[PATCH v3] misc: add a new test for gethostname

[Martin Coufal]

Add a simple test that runs in a UTS namespace: checks gethostname
after various sethostname values (including empty and HOST_NAME_MAX),
verifies ENAMETOOLONG when the buffer is too small for a maximal name,
EINVAL when sethostname is given a name longer than HOST_NAME_MAX.
---
v3 changes:
* call support_become_root and enter UTS namespace
* use sethostname instead of systemd commands
* clear errno
* changes to comply with the GNU coding standard
---
 misc/Makefile          |   1 +
 misc/tst-gethostname.c | 108 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 109 insertions(+)
 create mode 100644 misc/tst-gethostname.c

diff --git a/misc/Makefile b/misc/Makefile
index b38e983ed7..3937b5d5ea 100644
--- a/misc/Makefile
+++ b/misc/Makefile
@@ -245,6 +245,7 @@ tests := \
   tst-empty \
   tst-error1 \
   tst-fdset \
+  tst-gethostname \
   tst-getusershell \
   tst-hsearch \
   tst-insremque \
diff --git a/misc/tst-gethostname.c b/misc/tst-gethostname.c
new file mode 100644
index 0000000000..bbb58d421a
--- /dev/null
+++ b/misc/tst-gethostname.c
@@ -0,0 +1,108 @@
+/* Basic test for gethostname.
+   Copyright (C) 2024-2026 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 <limits.h>
+#include <sched.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <unistd.h>
+
+#ifndef HOST_NAME_MAX
+#define HOST_NAME_MAX 64
+#endif
+
+#define MAX_LEN_INCREASE 10
+
+
+static int
+do_test (void)
+{
+  char default_hostname[HOST_NAME_MAX + 1] = "\0";
+  char hostname[HOST_NAME_MAX + 1] = "\0";
+  char generated_hostname[HOST_NAME_MAX + MAX_LEN_INCREASE + 1] = "\0";
+
+  /* Become root and enter a UTS namespace.  */
+  support_become_root ();
+#ifdef CLONE_NEWUTS
+  if (unshare (CLONE_NEWUTS) != 0)
+    FAIL_UNSUPPORTED ("unshare (CLONE_NEWUTS): %m");
+#else
+  FAIL_UNSUPPORTED ("CLONE_NEWUTS not supported");
+#endif
+
+  /* Check default hostname is not an empty string.  */
+  TEST_VERIFY_EXIT (gethostname (default_hostname, sizeof (default_hostname))
+		    == 0);
+  TEST_VERIFY (strlen (default_hostname) > 0);
+
+  /* input: ''
+     expected output: '' */
+  TEST_VERIFY_EXIT (sethostname ("", 0) == 0);
+  TEST_VERIFY (gethostname (hostname, sizeof (hostname)) == 0);
+  TEST_VERIFY (strlen (hostname) == 0);
+
+  /* input: 'a'
+     expected output: 'a' */
+  TEST_VERIFY_EXIT (sethostname ("a", strlen ("a")) == 0);
+  TEST_VERIFY (gethostname (hostname, sizeof (hostname)) == 0);
+  TEST_COMPARE (strlen (hostname), 1);
+  TEST_VERIFY (strcmp (hostname, "a") == 0);
+
+  /* input: abc.def.ghi
+     expected output: abc.def.ghi */
+  TEST_VERIFY_EXIT (sethostname ("abc.def.ghi", strlen ("abc.def.ghi")) == 0);
+  TEST_VERIFY (gethostname (hostname, sizeof (hostname)) == 0);
+  TEST_COMPARE (strlen (hostname), strlen ("abc.def.ghi"));
+  TEST_VERIFY (strcmp (hostname, "abc.def.ghi") == 0);
+
+  /* input: exactly HOST_NAME_MAX
+     expected output: exactly HOST_NAME_MAX, gethostname returns -1, errno set */
+  for (int i = 0; i < HOST_NAME_MAX; i++)
+    {
+      /* Generate hostname of length == HOST_NAME_MAX.  */
+      generated_hostname[i] = 'a' + i % 26;
+    }
+  TEST_VERIFY_EXIT (sethostname
+		    (generated_hostname, strlen (generated_hostname)) == 0);
+  errno = 0;
+  TEST_VERIFY (gethostname (hostname, HOST_NAME_MAX) == -1);
+  TEST_COMPARE (errno, ENAMETOOLONG);
+  TEST_COMPARE (strlen (hostname), HOST_NAME_MAX);
+  TEST_VERIFY (strcmp (hostname, generated_hostname) == 0);
+
+  /* input: longer than HOST_NAME_MAX
+     expected output: sethostname fails, return -1, errno set */
+  for (int i = 0; i < HOST_NAME_MAX + MAX_LEN_INCREASE; i++)
+    {
+      /* Generate hostname LONGER than HOST_NAME_MAX.  */
+      generated_hostname[i] = 'a' + i % 26;
+    }
+  errno = 0;
+  TEST_VERIFY (sethostname (generated_hostname, strlen (generated_hostname))
+	       == -1);
+  TEST_COMPARE (errno, EINVAL);
+
+  /* Restore default hostname.  */
+  TEST_VERIFY_EXIT (sethostname (default_hostname, strlen (default_hostname))
+		    == 0);
+  return 0;
+}
+
+#include <support/test-driver.c>
-- 
2.53.0