shadow/lckpwdf.c

   1 /* Handle locking of password file.
   2    Copyright (C) 1996-2023 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <fcntl.h>
  20 #include <libc-lock.h>
  21 #include <shadow.h>
  22 #include <signal.h>
  23 #include <string.h>
  24 #include <unistd.h>
  25 #include <sys/file.h>
  26 #include <sigsetops.h>
  27
  28 #include <kernel-features.h>
  29
  30
  31 /* Name of the lock file.  */
  32 #define PWD_LOCKFILE "/etc/.pwd.lock"
  33
  34 /* How long to wait for getting the lock before returning with an
  35    error.  */
  36 #define TIMEOUT 15 /* sec */
  37
  38
  39 /* File descriptor for lock file.  */
  40 static int lock_fd = -1;
  41
  42 /* Prevent problems in multithreaded program by using mutex.  */
  43 __libc_lock_define_initialized (static, lock)
  44
  45
  46 /* Prototypes for local functions.  */
  47 static void noop_handler (int __sig);
  48
  49
  50 /* We cannot simply return in error cases.  We have to close the file
  51    and perhaps restore the signal handler.  */
  52 #define RETURN_CLOSE_FD(code)                                                 \
  53   do {                                                                        \
  54     if ((code) < 0 && lock_fd >= 0)                                           \
  55       {                                                                       \
  56         __close (lock_fd);                                                    \
  57         lock_fd = -1;                                                         \
  58       }                                                                       \
  59     __libc_lock_unlock (lock);                                                \
  60     return (code);                                                            \
  61   } while (0)
  62
  63 #define RETURN_RESTORE_HANDLER(code)                                          \
  64   do {                                                                        \
  65     /* Restore old action handler for alarm.  We don't need to know           \
  66        about the current one.  */                                             \
  67     __sigaction (SIGALRM, &saved_act, NULL);                                  \
  68     RETURN_CLOSE_FD (code);                                                   \
  69   } while (0)
  70
  71 #define RETURN_CLEAR_ALARM(code)                                              \
  72   do {                                                                        \
  73     /* Clear alarm.  */                                                       \
  74     alarm (0);                                                                \
  75     /* Restore old set of handled signals.  We don't need to know             \
  76        about the current one.*/                                               \
  77     __sigprocmask (SIG_SETMASK, &saved_set, NULL);                            \
  78     RETURN_RESTORE_HANDLER (code);                                            \
  79   } while (0)
  80
  81
  82 int
  83 __lckpwdf (void)
  84 {
  85   sigset_t saved_set;                   /* Saved set of caught signals.  */
  86   struct sigaction saved_act;           /* Saved signal action.  */
  87   sigset_t new_set;                     /* New set of caught signals.  */
  88   struct sigaction new_act;             /* New signal action.  */
  89   struct flock fl;                      /* Information struct for locking.  */
  90   int result;
  91
  92   if (lock_fd != -1)
  93     /* Still locked by own process.  */
  94     return -1;
  95
  96   /* Prevent problems caused by multiple threads.  */
  97   __libc_lock_lock (lock);
  98
  99   int oflags = O_WRONLY | O_CREAT | O_CLOEXEC;
 100   lock_fd = __open (PWD_LOCKFILE, oflags, 0600);
 101   if (lock_fd == -1)
 102     /* Cannot create lock file.  */
 103     RETURN_CLOSE_FD (-1);
 104
 105   /* Now we have to get exclusive write access.  Since multiple
 106      process could try this we won't stop when it first fails.
 107      Instead we set a timeout for the system call.  Once the timer
 108      expires it is likely that there are some problems which cannot be
 109      resolved by waiting.
 110
 111      It is important that we don't change the signal state.  We must
 112      restore the old signal behaviour.  */
 113   memset (&new_act, '\0', sizeof (struct sigaction));
 114   new_act.sa_handler = noop_handler;
 115   __sigfillset (&new_act.sa_mask);
 116   new_act.sa_flags = 0ul;
 117
 118   /* Install new action handler for alarm and save old.  */
 119   if (__sigaction (SIGALRM, &new_act, &saved_act) < 0)
 120     /* Cannot install signal handler.  */
 121     RETURN_CLOSE_FD (-1);
 122
 123   /* Now make sure the alarm signal is not blocked.  */
 124   __sigemptyset (&new_set);
 125   __sigaddset (&new_set, SIGALRM);
 126   if (__sigprocmask (SIG_UNBLOCK, &new_set, &saved_set) < 0)
 127     RETURN_RESTORE_HANDLER (-1);
 128
 129   /* Start timer.  If we cannot get the lock in the specified time we
 130      get a signal.  */
 131   alarm (TIMEOUT);
 132
 133   /* Try to get the lock.  */
 134   memset (&fl, '\0', sizeof (struct flock));
 135   fl.l_type = F_WRLCK;
 136   fl.l_whence = SEEK_SET;
 137   result = __fcntl (lock_fd, F_SETLKW, &fl);
 138
 139   RETURN_CLEAR_ALARM (result);
 140 }
 141 weak_alias (__lckpwdf, lckpwdf)
 142
 143
 144 int
 145 __ulckpwdf (void)
 146 {
 147   int result;
 148
 149   if (lock_fd == -1)
 150     /* There is no lock set.  */
 151     result = -1;
 152   else
 153     {
 154       /* Prevent problems caused by multiple threads.  */
 155       __libc_lock_lock (lock);
 156
 157       result = __close (lock_fd);
 158
 159       /* Mark descriptor as unused.  */
 160       lock_fd = -1;
 161
 162       /* Clear mutex.  */
 163       __libc_lock_unlock (lock);
 164     }
 165
 166   return result;
 167 }
 168 weak_alias (__ulckpwdf, ulckpwdf)
 169
 170
 171 static void
 172 noop_handler (int sig)
 173 {
 174   /* We simply return which makes the `fcntl' call return with an error.  */
 175 }