lib/fchmodat.c

   1 /* Change the protections of file relative to an open directory.
   2    Copyright (C) 2006, 2009-2024 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 <string.h>
  42 #include <unistd.h>
  43
  44 #ifdef __osf__
  45 /* Write "sys/stat.h" here, not <sys/stat.h>, otherwise OSF/1 5.1 DTK cc
  46    eliminates this include because of the preliminary #include <sys/stat.h>
  47    above.  */
  48 # include "sys/stat.h"
  49 #else
  50 # include <sys/stat.h>
  51 #endif
  52
  53 #include <intprops.h>
  54
  55 /* Invoke chmod or lchmod on FILE, using mode MODE, in the directory
  56    open on descriptor FD.  If possible, do it without changing the
  57    working directory.  Otherwise, resort to using save_cwd/fchdir,
  58    then (chmod|lchmod)/restore_cwd.  If either the save_cwd or the
  59    restore_cwd fails, then give a diagnostic and exit nonzero.
  60    Note that an attempt to use a FLAG value of AT_SYMLINK_NOFOLLOW
  61    on a system without lchmod support causes this function to fail.  */
  62
  63 #if HAVE_FCHMODAT
  64 int
  65 fchmodat (int dir, char const *file, mode_t mode, int flags)
  66 {
  67 # if HAVE_NEARLY_WORKING_FCHMODAT
  68   /* Correct the trailing slash handling.  */
  69   size_t len = strlen (file);
  70   if (len && file[len - 1] == '/')
  71     {
  72       struct stat st;
  73       if (fstatat (dir, file, &st, flags & AT_SYMLINK_NOFOLLOW) < 0)
  74         return -1;
  75       if (!S_ISDIR (st.st_mode))
  76         {
  77           errno = ENOTDIR;
  78           return -1;
  79         }
  80     }
  81 # endif
  82
  83 # if NEED_FCHMODAT_NONSYMLINK_FIX
  84   if (flags == AT_SYMLINK_NOFOLLOW)
  85     {
  86 #  if HAVE_READLINKAT
  87       char readlink_buf[1];
  88
  89 #   ifdef O_PATH
  90       /* Open a file descriptor with O_NOFOLLOW, to make sure we don't
  91          follow symbolic links, if /proc is mounted.  O_PATH is used to
  92          avoid a failure if the file is not readable.
  93          Cf. <https://sourceware.org/bugzilla/show_bug.cgi?id=14578>  */
  94       int fd = openat (dir, file, O_PATH | O_NOFOLLOW | O_CLOEXEC);
  95       if (fd < 0)
  96         return fd;
  97
  98       int err;
  99       if (0 <= readlinkat (fd, "", readlink_buf, sizeof readlink_buf))
 100         err = EOPNOTSUPP;
 101       else if (errno == EINVAL)
 102         {
 103           static char const fmt[] = "/proc/self/fd/%d";
 104           char buf[sizeof fmt - sizeof "%d" + INT_BUFSIZE_BOUND (int)];
 105           sprintf (buf, fmt, fd);
 106           err = chmod (buf, mode) == 0 ? 0 : errno == ENOENT ? -1 : errno;
 107         }
 108       else
 109         err = errno == ENOENT ? -1 : errno;
 110
 111       close (fd);
 112
 113       errno = err;
 114       if (0 <= err)
 115         return err == 0 ? 0 : -1;
 116 #   endif
 117
 118       /* O_PATH + /proc is not supported.  */
 119
 120       if (0 <= readlinkat (dir, file, readlink_buf, sizeof readlink_buf))
 121         {
 122           errno = EOPNOTSUPP;
 123           return -1;
 124         }
 125 #  endif
 126
 127       /* Fall back on orig_fchmodat with no flags, despite a possible race.  */
 128       flags = 0;
 129     }
 130 # endif
 131
 132   return orig_fchmodat (dir, file, mode, flags);
 133 }
 134 #else
 135 # define AT_FUNC_NAME fchmodat
 136 # define AT_FUNC_F1 lchmod
 137 # define AT_FUNC_F2 chmod
 138 # define AT_FUNC_USE_F1_COND AT_SYMLINK_NOFOLLOW
 139 # define AT_FUNC_POST_FILE_PARAM_DECLS , mode_t mode, int flag
 140 # define AT_FUNC_POST_FILE_ARGS        , mode
 141 # include "at-func.c"
 142 #endif