login/utmp_file.c

   1 /* Copyright (C) 1996-2024 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <assert.h>
  19 #include <errno.h>
  20 #include <fcntl.h>
  21 #include <signal.h>
  22 #include <stdbool.h>
  23 #include <stdio.h>
  24 #include <string.h>
  25 #include <unistd.h>
  26 #include <utmp.h>
  27 #include <not-cancel.h>
  28 #include <kernel-features.h>
  29 #include <sigsetops.h>
  30 #include <not-cancel.h>
  31
  32 #include "utmp-private.h"
  33 #include "utmp-equal.h"
  34
  35
  36 /* Descriptor for the file and position.  */
  37 static int file_fd = -1;
  38 static bool file_writable;
  39 static off64_t file_offset;
  40
  41 /* Cache for the last read entry.  */
  42 static struct utmp last_entry;
  43
  44 /* Returns true if *ENTRY matches last_entry, based on
  45    data->ut_type.  */
  46 static bool
  47 matches_last_entry (const struct utmp *data)
  48 {
  49   if (file_offset <= 0)
  50     /* Nothing has been read.  last_entry is stale and cannot match.  */
  51     return false;
  52
  53   if (data->ut_type == RUN_LVL
  54       || data->ut_type == BOOT_TIME
  55       || data->ut_type == OLD_TIME
  56       || data->ut_type == NEW_TIME)
  57     /* For some entry types, only a type match is required.  */
  58     return data->ut_type == last_entry.ut_type;
  59   else
  60     /* For the process-related entries, a full match is needed.  */
  61     return __utmp_equal (&last_entry, data);
  62 }
  63
  64 /* Locking timeout.  */
  65 #ifndef TIMEOUT
  66 # define TIMEOUT 10
  67 #endif
  68
  69 /* Do-nothing handler for locking timeout.  */
  70 static void timeout_handler (int signum) {};
  71
  72
  73 /* try_file_lock (LOCKING, FD, TYPE) returns true if the locking
  74    operation failed and recovery needs to be performed.
  75
  76    file_unlock (FD) removes the lock (which must have been
  77    successfully acquired). */
  78
  79 static bool
  80 try_file_lock (int fd, int type)
  81 {
  82   /* Cancel any existing alarm.  */
  83   int old_timeout = alarm (0);
  84
  85   /* Establish signal handler.  */
  86   struct sigaction old_action;
  87   struct sigaction action;
  88   action.sa_handler = timeout_handler;
  89   __sigemptyset (&action.sa_mask);
  90   action.sa_flags = 0;
  91   __sigaction (SIGALRM, &action, &old_action);
  92
  93   alarm (TIMEOUT);
  94
  95   /* Try to get the lock.  */
  96  struct flock64 fl =
  97    {
  98     .l_type = type,
  99     .l_whence = SEEK_SET,
 100    };
 101
 102  bool status = __fcntl64_nocancel (fd, F_SETLKW, &fl) < 0;
 103  int saved_errno = errno;
 104
 105  /* Reset the signal handler and alarm.  We must reset the alarm
 106     before resetting the handler so our alarm does not generate a
 107     spurious SIGALRM seen by the user.  However, we cannot just set
 108     the user's old alarm before restoring the handler, because then
 109     it's possible our handler could catch the user alarm's SIGARLM and
 110     then the user would never see the signal he expected.  */
 111   alarm (0);
 112   __sigaction (SIGALRM, &old_action, NULL);
 113   if (old_timeout != 0)
 114     alarm (old_timeout);
 115
 116   __set_errno (saved_errno);
 117   return status;
 118 }
 119
 120 static void
 121 file_unlock (int fd)
 122 {
 123   struct flock64 fl =
 124     {
 125       .l_type = F_UNLCK,
 126     };
 127   __fcntl64_nocancel (fd, F_SETLKW, &fl);
 128 }
 129
 130 #ifndef TRANSFORM_UTMP_FILE_NAME
 131 # define TRANSFORM_UTMP_FILE_NAME(file_name) (file_name)
 132 #endif
 133
 134 int
 135 __libc_setutent (void)
 136 {
 137   if (file_fd < 0)
 138     {
 139       const char *file_name;
 140
 141       file_name = TRANSFORM_UTMP_FILE_NAME (__libc_utmp_file_name);
 142
 143       file_writable = false;
 144       file_fd = __open_nocancel
 145         (file_name, O_RDONLY | O_LARGEFILE | O_CLOEXEC);
 146       if (file_fd == -1)
 147         return 0;
 148     }
 149
 150   __lseek64 (file_fd, 0, SEEK_SET);
 151   file_offset = 0;
 152
 153   return 1;
 154 }
 155
 156 /* Perform initialization if necessary.  */
 157 static bool
 158 maybe_setutent (void)
 159 {
 160   return file_fd >= 0 || __libc_setutent ();
 161 }
 162
 163 /* Reads the entry at file_offset, storing it in last_entry and
 164    updating file_offset on success.  Returns -1 for a read error, 0
 165    for EOF, and 1 for a successful read.  last_entry and file_offset
 166    are only updated on a successful and complete read.  */
 167 static ssize_t
 168 read_last_entry (void)
 169 {
 170   struct utmp buffer;
 171   ssize_t nbytes = __pread64_nocancel (file_fd, &buffer, sizeof (buffer),
 172                                        file_offset);
 173   if (nbytes < 0)
 174     return -1;
 175   else if (nbytes != sizeof (buffer))
 176     /* Assume EOF.  */
 177     return 0;
 178   else
 179     {
 180       last_entry = buffer;
 181       file_offset += sizeof (buffer);
 182       return 1;
 183     }
 184 }
 185
 186 int
 187 __libc_getutent_r (struct utmp *buffer, struct utmp **result)
 188 {
 189   int saved_errno = errno;
 190
 191   if (!maybe_setutent ())
 192     {
 193       /* Not available.  */
 194       *result = NULL;
 195       return -1;
 196     }
 197
 198   if (try_file_lock (file_fd, F_RDLCK))
 199     return -1;
 200
 201   ssize_t nbytes = read_last_entry ();
 202   file_unlock (file_fd);
 203
 204   if (nbytes <= 0)              /* Read error or EOF.  */
 205     {
 206       if (nbytes == 0)
 207         /* errno should be unchanged to indicate success.  A premature
 208            EOF is treated like an EOF (missing complete record at the
 209            end).  */
 210         __set_errno (saved_errno);
 211       *result = NULL;
 212       return -1;
 213     }
 214
 215   memcpy (buffer, &last_entry, sizeof (struct utmp));
 216   *result = buffer;
 217
 218   return 0;
 219 }
 220
 221
 222 /* Search for *ID, updating last_entry and file_offset.  Return 0 on
 223    success and -1 on failure.  Does not perform locking; for that see
 224    internal_getut_r below.  */
 225 static int
 226 internal_getut_nolock (const struct utmp *id)
 227 {
 228   while (1)
 229     {
 230       ssize_t nbytes = read_last_entry ();
 231       if (nbytes < 0)
 232         return -1;
 233       if (nbytes == 0)
 234         {
 235           /* End of file reached.  */
 236           __set_errno (ESRCH);
 237           return -1;
 238         }
 239
 240       if (matches_last_entry (id))
 241         break;
 242     }
 243
 244   return 0;
 245 }
 246
 247 /* Search for *ID, updating last_entry and file_offset.  Return 0 on
 248    success and -1 on failure.  If the locking operation failed, write
 249    true to *LOCK_FAILED.  */
 250 static int
 251 internal_getut_r (const struct utmp *id, bool *lock_failed)
 252 {
 253   if (try_file_lock (file_fd, F_RDLCK))
 254     {
 255       *lock_failed = true;
 256       return -1;
 257     }
 258
 259   int result = internal_getut_nolock (id);
 260   file_unlock (file_fd);
 261   return result;
 262 }
 263
 264 /* For implementing this function we don't use the getutent_r function
 265    because we can avoid the reposition on every new entry this way.  */
 266 int
 267 __libc_getutid_r (const struct utmp *id, struct utmp *buffer,
 268                   struct utmp **result)
 269 {
 270   if (!maybe_setutent ())
 271     {
 272       *result = NULL;
 273       return -1;
 274     }
 275
 276   /* We don't have to distinguish whether we can lock the file or
 277      whether there is no entry.  */
 278   bool lock_failed = false;
 279   if (internal_getut_r (id, &lock_failed) < 0)
 280     {
 281       *result = NULL;
 282       return -1;
 283     }
 284
 285   memcpy (buffer, &last_entry, sizeof (struct utmp));
 286   *result = buffer;
 287
 288   return 0;
 289 }
 290
 291 /* For implementing this function we don't use the getutent_r function
 292    because we can avoid the reposition on every new entry this way.  */
 293 int
 294 __libc_getutline_r (const struct utmp *line, struct utmp *buffer,
 295                     struct utmp **result)
 296 {
 297   if (!maybe_setutent ())
 298     {
 299       *result = NULL;
 300       return -1;
 301     }
 302
 303   if (try_file_lock (file_fd, F_RDLCK))
 304     {
 305       *result = NULL;
 306       return -1;
 307     }
 308
 309   while (1)
 310     {
 311       ssize_t nbytes = read_last_entry ();
 312       if (nbytes < 0)
 313         {
 314           file_unlock (file_fd);
 315           *result = NULL;
 316           return -1;
 317         }
 318       if (nbytes == 0)
 319         {
 320           /* End of file reached.  */
 321           file_unlock (file_fd);
 322           __set_errno (ESRCH);
 323           *result = NULL;
 324           return -1;
 325         }
 326
 327       /* Stop if we found a user or login entry.  */
 328       if ((last_entry.ut_type == USER_PROCESS
 329            || last_entry.ut_type == LOGIN_PROCESS)
 330           && (strncmp (line->ut_line, last_entry.ut_line, sizeof line->ut_line)
 331               == 0))
 332         break;
 333     }
 334
 335   file_unlock (file_fd);
 336   memcpy (buffer, &last_entry, sizeof (struct utmp));
 337   *result = buffer;
 338
 339   return 0;
 340 }
 341
 342
 343 struct utmp *
 344 __libc_pututline (const struct utmp *data)
 345 {
 346   if (!maybe_setutent ())
 347     return NULL;
 348
 349   struct utmp *pbuf;
 350
 351   if (! file_writable)
 352     {
 353       /* We must make the file descriptor writable before going on.  */
 354       const char *file_name = TRANSFORM_UTMP_FILE_NAME (__libc_utmp_file_name);
 355
 356       int new_fd = __open_nocancel
 357         (file_name, O_RDWR | O_LARGEFILE | O_CLOEXEC);
 358       if (new_fd == -1)
 359         return NULL;
 360
 361       if (__dup2 (new_fd, file_fd) < 0)
 362         {
 363           __close_nocancel_nostatus (new_fd);
 364           return NULL;
 365         }
 366       __close_nocancel_nostatus (new_fd);
 367       file_writable = true;
 368     }
 369
 370   /* Exclude other writers before validating the cache.  */
 371   if (try_file_lock (file_fd, F_WRLCK))
 372     return NULL;
 373
 374   /* Find the correct place to insert the data.  */
 375   bool found = false;
 376   if (matches_last_entry (data))
 377     {
 378       /* Read back the entry under the write lock.  */
 379       file_offset -= sizeof (last_entry);
 380       ssize_t nbytes = read_last_entry ();
 381       if (nbytes < 0)
 382         {
 383           file_unlock (file_fd);
 384           return NULL;
 385         }
 386
 387       if (nbytes == 0)
 388         /* End of file reached.  */
 389         found = false;
 390       else
 391         found = matches_last_entry (data);
 392     }
 393
 394   if (!found)
 395     /* Search forward for the entry.  */
 396     found = internal_getut_nolock (data) >= 0;
 397
 398   off64_t write_offset;
 399   if (!found)
 400     {
 401       /* We append the next entry.  */
 402       write_offset = __lseek64 (file_fd, 0, SEEK_END);
 403
 404       /* Round down to the next multiple of the entry size.  This
 405          ensures any partially-written record is overwritten by the
 406          new record.  */
 407       write_offset = (write_offset / sizeof (struct utmp)
 408                       * sizeof (struct utmp));
 409     }
 410   else
 411     /* Overwrite last_entry.  */
 412     write_offset = file_offset - sizeof (struct utmp);
 413
 414   /* Write the new data.  */
 415   ssize_t nbytes;
 416   if (__lseek64 (file_fd, write_offset, SEEK_SET) < 0
 417       || (nbytes = __write_nocancel (file_fd, data, sizeof (struct utmp))) < 0)
 418     {
 419       /* There is no need to recover the file position because all
 420          reads use pread64, and any future write is preceded by
 421          another seek.  */
 422       file_unlock (file_fd);
 423       return NULL;
 424     }
 425
 426   if (nbytes != sizeof (struct utmp))
 427     {
 428       /* If we appended a new record this is only partially written.
 429          Remove it.  */
 430       if (!found)
 431         (void) __ftruncate64 (file_fd, write_offset);
 432       file_unlock (file_fd);
 433       /* Assume that the write failure was due to missing disk
 434          space.  */
 435       __set_errno (ENOSPC);
 436       return NULL;
 437     }
 438
 439   file_unlock (file_fd);
 440   file_offset = write_offset + sizeof (struct utmp);
 441   pbuf = (struct utmp *) data;
 442
 443   return pbuf;
 444 }
 445
 446
 447 void
 448 __libc_endutent (void)
 449 {
 450   if (file_fd >= 0)
 451     {
 452       __close_nocancel_nostatus (file_fd);
 453       file_fd = -1;
 454     }
 455 }
 456
 457
 458 int
 459 __libc_updwtmp (const char *file, const struct utmp *utmp)
 460 {
 461   int result = -1;
 462   off64_t offset;
 463   int fd;
 464
 465   /* Open WTMP file.  */
 466   fd = __open_nocancel (file, O_WRONLY | O_LARGEFILE | O_CLOEXEC);
 467   if (fd < 0)
 468     return -1;
 469
 470   if (try_file_lock (fd, F_WRLCK))
 471     {
 472       __close_nocancel_nostatus (fd);
 473       return -1;
 474     }
 475
 476   /* Remember original size of log file.  */
 477   offset = __lseek64 (fd, 0, SEEK_END);
 478   if (offset % sizeof (struct utmp) != 0)
 479     {
 480       offset -= offset % sizeof (struct utmp);
 481       __ftruncate64 (fd, offset);
 482
 483       if (__lseek64 (fd, 0, SEEK_END) < 0)
 484         goto unlock_return;
 485     }
 486
 487   /* Write the entry.  If we can't write all the bytes, reset the file
 488      size back to the original size.  That way, no partial entries
 489      will remain.  */
 490   if (__write_nocancel (fd, utmp, sizeof (struct utmp))
 491       != sizeof (struct utmp))
 492     {
 493       __ftruncate64 (fd, offset);
 494       goto unlock_return;
 495     }
 496
 497   result = 0;
 498
 499 unlock_return:
 500   file_unlock (fd);
 501
 502   /* Close WTMP file.  */
 503   __close_nocancel_nostatus (fd);
 504
 505   return result;
 506 }