lib/fchdir.c

   1 /* fchdir replacement.
   2    Copyright (C) 2006-2018 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 "dosname.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 static char *
  89 get_name (char const *dir)
  90 {
  91   char *cwd;
  92   char *result;
  93   int saved_errno;
  94
  95   if (IS_ABSOLUTE_FILE_NAME (dir))
  96     return strdup (dir);
  97
  98   /* We often encounter "."; treat it as a special case.  */
  99   cwd = getcwd (NULL, 0);
 100   if (!cwd || (dir[0] == '.' && dir[1] == '\0'))
 101     return cwd;
 102
 103   result = mfile_name_concat (cwd, dir, NULL);
 104   saved_errno = errno;
 105   free (cwd);
 106   errno = saved_errno;
 107   return result;
 108 }
 109
 110 /* Hook into the gnulib replacements for open() and close() to keep track
 111    of the open file descriptors.  */
 112
 113 /* Close FD, cleaning up any fd to name mapping if fd was visiting a
 114    directory.  */
 115 void
 116 _gl_unregister_fd (int fd)
 117 {
 118   if (fd >= 0 && fd < dirs_allocated)
 119     {
 120       free (dirs[fd].name);
 121       dirs[fd].name = NULL;
 122     }
 123 }
 124
 125 /* Mark FD as visiting FILENAME.  FD must be non-negative, and refer
 126    to an open file descriptor.  If REPLACE_OPEN_DIRECTORY is non-zero,
 127    this should only be called if FD is visiting a directory.  Close FD
 128    and return -1 if there is insufficient memory to track the
 129    directory name; otherwise return FD.  */
 130 int
 131 _gl_register_fd (int fd, const char *filename)
 132 {
 133   struct stat statbuf;
 134
 135   assure (0 <= fd);
 136   if (REPLACE_OPEN_DIRECTORY
 137       || (fstat (fd, &statbuf) == 0 && S_ISDIR (statbuf.st_mode)))
 138     {
 139       if (!ensure_dirs_slot (fd)
 140           || (dirs[fd].name = get_name (filename)) == NULL)
 141         {
 142           int saved_errno = errno;
 143           close (fd);
 144           errno = saved_errno;
 145           return -1;
 146         }
 147     }
 148   return fd;
 149 }
 150
 151 /* Mark NEWFD as a duplicate of OLDFD; useful from dup, dup2, dup3,
 152    and fcntl.  Both arguments must be valid and distinct file
 153    descriptors.  Close NEWFD and return -1 if OLDFD is tracking a
 154    directory, but there is insufficient memory to track the same
 155    directory in NEWFD; otherwise return NEWFD.  */
 156 int
 157 _gl_register_dup (int oldfd, int newfd)
 158 {
 159   assure (0 <= oldfd && 0 <= newfd && oldfd != newfd);
 160   if (oldfd < dirs_allocated && dirs[oldfd].name)
 161     {
 162       /* Duplicated a directory; must ensure newfd is allocated.  */
 163       if (!ensure_dirs_slot (newfd)
 164           || (dirs[newfd].name = strdup (dirs[oldfd].name)) == NULL)
 165         {
 166           int saved_errno = errno;
 167           close (newfd);
 168           errno = saved_errno;
 169           newfd = -1;
 170         }
 171     }
 172   else if (newfd < dirs_allocated)
 173     {
 174       /* Duplicated a non-directory; ensure newfd is cleared.  */
 175       free (dirs[newfd].name);
 176       dirs[newfd].name = NULL;
 177     }
 178   return newfd;
 179 }
 180
 181 /* If FD is currently visiting a directory, then return the name of
 182    that directory.  Otherwise, return NULL and set errno.  */
 183 const char *
 184 _gl_directory_name (int fd)
 185 {
 186   if (0 <= fd && fd < dirs_allocated && dirs[fd].name != NULL)
 187     return dirs[fd].name;
 188   /* At this point, fd is either invalid, or open but not a directory.
 189      If dup2 fails, errno is correctly EBADF.  */
 190   if (0 <= fd)
 191     {
 192       if (dup2 (fd, fd) == fd)
 193         errno = ENOTDIR;
 194     }
 195   else
 196     errno = EBADF;
 197   return NULL;
 198 }
 199
 200
 201 /* Implement fchdir() in terms of chdir().  */
 202
 203 int
 204 fchdir (int fd)
 205 {
 206   const char *name = _gl_directory_name (fd);
 207   return name ? chdir (name) : -1;
 208 }