lib/savewd.c

   1 /* Save and restore the working directory, possibly using a child process.
   2
   3    Copyright (C) 2006-2007, 2009-2019 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program 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
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert.  */
  19
  20 #include <config.h>
  21
  22 #define SAVEWD_INLINE _GL_EXTERN_INLINE
  23
  24 #include "savewd.h"
  25
  26 #include <errno.h>
  27 #include <fcntl.h>
  28 #include <signal.h>
  29 #include <stdbool.h>
  30 #include <stdlib.h>
  31 #include <sys/types.h>
  32 #include <sys/wait.h>
  33 #include <unistd.h>
  34
  35 #include "assure.h"
  36 #include "dosname.h"
  37 #include "fcntl-safer.h"
  38
  39 #ifndef FALLTHROUGH
  40 # if __GNUC__ < 7
  41 #  define FALLTHROUGH ((void) 0)
  42 # else
  43 #  define FALLTHROUGH __attribute__ ((__fallthrough__))
  44 # endif
  45 #endif
  46
  47 /* Save the working directory into *WD, if it hasn't been saved
  48    already.  Return true if a child has been forked to do the real
  49    work.  */
  50 static bool
  51 savewd_save (struct savewd *wd)
  52 {
  53   switch (wd->state)
  54     {
  55     case INITIAL_STATE:
  56       /* Save the working directory, or prepare to fall back if possible.  */
  57       {
  58         int fd = open_safer (".", O_SEARCH);
  59         if (0 <= fd)
  60           {
  61             wd->state = FD_STATE;
  62             wd->val.fd = fd;
  63             break;
  64           }
  65         if (errno != EACCES && errno != ESTALE)
  66           {
  67             wd->state = ERROR_STATE;
  68             wd->val.errnum = errno;
  69             break;
  70           }
  71       }
  72       wd->state = FORKING_STATE;
  73       wd->val.child = -1;
  74       FALLTHROUGH;
  75     case FORKING_STATE:
  76       if (wd->val.child < 0)
  77         {
  78           /* "Save" the initial working directory by forking a new
  79              subprocess that will attempt all the work from the chdir
  80              until the next savewd_restore.  */
  81           wd->val.child = fork ();
  82           if (wd->val.child != 0)
  83             {
  84               if (0 < wd->val.child)
  85                 return true;
  86               wd->state = ERROR_STATE;
  87               wd->val.errnum = errno;
  88             }
  89         }
  90       break;
  91
  92     case FD_STATE:
  93     case FD_POST_CHDIR_STATE:
  94     case ERROR_STATE:
  95     case FINAL_STATE:
  96       break;
  97
  98     default:
  99       assure (false);
 100     }
 101
 102   return false;
 103 }
 104
 105 int
 106 savewd_chdir (struct savewd *wd, char const *dir, int options,
 107               int open_result[2])
 108 {
 109   int fd = -1;
 110   int result = 0;
 111
 112   /* Open the directory if requested, or if avoiding a race condition
 113      is requested and possible.  */
 114   if (open_result
 115       || (options & (HAVE_WORKING_O_NOFOLLOW ? SAVEWD_CHDIR_NOFOLLOW : 0)))
 116     {
 117       fd = open (dir,
 118                  (O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK
 119                   | (options & SAVEWD_CHDIR_NOFOLLOW ? O_NOFOLLOW : 0)));
 120
 121       if (open_result)
 122         {
 123           open_result[0] = fd;
 124           open_result[1] = errno;
 125         }
 126
 127       if (fd < 0 && errno != EACCES)
 128         result = -1;
 129     }
 130
 131   if (result == 0 && ! (0 <= fd && options & SAVEWD_CHDIR_SKIP_READABLE))
 132     {
 133       if (savewd_save (wd))
 134         {
 135           open_result = NULL;
 136           result = -2;
 137         }
 138       else
 139         {
 140           result = (fd < 0 ? chdir (dir) : fchdir (fd));
 141
 142           if (result == 0)
 143             switch (wd->state)
 144               {
 145               case FD_STATE:
 146                 wd->state = FD_POST_CHDIR_STATE;
 147                 break;
 148
 149               case ERROR_STATE:
 150               case FD_POST_CHDIR_STATE:
 151               case FINAL_STATE:
 152                 break;
 153
 154               case FORKING_STATE:
 155                 assure (wd->val.child == 0);
 156                 break;
 157
 158               default:
 159                 assure (false);
 160               }
 161         }
 162     }
 163
 164   if (0 <= fd && ! open_result)
 165     {
 166       int e = errno;
 167       close (fd);
 168       errno = e;
 169     }
 170
 171   return result;
 172 }
 173
 174 int
 175 savewd_restore (struct savewd *wd, int status)
 176 {
 177   switch (wd->state)
 178     {
 179     case INITIAL_STATE:
 180     case FD_STATE:
 181       /* The working directory is the desired directory, so there's no
 182          work to do.  */
 183       break;
 184
 185     case FD_POST_CHDIR_STATE:
 186       /* Restore the working directory using fchdir.  */
 187       if (fchdir (wd->val.fd) == 0)
 188         {
 189           wd->state = FD_STATE;
 190           break;
 191         }
 192       else
 193         {
 194           int chdir_errno = errno;
 195           close (wd->val.fd);
 196           wd->state = ERROR_STATE;
 197           wd->val.errnum = chdir_errno;
 198         }
 199       FALLTHROUGH;
 200     case ERROR_STATE:
 201       /* Report an error if asked to restore the working directory.  */
 202       errno = wd->val.errnum;
 203       return -1;
 204
 205     case FORKING_STATE:
 206       /* "Restore" the working directory by waiting for the subprocess
 207          to finish.  */
 208       {
 209         pid_t child = wd->val.child;
 210         if (child == 0)
 211           _exit (status);
 212         if (0 < child)
 213           {
 214             int child_status;
 215             while (waitpid (child, &child_status, 0) < 0)
 216               assure (errno == EINTR);
 217             wd->val.child = -1;
 218             if (! WIFEXITED (child_status))
 219               raise (WTERMSIG (child_status));
 220             return WEXITSTATUS (child_status);
 221           }
 222       }
 223       break;
 224
 225     default:
 226       assure (false);
 227     }
 228
 229   return 0;
 230 }
 231
 232 void
 233 savewd_finish (struct savewd *wd)
 234 {
 235   switch (wd->state)
 236     {
 237     case INITIAL_STATE:
 238     case ERROR_STATE:
 239       break;
 240
 241     case FD_STATE:
 242     case FD_POST_CHDIR_STATE:
 243       close (wd->val.fd);
 244       break;
 245
 246     case FORKING_STATE:
 247       assure (wd->val.child < 0);
 248       break;
 249
 250     default:
 251       assure (false);
 252     }
 253
 254   wd->state = FINAL_STATE;
 255 }
 256
 257 /* Return true if the actual work is currently being done by a
 258    subprocess.
 259
 260    A true return means that the caller and the subprocess should
 261    resynchronize later with savewd_restore, using only their own
 262    memory to decide when to resynchronize; they should not consult the
 263    file system to decide, because that might lead to race conditions.
 264    This is why savewd_chdir is broken out into another function;
 265    savewd_chdir's callers _can_ inspect the file system to decide
 266    whether to call savewd_chdir.  */
 267 static bool
 268 savewd_delegating (struct savewd const *wd)
 269 {
 270   return wd->state == FORKING_STATE && 0 < wd->val.child;
 271 }
 272
 273 int
 274 savewd_process_files (int n_files, char **file,
 275                       int (*act) (char *, struct savewd *, void *),
 276                       void *options)
 277 {
 278   int i = 0;
 279   int last_relative;
 280   int exit_status = EXIT_SUCCESS;
 281   struct savewd wd;
 282   savewd_init (&wd);
 283
 284   for (last_relative = n_files - 1; 0 <= last_relative; last_relative--)
 285     if (! IS_ABSOLUTE_FILE_NAME (file[last_relative]))
 286       break;
 287
 288   for (; i < last_relative; i++)
 289     {
 290       if (! savewd_delegating (&wd))
 291         {
 292           int s = act (file[i], &wd, options);
 293           if (exit_status < s)
 294             exit_status = s;
 295         }
 296
 297       if (! IS_ABSOLUTE_FILE_NAME (file[i + 1]))
 298         {
 299           int r = savewd_restore (&wd, exit_status);
 300           if (exit_status < r)
 301             exit_status = r;
 302         }
 303     }
 304
 305   savewd_finish (&wd);
 306
 307   for (; i < n_files; i++)
 308     {
 309       int s = act (file[i], &wd, options);
 310       if (exit_status < s)
 311         exit_status = s;
 312     }
 313
 314   return exit_status;
 315 }