lib/lchmod.c

   1 /* Implement lchmod on platforms where it does not work correctly.
   2
   3    Copyright 2020-2025 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation, either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* written by Paul Eggert */
  19
  20 #include <config.h>
  21
  22 /* Specification.  */
  23 #include <sys/stat.h>
  24
  25 #include <errno.h>
  26 #include <fcntl.h>
  27 #include <stdio.h>
  28 #include <string.h>
  29 #include <unistd.h>
  30
  31 #include <intprops.h>
  32
  33 /* Work like chmod, except when FILE is a symbolic link.
  34    In that case, on systems where permissions on symbolic links are unsupported
  35    (such as Linux), set errno to EOPNOTSUPP and return -1.  */
  36
  37 int
  38 lchmod (char const *file, mode_t mode)
  39 {
  40   char readlink_buf[1];
  41
  42 #ifdef O_PATH
  43   /* Open a file descriptor with O_NOFOLLOW, to make sure we don't
  44      follow symbolic links, if /proc is mounted.  O_PATH is used to
  45      avoid a failure if the file is not readable.
  46      Cf. <https://sourceware.org/bugzilla/show_bug.cgi?id=14578>  */
  47   int fd = open (file, O_PATH | O_NOFOLLOW | O_CLOEXEC);
  48   if (fd < 0)
  49     return fd;
  50
  51   int err;
  52   if (0 <= readlinkat (fd, "", readlink_buf, sizeof readlink_buf))
  53     err = EOPNOTSUPP;
  54   else if (errno == EINVAL)
  55     {
  56       static char const fmt[] = "/proc/self/fd/%d";
  57       char buf[sizeof fmt - sizeof "%d" + INT_BUFSIZE_BOUND (int)];
  58       sprintf (buf, fmt, fd);
  59       err = chmod (buf, mode) == 0 ? 0 : errno == ENOENT ? -1 : errno;
  60     }
  61   else
  62     err = errno == ENOENT ? -1 : errno;
  63
  64   close (fd);
  65
  66   errno = err;
  67   if (0 <= err)
  68     return err == 0 ? 0 : -1;
  69 #endif
  70
  71   size_t len = strlen (file);
  72   if (len && file[len - 1] == '/')
  73     {
  74       struct stat st;
  75       if (lstat (file, &st) < 0)
  76         return -1;
  77       if (!S_ISDIR (st.st_mode))
  78         {
  79           errno = ENOTDIR;
  80           return -1;
  81         }
  82     }
  83
  84   /* O_PATH + /proc is not supported.  */
  85
  86   if (0 <= readlink (file, readlink_buf, sizeof readlink_buf))
  87     {
  88       errno = EOPNOTSUPP;
  89       return -1;
  90     }
  91
  92   /* Fall back on chmod, despite a possible race.  */
  93   return chmod (file, mode);
  94 }