lib/fchmodat.c

   1 /* Change the protections of file relative to an open directory.
   2    Copyright (C) 2006, 2009-2020 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 /* written by Jim Meyering and Paul Eggert */
  18
  19 /* If the user's config.h happens to include <sys/stat.h>, let it include only
  20    the system's <sys/stat.h> here, so that orig_fchmodat doesn't recurse to
  21    rpl_fchmodat.  */
  22 #define __need_system_sys_stat_h
  23 #include <config.h>
  24
  25 /* Specification.  */
  26 #include <sys/stat.h>
  27 #undef __need_system_sys_stat_h
  28
  29 #if HAVE_FCHMODAT
  30 static int
  31 orig_fchmodat (int dir, char const *file, mode_t mode, int flags)
  32 {
  33   return fchmodat (dir, file, mode, flags);
  34 }
  35 #endif
  36
  37 #include <errno.h>
  38 #include <fcntl.h>
  39 #include <stdio.h>
  40 #include <stdlib.h>
  41 #include <unistd.h>
  42
  43 #ifdef __osf__
  44 /* Write "sys/stat.h" here, not <sys/stat.h>, otherwise OSF/1 5.1 DTK cc
  45    eliminates this include because of the preliminary #include <sys/stat.h>
  46    above.  */
  47 # include "sys/stat.h"
  48 #else
  49 # include <sys/stat.h>
  50 #endif
  51
  52 #include <intprops.h>
  53
  54 /* Invoke chmod or lchmod on FILE, using mode MODE, in the directory
  55    open on descriptor FD.  If possible, do it without changing the
  56    working directory.  Otherwise, resort to using save_cwd/fchdir,
  57    then (chmod|lchmod)/restore_cwd.  If either the save_cwd or the
  58    restore_cwd fails, then give a diagnostic and exit nonzero.
  59    Note that an attempt to use a FLAG value of AT_SYMLINK_NOFOLLOW
  60    on a system without lchmod support causes this function to fail.  */
  61
  62 #if HAVE_FCHMODAT
  63 int
  64 fchmodat (int dir, char const *file, mode_t mode, int flags)
  65 {
  66 # if NEED_FCHMODAT_NONSYMLINK_FIX
  67   if (flags == AT_SYMLINK_NOFOLLOW)
  68     {
  69       struct stat st;
  70
  71 #  if defined O_PATH && defined AT_EMPTY_PATH
  72       /* Open a file descriptor with O_NOFOLLOW, to make sure we don't
  73          follow symbolic links, if /proc is mounted.  O_PATH is used to
  74          avoid a failure if the file is not readable.
  75          Cf. <https://sourceware.org/bugzilla/show_bug.cgi?id=14578>  */
  76       int fd = openat (dir, file, O_PATH | O_NOFOLLOW | O_CLOEXEC);
  77       if (fd < 0)
  78         return fd;
  79
  80       /* Up to Linux 5.3 at least, when FILE refers to a symbolic link, the
  81          chmod call below will change the permissions of the symbolic link
  82          - which is undesired - and on many file systems (ext4, btrfs, jfs,
  83          xfs, ..., but not reiserfs) fail with error EOPNOTSUPP - which is
  84          misleading.  Therefore test for a symbolic link explicitly.
  85          Use fstatat because fstat does not work on O_PATH descriptors
  86          before Linux 3.6.  */
  87       if (fstatat (fd, "", &st, AT_EMPTY_PATH) != 0)
  88         {
  89           int stat_errno = errno;
  90           close (fd);
  91           errno = stat_errno;
  92           return -1;
  93         }
  94       if (S_ISLNK (st.st_mode))
  95         {
  96           close (fd);
  97           errno = EOPNOTSUPP;
  98           return -1;
  99         }
 100
 101 #   if defined __linux__ || defined __ANDROID__ || defined __CYGWIN__
 102       static char const fmt[] = "/proc/self/fd/%d";
 103       char buf[sizeof fmt - sizeof "%d" + INT_BUFSIZE_BOUND (int)];
 104       sprintf (buf, fmt, fd);
 105       int chmod_result = chmod (buf, mode);
 106       int chmod_errno = errno;
 107       close (fd);
 108       if (chmod_result == 0)
 109         return chmod_result;
 110       if (chmod_errno != ENOENT)
 111         {
 112           errno = chmod_errno;
 113           return chmod_result;
 114         }
 115 #   endif
 116       /* /proc is not mounted or would not work as in GNU/Linux.  */
 117
 118 #  else
 119       int fstatat_result = fstatat (dir, file, &st, AT_SYMLINK_NOFOLLOW);
 120       if (fstatat_result != 0)
 121         return fstatat_result;
 122       if (S_ISLNK (st.st_mode))
 123         {
 124           errno = EOPNOTSUPP;
 125           return -1;
 126         }
 127 #  endif
 128
 129       /* Fall back on orig_fchmodat with no flags, despite a possible race.  */
 130       flags = 0;
 131     }
 132 # endif
 133
 134   return orig_fchmodat (dir, file, mode, flags);
 135 }
 136 #else
 137 # define AT_FUNC_NAME fchmodat
 138 # define AT_FUNC_F1 lchmod
 139 # define AT_FUNC_F2 chmod
 140 # define AT_FUNC_USE_F1_COND AT_SYMLINK_NOFOLLOW
 141 # define AT_FUNC_POST_FILE_PARAM_DECLS , mode_t mode, int flag
 142 # define AT_FUNC_POST_FILE_ARGS        , mode
 143 # include "at-func.c"
 144 #endif