glibc/sysdeps/unix/sysv/linux/fchmodat.c

/* Change the protections of file relative to open directory.  Linux version.
   Copyright (C) 2006-2020 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#include <errno.h>
#include <fcntl.h>
#include <not-cancel.h>
#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sysdep.h>
#include <unistd.h>

int
fchmodat (int fd, const char *file, mode_t mode, int flag)
{
  if (flag == 0)
    return INLINE_SYSCALL (fchmodat, 3, fd, file, mode);
  else if (flag != AT_SYMLINK_NOFOLLOW)
    return INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL);
  else
    {
      /* The kernel system call does not have a mode argument.
	 However, we can create an O_PATH descriptor and change that
	 via /proc (which does not resolve symbolic links).  */

      int pathfd = __openat_nocancel (fd, file,
				      O_PATH | O_NOFOLLOW | O_CLOEXEC);
      if (pathfd < 0)
	/* This may report errors such as ENFILE and EMFILE.  The
	   caller can treat them as temporary if necessary.  */
	return pathfd;

      char buf[32];
      if (__snprintf (buf, sizeof (buf), "/proc/self/fd/%d", pathfd) < 0)
	{
	  /* This also may report strange error codes to the caller
	     (although snprintf really should not fail).  */
	  __close_nocancel (pathfd);
	  return -1;
	}

      /* This operates directly on the symbolic link if it is one.
	 /proc/self/fd files look like symbolic links, but they are
	 not.  (fchmod and fchmodat do not work on O_PATH descriptors,
	 similar to fstat before Linux 3.6.)  */
      int ret = __chmod (buf, mode);
      if (ret != 0)
	{
	  if (errno == ENOENT)
	    /* /proc has not been mounted.  Without /proc, there is no
	       way to upgrade the O_PATH descriptor to a full
	       descriptor.  It is also not possible to re-open the
	       file without O_PATH because the file name may refer to
	       another file, and opening that without O_PATH may have
	       side effects (such as blocking, device rewinding, or
	       releasing POSIX locks).  */
	    __set_errno (EOPNOTSUPP);
	}
      __close_nocancel (pathfd);
      return ret;
    }
}
libc_hidden_def (fchmodat)