lib/lchmod.c

   1 /* Implement lchmod on platforms where it does not work correctly.
   2
   3    Copyright 2020 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 <unistd.h>
  29
  30 #ifdef __osf__
  31 /* Write "sys/stat.h" here, not <sys/stat.h>, otherwise OSF/1 5.1 DTK cc
  32    eliminates this include because of the preliminary #include <sys/stat.h>
  33    above.  */
  34 # include "sys/stat.h"
  35 #else
  36 # include <sys/stat.h>
  37 #endif
  38
  39 #include <intprops.h>
  40
  41 /* Work like chmod, except when FILE is a symbolic link.
  42    In that case, on systems where permissions on symbolic links are unsupported
  43    (such as Linux), set errno to EOPNOTSUPP and return -1.  */
  44
  45 int
  46 lchmod (char const *file, mode_t mode)
  47 {
  48 #if defined O_PATH && defined AT_EMPTY_PATH
  49   /* Open a file descriptor with O_NOFOLLOW, to make sure we don't
  50      follow symbolic links, if /proc is mounted.  O_PATH is used to
  51      avoid a failure if the file is not readable.
  52      Cf. <https://sourceware.org/bugzilla/show_bug.cgi?id=14578>  */
  53   int fd = open (file, O_PATH | O_NOFOLLOW | O_CLOEXEC);
  54   if (fd < 0)
  55     return fd;
  56
  57   /* Up to Linux 5.3 at least, when FILE refers to a symbolic link, the
  58      chmod call below will change the permissions of the symbolic link
  59      - which is undesired - and on many file systems (ext4, btrfs, jfs,
  60      xfs, ..., but not reiserfs) fail with error EOPNOTSUPP - which is
  61      misleading.  Therefore test for a symbolic link explicitly.
  62      Use fstatat because fstat does not work on O_PATH descriptors
  63      before Linux 3.6.  */
  64   struct stat st;
  65   if (fstatat (fd, "", &st, AT_EMPTY_PATH) != 0)
  66     {
  67       int stat_errno = errno;
  68       close (fd);
  69       errno = stat_errno;
  70       return -1;
  71     }
  72   if (S_ISLNK (st.st_mode))
  73     {
  74       close (fd);
  75       errno = EOPNOTSUPP;
  76       return -1;
  77     }
  78
  79 # if defined __linux__ || defined __ANDROID__ || defined __CYGWIN__
  80   static char const fmt[] = "/proc/self/fd/%d";
  81   char buf[sizeof fmt - sizeof "%d" + INT_BUFSIZE_BOUND (int)];
  82   sprintf (buf, fmt, fd);
  83   int chmod_result = chmod (buf, mode);
  84   int chmod_errno = errno;
  85   close (fd);
  86   if (chmod_result == 0)
  87     return chmod_result;
  88   if (chmod_errno != ENOENT)
  89     {
  90       errno = chmod_errno;
  91       return chmod_result;
  92     }
  93 # endif
  94   /* /proc is not mounted or would not work as in GNU/Linux.  */
  95
  96 #elif HAVE_LSTAT
  97   struct stat st;
  98   int lstat_result = lstat (file, &st);
  99   if (lstat_result != 0)
 100     return lstat_result;
 101   if (S_ISLNK (st.st_mode))
 102     {
 103       errno = EOPNOTSUPP;
 104       return -1;
 105     }
 106 #endif
 107
 108   /* Fall back on chmod, despite a possible race.  */
 109   return chmod (file, mode);
 110 }