lib/fchdir.c

   1 /* fchdir replacement.
   2    Copyright (C) 2006-2025 Free Software Foundation, Inc.
   3
   4    This file is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU Lesser General Public License as
   6    published by the Free Software Foundation, either version 3 of the
   7    License, or (at your option) any later version.
   8
   9    This file 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 Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser 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 <stdlib.h>
  26 #include <string.h>
  27 #include <sys/types.h>
  28 #include <sys/stat.h>
  29
  30 #include "assure.h"
  31 #include "filename.h"
  32 #include "filenamecat.h"
  33
  34 #ifndef REPLACE_OPEN_DIRECTORY
  35 # define REPLACE_OPEN_DIRECTORY 0
  36 #endif
  37
  38 /* This replacement assumes that a directory is not renamed while opened
  39    through a file descriptor.
  40
  41    FIXME: On mingw, this would be possible to enforce if we were to
  42    also open a HANDLE to each directory currently visited by a file
  43    descriptor, since mingw refuses to rename any in-use file system
  44    object.  */
  45
  46 /* Array of file descriptors opened.  If REPLACE_OPEN_DIRECTORY or if it points
  47    to a directory, it stores info about this directory.  */
  48 typedef struct
  49 {
  50   char *name;       /* Absolute name of the directory, or NULL.  */
  51 } dir_info_t;
  52 static dir_info_t *dirs;
  53 static size_t dirs_allocated;
  54
  55 /* Try to ensure dirs has enough room for a slot at index fd; free any
  56    contents already in that slot.  Return false and set errno to
  57    ENOMEM on allocation failure.  */
  58 static bool
  59 ensure_dirs_slot (size_t fd)
  60 {
  61   if (fd < dirs_allocated)
  62     free (dirs[fd].name);
  63   else
  64     {
  65       size_t new_allocated;
  66       dir_info_t *new_dirs;
  67
  68       new_allocated = 2 * dirs_allocated + 1;
  69       if (new_allocated <= fd)
  70         new_allocated = fd + 1;
  71       new_dirs =
  72         (dirs != NULL
  73          ? (dir_info_t *) realloc (dirs, new_allocated * sizeof *dirs)
  74          : (dir_info_t *) malloc (new_allocated * sizeof *dirs));
  75       if (new_dirs == NULL)
  76         return false;
  77       memset (new_dirs + dirs_allocated, 0,
  78               (new_allocated - dirs_allocated) * sizeof *dirs);
  79       dirs = new_dirs;
  80       dirs_allocated = new_allocated;
  81     }
  82   return true;
  83 }
  84
  85 /* Return an absolute name of DIR in malloc'd storage.
  86    Upon failure, return NULL with errno set.  */
  87 static char *
  88 get_name (char const *dir)
  89 {
  90   char *cwd;
  91   char *result;
  92
  93   if (IS_ABSOLUTE_FILE_NAME (dir))
  94     return strdup (dir);
  95
  96   /* We often encounter "."; treat it as a special case.  */
  97   cwd = getcwd (NULL, 0);
  98   if (!cwd || (dir[0] == '.' && dir[1] == '\0'))
  99     return cwd;
 100
 101   result = mfile_name_concat (cwd, dir, NULL);
 102   free (cwd);
 103   return result;
 104 }
 105
 106 /* Hook into the gnulib replacements for open() and close() to keep track
 107    of the open file descriptors.  */
 108
 109 /* Close FD, cleaning up any fd to name mapping if fd was visiting a
 110    directory.  */
 111 void
 112 _gl_unregister_fd (int fd)
 113 {
 114   if (fd >= 0 && fd < dirs_allocated)
 115     {
 116       free (dirs[fd].name);
 117       dirs[fd].name = NULL;
 118     }
 119 }
 120
 121 /* Mark FD as visiting FILENAME.  FD must be non-negative, and refer
 122    to an open file descriptor.  If REPLACE_OPEN_DIRECTORY is non-zero,
 123    this should only be called if FD is visiting a directory.  Close FD
 124    and return -1 with errno set if there is insufficient memory to track
 125    the directory name; otherwise return FD.  */
 126 int
 127 _gl_register_fd (int fd, const char *filename)
 128 {
 129   struct stat statbuf;
 130
 131   assure (0 <= fd);
 132   if (REPLACE_OPEN_DIRECTORY
 133       || (fstat (fd, &statbuf) == 0 && S_ISDIR (statbuf.st_mode)))
 134     {
 135       if (!ensure_dirs_slot (fd)
 136           || (dirs[fd].name = get_name (filename)) == NULL)
 137         {
 138           int saved_errno = errno;
 139           close (fd);
 140           errno = saved_errno;
 141           return -1;
 142         }
 143     }
 144   return fd;
 145 }
 146
 147 /* Mark NEWFD as a duplicate of OLDFD; useful from dup, dup2, dup3,
 148    and fcntl.  Both arguments must be valid and distinct file
 149    descriptors.  Close NEWFD and return -1 if OLDFD is tracking a
 150    directory, but there is insufficient memory to track the same
 151    directory in NEWFD; otherwise return NEWFD.  */
 152 int
 153 _gl_register_dup (int oldfd, int newfd)
 154 {
 155   assure (0 <= oldfd && 0 <= newfd && oldfd != newfd);
 156   if (oldfd < dirs_allocated && dirs[oldfd].name)
 157     {
 158       /* Duplicated a directory; must ensure newfd is allocated.  */
 159       if (!ensure_dirs_slot (newfd)
 160           || (dirs[newfd].name = strdup (dirs[oldfd].name)) == NULL)
 161         {
 162           int saved_errno = errno;
 163           close (newfd);
 164           errno = saved_errno;
 165           newfd = -1;
 166         }
 167     }
 168   else if (newfd < dirs_allocated)
 169     {
 170       /* Duplicated a non-directory; ensure newfd is cleared.  */
 171       free (dirs[newfd].name);
 172       dirs[newfd].name = NULL;
 173     }
 174   return newfd;
 175 }
 176
 177 /* If FD is currently visiting a directory, then return the name of
 178    that directory.  Otherwise, return NULL and set errno.  */
 179 const char *
 180 _gl_directory_name (int fd)
 181 {
 182   if (0 <= fd && fd < dirs_allocated && dirs[fd].name != NULL)
 183     return dirs[fd].name;
 184   /* At this point, fd is either invalid, or open but not a directory.
 185      If dup2 fails, errno is correctly EBADF.  */
 186   if (0 <= fd)
 187     {
 188       if (dup2 (fd, fd) == fd)
 189         errno = ENOTDIR;
 190     }
 191   else
 192     errno = EBADF;
 193   return NULL;
 194 }
 195
 196
 197 /* Implement fchdir() in terms of chdir().  */
 198
 199 int
 200 fchdir (int fd)
 201 {
 202   const char *name = _gl_directory_name (fd);
 203   return name ? chdir (name) : -1;
 204 }