login: Acquire write lock early in pututline [BZ #24882]

It has been reported that due to lack of fairness in POSIX file
locking, the current reader-to-writer lock upgrade can result in
lack of forward progress.  Acquiring the write lock directly
hopefully avoids this issue if there are only writers.

This also fixes bug 24882 due to the cache revalidation in
__libc_pututline.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Change-Id: I57e31ae30719e609a53505a0924dda101d46372e

3 files changed, 238 insertions, 31 deletions

diff --git a/login/Makefile b/login/Makefile
index 0183db1..6a0182b 100644
--- a/login/Makefile
+++ b/login/Makefile
@@ -44,7 +44,7 @@ subdir-dirs = programs
 vpath %.c programs
 
 tests := tst-utmp tst-utmpx tst-grantpt tst-ptsname tst-getlogin tst-updwtmpx \
-  tst-pututxline-lockfail
+  tst-pututxline-lockfail tst-pututxline-cache
 
 # Build the -lutil library with these extra functions.
 extra-libs      := libutil
@@ -74,3 +74,4 @@ $(inst_libexecdir)/pt_chown: $(objpfx)pt_chown $(+force)
 	-$(INSTALL_PROGRAM) -m 4755 -o root $< $@
 
 $(objpfx)tst-pututxline-lockfail: $(shared-thread-library)
+$(objpfx)tst-pututxline-cache: $(shared-thread-library)
diff --git a/login/tst-pututxline-cache.c b/login/tst-pututxline-cache.c
new file mode 100644
index 0000000..3f30dd1
--- /dev/null
+++ b/login/tst-pututxline-cache.c
@@ -0,0 +1,193 @@
+/* Test case for cache invalidation after concurrent write (bug 24882).
+   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/>.  */
+
+/* This test writes an entry to the utmpx file, reads it (so that it
+   is cached) in process1, and overwrites the same entry in process2
+   with something that does not match the search criteria.  At this
+   point, the cache of the first process is stale, and when process1
+   attempts to write a new record which would have gone to the same
+   place (as indicated by the cache), it needs to realize that it has
+   to pick a different slot because the old slot is now used for
+   something else.  */
+
+#include <errno.h>
+#include <stdlib.h>
+#include <string.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 <utmp.h>
+#include <utmpx.h>
+
+/* Set to the path of the utmp file.  */
+static char *utmp_file;
+
+/* Used to synchronize the subprocesses.  The barrier itself is
+   allocated in shared memory.  */
+static pthread_barrier_t *barrier;
+
+/* setutxent with error checking.  */
+static void
+xsetutxent (void)
+{
+  errno = 0;
+  setutxent ();
+  TEST_COMPARE (errno, 0);
+}
+
+/* getutxent with error checking.  */
+static struct utmpx *
+xgetutxent (void)
+{
+  errno = 0;
+  struct utmpx *result = getutxent ();
+  if (result == NULL)
+    FAIL_EXIT1 ("getutxent: %m");
+  return result;
+}
+
+static void
+put_entry (const char *id, pid_t pid, const char *user, const char *line)
+{
+  struct utmpx ut =
+    {
+     .ut_type = LOGIN_PROCESS,
+     .ut_pid = pid,
+     .ut_host = "localhost",
+    };
+  strcpy (ut.ut_id, id);
+  strncpy (ut.ut_user, user, sizeof (ut.ut_user));
+  strncpy (ut.ut_line, line, sizeof (ut.ut_line));
+  TEST_VERIFY (pututxline (&ut) != NULL);
+}
+
+/* Use two cooperating subprocesses to avoid issues related to
+   unlock-on-close semantics of POSIX advisory locks.  */
+
+static __attribute__ ((noreturn)) void
+process1 (void)
+{
+  TEST_COMPARE (utmpname (utmp_file), 0);
+
+  /* Create an entry.  */
+  xsetutxent ();
+  put_entry ("1", 101, "root", "process1");
+
+  /* Retrieve the entry.  This will fill the internal cache.  */
+  {
+    errno = 0;
+    setutxent ();
+    TEST_COMPARE (errno, 0);
+    struct utmpx ut =
+      {
+       .ut_type = LOGIN_PROCESS,
+       .ut_line = "process1",
+      };
+    struct utmpx *result = getutxline (&ut);
+    if (result == NULL)
+      FAIL_EXIT1 ("getutxline (\"process1\"): %m");
+    TEST_COMPARE (result->ut_pid, 101);
+  }
+
+  /* Signal the other process to overwrite the entry.  */
+  xpthread_barrier_wait (barrier);
+
+  /* Wait for the other process to complete the write operation.  */
+  xpthread_barrier_wait (barrier);
+
+  /* Add another entry.  Note: This time, there is no setutxent call.  */
+  put_entry ("1", 103, "root", "process1");
+
+  _exit (0);
+}
+
+static void
+process2 (void *closure)
+{
+  /* Wait for the first process to write its entry.  */
+  xpthread_barrier_wait (barrier);
+
+  /* Truncate the file.  The glibc interface does not support
+     re-purposing records, but an external expiration mechanism may
+     trigger this.  */
+  TEST_COMPARE (truncate64 (utmp_file, 0), 0);
+
+  /* Write the replacement entry.  */
+  TEST_COMPARE (utmpname (utmp_file), 0);
+  xsetutxent ();
+  put_entry ("2", 102, "user", "process2");
+
+  /* Signal the other process that the entry has been replaced.  */
+  xpthread_barrier_wait (barrier);
+}
+
+static int
+do_test (void)
+{
+  xclose (create_temp_file ("tst-tumpx-cache-write-", &utmp_file));
+  {
+    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);
+  }
+
+  /* Run both subprocesses in parallel.  */
+  {
+    pid_t pid1 = xfork ();
+    if (pid1 == 0)
+      process1 ();
+    support_isolate_in_subprocess (process2, NULL);
+    int status;
+    xwaitpid (pid1, &status, 0);
+    TEST_COMPARE (status, 0);
+  }
+
+  /* Check that the utmpx database contains the expected records.  */
+  {
+    TEST_COMPARE (utmpname (utmp_file), 0);
+    xsetutxent ();
+
+    struct utmpx *ut = xgetutxent ();
+    TEST_COMPARE_STRING (ut->ut_id, "2");
+    TEST_COMPARE (ut->ut_pid, 102);
+    TEST_COMPARE_STRING (ut->ut_user, "user");
+    TEST_COMPARE_STRING (ut->ut_line, "process2");
+
+    ut = xgetutxent ();
+    TEST_COMPARE_STRING (ut->ut_id, "1");
+    TEST_COMPARE (ut->ut_pid, 103);
+    TEST_COMPARE_STRING (ut->ut_user, "root");
+    TEST_COMPARE_STRING (ut->ut_line, "process1");
+
+    if (getutxent () != NULL)
+      FAIL_EXIT1 ("additional utmpx entry");
+  }
+
+  xpthread_barrier_destroy (barrier);
+  support_shared_free (barrier);
+  free (utmp_file);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/login/utmp_file.c b/login/utmp_file.c
index e627233..917c4c5 100644
--- a/login/utmp_file.c
+++ b/login/utmp_file.c
@@ -186,19 +186,11 @@ __libc_getutent_r (struct utmp *buffer, struct utmp **result)
 
 
 /* Search for *ID, updating last_entry and file_offset.  Return 0 on
-   success and -1 on failure.  If the locking operation failed, write
-   true to *LOCK_FAILED.  */
+   success and -1 on failure.  Does not perform locking; for that see
+   internal_getut_r below.  */
 static int
-internal_getut_r (const struct utmp *id, bool *lock_failed)
+internal_getut_nolock (const struct utmp *id)
 {
-  int result = -1;
-
-  if (try_file_lock (file_fd, F_RDLCK))
-    {
-      *lock_failed = true;
-      return -1;
-    }
-
   if (id->ut_type == RUN_LVL || id->ut_type == BOOT_TIME
       || id->ut_type == OLD_TIME || id->ut_type == NEW_TIME)
     {
@@ -213,7 +205,7 @@ internal_getut_r (const struct utmp *id, bool *lock_failed)
 	    {
 	      __set_errno (ESRCH);
 	      file_offset = -1l;
-	      goto unlock_return;
+	      return -1;
 	    }
 	  file_offset += sizeof (struct utmp);
 
@@ -234,7 +226,7 @@ internal_getut_r (const struct utmp *id, bool *lock_failed)
 	    {
 	      __set_errno (ESRCH);
 	      file_offset = -1l;
-	      goto unlock_return;
+	      return -1;
 	    }
 	  file_offset += sizeof (struct utmp);
 
@@ -243,15 +235,26 @@ internal_getut_r (const struct utmp *id, bool *lock_failed)
 	}
     }
 
-  result = 0;
+  return 0;
+}
 
-unlock_return:
-  file_unlock (file_fd);
+/* Search for *ID, updating last_entry and file_offset.  Return 0 on
+   success and -1 on failure.  If the locking operation failed, write
+   true to *LOCK_FAILED.  */
+static int
+internal_getut_r (const struct utmp *id, bool *lock_failed)
+{
+  if (try_file_lock (file_fd, F_RDLCK))
+    {
+      *lock_failed = true;
+      return -1;
+    }
 
+  int result = internal_getut_nolock (id);
+  file_unlock (file_fd);
   return result;
 }
 
-
 /* For implementing this function we don't use the getutent_r function
    because we can avoid the reposition on every new entry this way.  */
 int
@@ -279,7 +282,6 @@ __libc_getutid_r (const struct utmp *id, struct utmp *buffer,
   return 0;
 }
 
-
 /* For implementing this function we don't use the getutent_r function
    because we can avoid the reposition on every new entry this way.  */
 int
@@ -336,7 +338,6 @@ __libc_pututline (const struct utmp *data)
     return NULL;
 
   struct utmp *pbuf;
-  int found;
 
   if (! file_writable)
     {
@@ -358,7 +359,12 @@ __libc_pututline (const struct utmp *data)
       file_writable = true;
     }
 
+  /* Exclude other writers before validating the cache.  */
+  if (try_file_lock (file_fd, F_WRLCK))
+    return NULL;
+
   /* Find the correct place to insert the data.  */
+  bool found = false;
   if (file_offset > 0
       && ((last_entry.ut_type == data->ut_type
 	   && (last_entry.ut_type == RUN_LVL
@@ -366,23 +372,30 @@ __libc_pututline (const struct utmp *data)
 	       || last_entry.ut_type == OLD_TIME
 	       || last_entry.ut_type == NEW_TIME))
 	  || __utmp_equal (&last_entry, data)))
-    found = 1;
-  else
     {
-      bool lock_failed = false;
-      found = internal_getut_r (data, &lock_failed);
-
-      if (__builtin_expect (lock_failed, false))
+      if (__lseek64 (file_fd, file_offset, SEEK_SET) < 0)
 	{
-	  __set_errno (EAGAIN);
+	  file_unlock (file_fd);
 	  return NULL;
 	}
+      if (__read_nocancel (file_fd, &last_entry, sizeof (last_entry))
+	  != sizeof (last_entry))
+	{
+	  if (__lseek64 (file_fd, file_offset, SEEK_SET) < 0)
+	    {
+	      file_unlock (file_fd);
+	      return NULL;
+	    }
+	  found = false;
+	}
+      else
+	found = __utmp_equal (&last_entry, data);
     }
 
-  if (try_file_lock (file_fd, F_WRLCK))
-    return NULL;
+  if (!found)
+    found = internal_getut_nolock (data) >= 0;
 
-  if (found < 0)
+  if (!found)
     {
       /* We append the next entry.  */
       file_offset = __lseek64 (file_fd, 0, SEEK_END);
@@ -411,7 +424,7 @@ __libc_pututline (const struct utmp *data)
     {
       /* If we appended a new record this is only partially written.
 	 Remove it.  */
-      if (found < 0)
+      if (!found)
 	(void) __ftruncate64 (file_fd, file_offset);
       pbuf = NULL;
     }