
+
+/// GNU `head` prints the `==> name <==` header only after the file is
+/// successfully opened. A file that exists but cannot be opened (e.g. no read
+/// permission) must therefore produce only an error and no header.
+#[cfg(unix)]
+#[test]
+fn test_unreadable_file_prints_no_header() {
+    let ts = TestScenario::new(util_name!());
+    let at = &ts.fixtures;
+    at.write("unreadable", "secret\n");
+    at.write("readable", "hello\n");
+    at.set_mode("unreadable", 0o000);
+
+    // Running as root bypasses permission checks, so the open would succeed.
+    if std::fs::File::open(at.plus("unreadable")).is_ok() {
+        return;
+    }
+
+    ts.ucmd()
+        .args(&["-c", "5", "unreadable", "readable"])
+        .fails_with_code(1)
+        .stdout_is("==> readable <==\nhello")
+        .stdout_does_not_contain("==> unreadable <==")
+        .stderr_contains("cannot open 'unreadable' for reading: Permission denied");
+}

util/check-safe-traversal.sh

@@ -191,7 +191,7 @@ if [ "$USE_MULTICALL" -eq 1 ]; then
     AVAILABLE_UTILS=$($COREUTILS_BIN --list)
 else
     AVAILABLE_UTILS=""
-    for util in rm chmod chown chgrp du mv cp touch; do
+    for util in rm chmod chown chgrp du mv cp touch head; do
         if [ -f "$PROJECT_ROOT/target/${PROFILE}/$util" ]; then
             AVAILABLE_UTILS="$AVAILABLE_UTILS $util"
         fi
@@ -409,6 +409,33 @@ if echo "$AVAILABLE_UTILS" | grep -q "touch"; then
     echo "✓ touch creates with O_CREAT and without O_TRUNC"
 fi
 
+# Test head - the is-a-directory check must derive from the already-open
+# descriptor (fstat/statx on the fd), not from a separate path-based stat
+# performed before the open. A path stat followed by an open is a TOCTOU
+# window (#11972): the object named by the path can be swapped in between.
+if echo "$AVAILABLE_UTILS" | grep -q "head"; then
+    echo ""
+    echo "Testing head (fstat_after_open)..."
+    if [ "$USE_MULTICALL" -eq 1 ]; then
+        head_cmd="$COREUTILS_BIN head"
+    else
+        head_cmd="$PROJECT_ROOT/target/${PROFILE}/head"
+    fi
+    echo "headtest" > test_head_file.txt
+    strace -f -e trace=openat,fstat,newfstatat,statx,stat,lstat \
+        -o strace_head_metadata.log $head_cmd -c 4 test_head_file.txt 2>/dev/null || true
+    cat strace_head_metadata.log
+    if ! grep -q 'openat(AT_FDCWD, "test_head_file.txt"' strace_head_metadata.log; then
+        fail_immediately "head did not open test_head_file.txt via openat"
+    fi
+    # The filename should appear only in the openat; any stat-family call naming
+    # the path means head stat'd it before opening - the TOCTOU window.
+    if grep '"test_head_file.txt"' strace_head_metadata.log | grep -qv 'openat('; then
+        fail_immediately "head stat'd the path before opening it - TOCTOU window (#11972); metadata must come from the open descriptor"
+    fi
+    echo "✓ head reads metadata from the open descriptor, not a path stat"
+fi
+
 echo ""
 echo "✓ Basic safe traversal verification completed"
 echo ""