lib/fchdir.c

   1 /* fchdir replacement.
   2    Copyright (C) 2006-2021 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program 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
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 #include <config.h>
  18
  19 /* Specification.  */
  20 #include <unistd.h>
  21
  22 #include <dirent.h>
  23 #include <errno.h>
  24 #include <fcntl.h>
  25 #include <stdbool.h>
  26 #include <stdlib.h>
  27 #include <string.h>
  28 #include <sys/types.h>
  29 #include <sys/stat.h>
  30
  31 #include "assure.h"
  32 #include "filename.h"
  33 #include "filenamecat.h"
  34
  35 #ifndef REPLACE_OPEN_DIRECTORY
  36 # define REPLACE_OPEN_DIRECTORY 0
  37 #endif
  38
  39 /* This replacement assumes that a directory is not renamed while opened
  40    through a file descriptor.
  41
  42    FIXME: On mingw, this would be possible to enforce if we were to
  43    also open a HANDLE to each directory currently visited by a file
  44    descriptor, since mingw refuses to rename any in-use file system
  45    object.  */
  46
  47 /* Array of file descriptors opened.  If REPLACE_OPEN_DIRECTORY or if it points
  48    to a directory, it stores info about this directory.  */
  49 typedef struct
  50 {
  51   char *name;       /* Absolute name of the directory, or NULL.  */
  52   /* FIXME - add a DIR* member to make dirfd possible on mingw?  */
  53 } dir_info_t;
  54 static dir_info_t *dirs;
  55 static size_t dirs_allocated;
  56
  57 /* Try to ensure dirs has enough room for a slot at index fd; free any
  58    contents already in that slot.  Return false and set errno to
  59    ENOMEM on allocation failure.  */
  60 static bool
  61 ensure_dirs_slot (size_t fd)
  62 {
  63   if (fd < dirs_allocated)
  64     free (dirs[fd].name);
  65   else
  66     {
  67       size_t new_allocated;
  68       dir_info_t *new_dirs;
  69
  70       new_allocated = 2 * dirs_allocated + 1;
  71       if (new_allocated <= fd)
  72         new_allocated = fd + 1;
  73       new_dirs =
  74         (dirs != NULL
  75          ? (dir_info_t *) realloc (dirs, new_allocated * sizeof *dirs)
  76          : (dir_info_t *) malloc (new_allocated * sizeof *dirs));
  77       if (new_dirs == NULL)
  78         return false;
  79       memset (new_dirs + dirs_allocated, 0,
  80               (new_allocated - dirs_allocated) * sizeof *dirs);
  81       dirs = new_dirs;
  82       dirs_allocated = new_allocated;
  83     }
  84   return true;
  85 }
  86
  87 /* Return an absolute name of DIR in malloc'd storage.
  88    Upon failure, return NULL with errno set.  */
  89 static char *
  90 get_name (char const *dir)
  91 {
  92   char *cwd;
  93   char *result;
  94   int saved_errno;
  95
  96   if (IS_ABSOLUTE_FILE_NAME (dir))
  97     return strdup (dir);
  98
  99   /* We often encounter "."; treat it as a special case.  */
 100   cwd = getcwd (NULL, 0);
 101   if (!cwd || (dir[0] == '.' && dir[1] == '\0'))
 102     return cwd;
 103
 104   result = mfile_name_concat (cwd, dir, NULL);
 105   saved_errno = errno;
 106   free (cwd);
 107   errno = saved_errno;
 108   return result;
 109 }
 110
 111 /* Hook into the gnulib replacements for open() and close() to keep track
 112    of the open file descriptors.  */
 113
 114 /* Close FD, cleaning up any fd to name mapping if fd was visiting a
 115    directory.  */
 116 void
 117 _gl_unregister_fd (int fd)
 118 {
 119   if (fd >= 0 && fd < dirs_allocated)
 120     {
 121       free (dirs[fd].name);
 122       dirs[fd].name = NULL;
 123     }
 124 }
 125
 126 /* Mark FD as visiting FILENAME.  FD must be non-negative, and refer
 127    to an open file descriptor.  If REPLACE_OPEN_DIRECTORY is non-zero,
 128    this should only be called if FD is visiting a directory.  Close FD
 129    and return -1 with errno set if there is insufficient memory to track
 130    the directory name; otherwise return FD.  */
 131 int
 132 _gl_register_fd (int fd, const char *filename)
 133 {
 134   struct stat statbuf;
 135
 136   assure (0 <= fd);
 137   if (REPLACE_OPEN_DIRECTORY
 138       || (fstat (fd, &statbuf) == 0 && S_ISDIR (statbuf.st_mode)))
 139     {
 140       if (!ensure_dirs_slot (fd)
 141           || (dirs[fd].name = get_name (filename)) == NULL)
 142         {
 143           int saved_errno = errno;
 144           close (fd);
 145           errno = saved_errno;
 146           return -1;
 147         }
 148     }
 149   return fd;
 150 }
 151
 152 /* Mark NEWFD as a duplicate of OLDFD; useful from dup, dup2, dup3,
 153    and fcntl.  Both arguments must be valid and distinct file
 154    descriptors.  Close NEWFD and return -1 if OLDFD is tracking a
 155    directory, but there is insufficient memory to track the same
 156    directory in NEWFD; otherwise return NEWFD.  */
 157 int
 158 _gl_register_dup (int oldfd, int newfd)
 159 {
 160   assure (0 <= oldfd && 0 <= newfd && oldfd != newfd);
 161   if (oldfd < dirs_allocated && dirs[oldfd].name)
 162     {
 163       /* Duplicated a directory; must ensure newfd is allocated.  */
 164       if (!ensure_dirs_slot (newfd)
 165           || (dirs[newfd].name = strdup (dirs[oldfd].name)) == NULL)
 166         {
 167           int saved_errno = errno;
 168           close (newfd);
 169           errno = saved_errno;
 170           newfd = -1;
 171         }
 172     }
 173   else if (newfd < dirs_allocated)
 174     {
 175       /* Duplicated a non-directory; ensure newfd is cleared.  */
 176       free (dirs[newfd].name);
 177       dirs[newfd].name = NULL;
 178     }
 179   return newfd;
 180 }
 181
 182 /* If FD is currently visiting a directory, then return the name of
 183    that directory.  Otherwise, return NULL and set errno.  */
 184 const char *
 185 _gl_directory_name (int fd)
 186 {
 187   if (0 <= fd && fd < dirs_allocated && dirs[fd].name != NULL)
 188     return dirs[fd].name;
 189   /* At this point, fd is either invalid, or open but not a directory.
 190      If dup2 fails, errno is correctly EBADF.  */
 191   if (0 <= fd)
 192     {
 193       if (dup2 (fd, fd) == fd)
 194         errno = ENOTDIR;
 195     }
 196   else
 197     errno = EBADF;
 198   return NULL;
 199 }
 200
 201
 202 /* Implement fchdir() in terms of chdir().  */
 203
 204 int
 205 fchdir (int fd)
 206 {
 207   const char *name = _gl_directory_name (fd);
 208   return name ? chdir (name) : -1;
 209 }