shadow/lckpwdf.c

   1 /* Handle locking of password file.
   2    Copyright (C) 1996-2017 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Contributed by Ulrich Drepper <drepper@cygnus.com>, 1996.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <http://www.gnu.org/licenses/>.  */
  19
  20 #include <fcntl.h>
  21 #include <libc-lock.h>
  22 #include <shadow.h>
  23 #include <signal.h>
  24 #include <string.h>
  25 #include <unistd.h>
  26 #include <sys/file.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;
 100 #ifdef O_CLOEXEC
 101   oflags |= O_CLOEXEC;
 102 #endif
 103   lock_fd = __open (PWD_LOCKFILE, oflags, 0600);
 104   if (lock_fd == -1)
 105     /* Cannot create lock file.  */
 106     RETURN_CLOSE_FD (-1);
 107
 108 #ifndef __ASSUME_O_CLOEXEC
 109 # ifdef O_CLOEXEC
 110   if (__have_o_cloexec <= 0)
 111 # endif
 112     {
 113       /* Make sure file gets correctly closed when process finished.  */
 114       int flags = __fcntl (lock_fd, F_GETFD, 0);
 115       if (flags == -1)
 116         /* Cannot get file flags.  */
 117         RETURN_CLOSE_FD (-1);
 118 # ifdef O_CLOEXEC
 119       if (__have_o_cloexec == 0)
 120         __have_o_cloexec = (flags & FD_CLOEXEC) == 0 ? -1 : 1;
 121       if (__have_o_cloexec < 0)
 122 # endif
 123         {
 124           flags |= FD_CLOEXEC;          /* Close on exit.  */
 125           if (__fcntl (lock_fd, F_SETFD, flags) < 0)
 126             /* Cannot set new flags.  */
 127             RETURN_CLOSE_FD (-1);
 128         }
 129     }
 130 #endif
 131
 132   /* Now we have to get exclusive write access.  Since multiple
 133      process could try this we won't stop when it first fails.
 134      Instead we set a timeout for the system call.  Once the timer
 135      expires it is likely that there are some problems which cannot be
 136      resolved by waiting.
 137
 138      It is important that we don't change the signal state.  We must
 139      restore the old signal behaviour.  */
 140   memset (&new_act, '\0', sizeof (struct sigaction));
 141   new_act.sa_handler = noop_handler;
 142   __sigfillset (&new_act.sa_mask);
 143   new_act.sa_flags = 0ul;
 144
 145   /* Install new action handler for alarm and save old.  */
 146   if (__sigaction (SIGALRM, &new_act, &saved_act) < 0)
 147     /* Cannot install signal handler.  */
 148     RETURN_CLOSE_FD (-1);
 149
 150   /* Now make sure the alarm signal is not blocked.  */
 151   __sigemptyset (&new_set);
 152   __sigaddset (&new_set, SIGALRM);
 153   if (__sigprocmask (SIG_UNBLOCK, &new_set, &saved_set) < 0)
 154     RETURN_RESTORE_HANDLER (-1);
 155
 156   /* Start timer.  If we cannot get the lock in the specified time we
 157      get a signal.  */
 158   alarm (TIMEOUT);
 159
 160   /* Try to get the lock.  */
 161   memset (&fl, '\0', sizeof (struct flock));
 162   fl.l_type = F_WRLCK;
 163   fl.l_whence = SEEK_SET;
 164   result = __fcntl (lock_fd, F_SETLKW, &fl);
 165
 166   RETURN_CLEAR_ALARM (result);
 167 }
 168 weak_alias (__lckpwdf, lckpwdf)
 169
 170
 171 int
 172 __ulckpwdf (void)
 173 {
 174   int result;
 175
 176   if (lock_fd == -1)
 177     /* There is no lock set.  */
 178     result = -1;
 179   else
 180     {
 181       /* Prevent problems caused by multiple threads.  */
 182       __libc_lock_lock (lock);
 183
 184       result = __close (lock_fd);
 185
 186       /* Mark descriptor as unused.  */
 187       lock_fd = -1;
 188
 189       /* Clear mutex.  */
 190       __libc_lock_unlock (lock);
 191     }
 192
 193   return result;
 194 }
 195 weak_alias (__ulckpwdf, ulckpwdf)
 196
 197
 198 static void
 199 noop_handler (int sig)
 200 {
 201   /* We simply return which makes the `fcntl' call return with an error.  */
 202 }