io: Add error tests for fchmod

On Linux most descriptors that do not correspond to file system
entities (such as anonymous pipes and sockets) have file permissions
that can be changed.  While it is possible to create a custom file
system that returns (say) EINVAL for an fchmod attempt, testing this
does not appear to be useful.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

2 files changed, 57 insertions, 0 deletions

diff --git a/io/Makefile b/io/Makefile
index 19932d50f7..47666a1deb 100644
--- a/io/Makefile
+++ b/io/Makefile
@@ -188,6 +188,7 @@ tests := \
   tst-closefrom \
   tst-copy_file_range \
   tst-faccessat \
+  tst-fchmod-errors \
   tst-fchmodat \
   tst-fchownat \
   tst-fcntl \
diff --git a/io/tst-fchmod-errors.c b/io/tst-fchmod-errors.c
new file mode 100644
index 0000000000..ee15300fc3
--- /dev/null
+++ b/io/tst-fchmod-errors.c
@@ -0,0 +1,56 @@
+/* Test various fchmod error cases.
+   Copyright (C) 2024 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <support/check.h>
+#include <support/xunistd.h>
+
+static int
+do_test (void)
+{
+  {
+    /* Permissions on /dev/null (the opened descriptor) cannot be changed.  */
+    int fd = xopen ("/dev/null", O_RDWR, 0);
+    errno = 0;
+    TEST_COMPARE (fchmod (fd, 0), -1);
+    TEST_COMPARE (errno, EPERM);
+    xclose (fd);
+
+    /* Now testing an invalid file descriptor.   */
+    errno = 0;
+    TEST_COMPARE (fchmod (fd, 0600), -1);
+    TEST_COMPARE (errno, EBADF);
+  }
+
+  errno = 0;
+  TEST_COMPARE (fchmod (-1, 0600), -1);
+  TEST_COMPARE (errno, EBADF);
+
+  errno = 0;
+  TEST_COMPARE (fchmod (AT_FDCWD, 0600), -1);
+  TEST_COMPARE (errno, EBADF);
+
+  /* Linux supports fchmod on pretty much all file descriptors, so
+     there is no check for failure on specific types of descriptors
+     here.  */
+
+  return 0;
+}
+
+#include <support/test-driver.c>