login: pututxline could fail to overwrite existing entries [BZ #24902]

The internal_getut_r function updates the file_offset variable and
therefore must always update last_entry as well.

Previously, if pututxline could not upgrade the read lock to a
write lock, internal_getut_r would update file_offset only,
without updating last_entry, and a subsequent call would not
overwrite the existing utmpx entry at file_offset, instead
creating a new entry.  This has been observed to cause unbounded
file growth in high-load situations.

This commit removes the buffer argument to internal_getut_r and
updates the last_entry variable directly, along with file_offset.

Initially reported and fixed by Ondřej Lysoněk.

Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>

1 files changed, 176 insertions, 0 deletions

diff --git a/login/tst-pututxline-lockfail.c b/login/tst-pututxline-lockfail.c
new file mode 100644
index 0000000000..47c25dc065
--- /dev/null
+++ b/login/tst-pututxline-lockfail.c
@@ -0,0 +1,176 @@
+/* Test the lock upgrade path in tst-pututxline.
+   Copyright (C) 2019 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; see the file COPYING.LIB.  If
+   not, see <http://www.gnu.org/licenses/>.  */
+
+/* pututxline upgrades the read lock on the file to a write lock.
+   This test verifies that if the lock upgrade fails, the utmp
+   subsystem remains in a consistent state, so that pututxline can be
+   called again.  */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <support/xthread.h>
+#include <support/xunistd.h>
+#include <unistd.h>
+#include <utmp.h>
+#include <utmpx.h>
+
+/* Path to the temporary utmp file.   */
+static char *path;
+
+/* Used to synchronize the subprocesses.  The barrier itself is
+   allocated in shared memory.  */
+static pthread_barrier_t *barrier;
+
+/* Use pututxline to write an entry for PID.  */
+static struct utmpx *
+write_entry (pid_t pid)
+{
+  struct utmpx ut =
+    {
+     .ut_type = LOGIN_PROCESS,
+     .ut_id = "1",
+     .ut_user = "root",
+     .ut_pid = pid,
+     .ut_line = "entry",
+     .ut_host = "localhost",
+    };
+  return pututxline (&ut);
+}
+
+/* Create the initial entry in a subprocess, so that the utmp
+   subsystem in the original process is not disturbed.  */
+static void
+subprocess_create_entry (void *closure)
+{
+  TEST_COMPARE (utmpname (path), 0);
+  TEST_VERIFY (write_entry (101) != NULL);
+}
+
+/* Acquire an advisory read lock on PATH.  */
+__attribute__ ((noreturn)) static void
+subprocess_lock_file (void)
+{
+  int fd = xopen (path, O_RDONLY, 0);
+
+  struct flock64 fl =
+    {
+     .l_type = F_RDLCK,
+     fl.l_whence = SEEK_SET,
+    };
+  TEST_COMPARE (fcntl64 (fd, F_SETLKW, &fl), 0);
+
+  /* Signal to the main process that the lock has been acquired.  */
+  xpthread_barrier_wait (barrier);
+
+  /* Wait for the unlock request from the main process.  */
+  xpthread_barrier_wait (barrier);
+
+  /* Implicitly unlock the file.  */
+  xclose (fd);
+
+  /* Overwrite the existing entry.  */
+  TEST_COMPARE (utmpname (path), 0);
+  errno = 0;
+  setutxent ();
+  TEST_COMPARE (errno, 0);
+  TEST_VERIFY (write_entry (102) != NULL);
+  errno = 0;
+  endutxent ();
+  TEST_COMPARE (errno, 0);
+
+  _exit (0);
+}
+
+static int
+do_test (void)
+{
+  xclose (create_temp_file ("tst-pututxline-lockfail-", &path));
+
+  {
+    pthread_barrierattr_t attr;
+    xpthread_barrierattr_init (&attr);
+    xpthread_barrierattr_setpshared (&attr, PTHREAD_SCOPE_PROCESS);
+    barrier = support_shared_allocate (sizeof (*barrier));
+    xpthread_barrier_init (barrier, &attr, 2);
+    xpthread_barrierattr_destroy (&attr);
+  }
+
+  /* Write the initial entry.  */
+  support_isolate_in_subprocess (subprocess_create_entry, NULL);
+
+  pid_t locker_pid = xfork ();
+  if (locker_pid == 0)
+    subprocess_lock_file ();
+
+  /* Wait for the file locking to complete.  */
+  xpthread_barrier_wait (barrier);
+
+  /* Try to add another entry.  This attempt will fail, with EINTR or
+     EAGAIN.  */
+  TEST_COMPARE (utmpname (path), 0);
+  TEST_VERIFY (write_entry (102) == NULL);
+  if (errno != EINTR)
+    TEST_COMPARE (errno, EAGAIN);
+
+  /* Signal the subprocess to overwrite the entry.  */
+  xpthread_barrier_wait (barrier);
+
+  /* Wait for write and unlock to complete.  */
+  {
+    int status;
+    xwaitpid (locker_pid, &status, 0);
+    TEST_COMPARE (status, 0);
+  }
+
+  /* The file is no longer locked, so this operation will succeed.  */
+  TEST_VERIFY (write_entry (103) != NULL);
+  errno = 0;
+  endutxent ();
+  TEST_COMPARE (errno, 0);
+
+  /* Check that there is just one entry with the expected contents.
+     If pututxline becomes desynchronized internally, the entry is not
+     overwritten (bug 24902).  */
+  errno = 0;
+  setutxent ();
+  TEST_COMPARE (errno, 0);
+  struct utmpx *ut = getutxent ();
+  TEST_VERIFY_EXIT (ut != NULL);
+  TEST_COMPARE (ut->ut_type, LOGIN_PROCESS);
+  TEST_COMPARE_STRING (ut->ut_id, "1");
+  TEST_COMPARE_STRING (ut->ut_user, "root");
+  TEST_COMPARE (ut->ut_pid, 103);
+  TEST_COMPARE_STRING (ut->ut_line, "entry");
+  TEST_COMPARE_STRING (ut->ut_host, "localhost");
+  TEST_VERIFY (getutxent () == NULL);
+  errno = 0;
+  endutxent ();
+  TEST_COMPARE (errno, 0);
+
+  xpthread_barrier_destroy (barrier);
+  support_shared_free (barrier);
+  free (path);
+  return 0;
+}
+
+#include <support/test-driver.c>