sysdeps/unix/sysv/linux/fchmodat.c

   1 /* Change the protections of file relative to open directory.  Linux version.
   2    Copyright (C) 2006-2021 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library 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 GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <errno.h>
  20 #include <fcntl.h>
  21 #include <not-cancel.h>
  22 #include <stdio.h>
  23 #include <sys/stat.h>
  24 #include <sys/types.h>
  25 #include <sysdep.h>
  26 #include <unistd.h>
  27
  28 int
  29 fchmodat (int fd, const char *file, mode_t mode, int flag)
  30 {
  31   if (flag == 0)
  32     return INLINE_SYSCALL (fchmodat, 3, fd, file, mode);
  33   else if (flag != AT_SYMLINK_NOFOLLOW)
  34     return INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL);
  35   else
  36     {
  37       /* The kernel system call does not have a mode argument.
  38          However, we can create an O_PATH descriptor and change that
  39          via /proc (which does not resolve symbolic links).  */
  40
  41       int pathfd = __openat_nocancel (fd, file,
  42                                       O_PATH | O_NOFOLLOW | O_CLOEXEC);
  43       if (pathfd < 0)
  44         /* This may report errors such as ENFILE and EMFILE.  The
  45            caller can treat them as temporary if necessary.  */
  46         return pathfd;
  47
  48       /* Use fstatat because fstat does not work on O_PATH descriptors
  49          before Linux 3.6.  */
  50       struct stat64 st;
  51       if (__fstatat64 (pathfd, "", &st, AT_EMPTY_PATH) != 0)
  52         {
  53           __close_nocancel (pathfd);
  54           return -1;
  55         }
  56
  57       /* Some Linux versions with some file systems can actually
  58          change symbolic link permissions via /proc, but this is not
  59          intentional, and it gives inconsistent results (e.g., error
  60          return despite mode change).  The expected behavior is that
  61          symbolic link modes cannot be changed at all, and this check
  62          enforces that.  */
  63       if (S_ISLNK (st.st_mode))
  64         {
  65           __close_nocancel (pathfd);
  66           __set_errno (EOPNOTSUPP);
  67           return -1;
  68         }
  69
  70       /* For most file systems, fchmod does not operate on O_PATH
  71          descriptors, so go through /proc.  */
  72       char buf[32];
  73       if (__snprintf (buf, sizeof (buf), "/proc/self/fd/%d", pathfd) < 0)
  74         {
  75           /* This also may report strange error codes to the caller
  76              (although snprintf really should not fail).  */
  77           __close_nocancel (pathfd);
  78           return -1;
  79         }
  80
  81       int ret = __chmod (buf, mode);
  82       if (ret != 0)
  83         {
  84           if (errno == ENOENT)
  85             /* /proc has not been mounted.  Without /proc, there is no
  86                way to upgrade the O_PATH descriptor to a full
  87                descriptor.  It is also not possible to re-open the
  88                file without O_PATH because the file name may refer to
  89                another file, and opening that without O_PATH may have
  90                side effects (such as blocking, device rewinding, or
  91                releasing POSIX locks).  */
  92             __set_errno (EOPNOTSUPP);
  93         }
  94       __close_nocancel (pathfd);
  95       return ret;
  96     }
  97 }
  98 libc_hidden_def (fchmodat)