/* Create /proc/self/fd-related names for subfiles of open directories.

   Copyright (C) 2006 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program 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 General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

/* Written by Paul Eggert.  */

#include <config.h>

#include "openat-priv.h"

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <stdio.h>
#include <string.h>

#include "dirname.h"
#include "intprops.h"
#include "same-inode.h"
#include "xalloc.h"

/* The results of open() in this file are not used with fchdir,
   therefore save some unnecessary work in fchdir.c.  */
#undef open
#undef close

#define PROC_SELF_FD_FORMAT "/proc/self/fd/%d/%s"

#define PROC_SELF_FD_NAME_SIZE_BOUND(len) \
  (sizeof PROC_SELF_FD_FORMAT - sizeof "%d%s" \
   + INT_STRLEN_BOUND (int) + (len) + 1)


/* Set BUF to the expansion of PROC_SELF_FD_FORMAT, using FD and FILE
   respectively for %d and %s.  If successful, return BUF if the
   result fits in BUF, dynamically allocated memory otherwise.  But
   return NULL if /proc is not reliable.  */
char *
openat_proc_name (char buf[OPENAT_BUFFER_SIZE], int fd, char const *file)
{
  static int proc_status = 0;

  if (! proc_status)
    {
      /* Set PROC_STATUS to a positive value if /proc/self/fd is
	 reliable, and a negative value otherwise.  Solaris 10
	 /proc/self/fd mishandles "..", and any file name might expand
	 to ".." after symbolic link expansion, so avoid /proc/self/fd
	 if it mishandles "..".  Solaris 10 has openat, but this
	 problem is exhibited on code that built on Solaris 8 and
	 running on Solaris 10.  */

      int proc_self_fd = open ("/proc/self/fd", O_RDONLY);
      if (proc_self_fd < 0)
	proc_status = -1;
      else
	{
	  struct stat proc_self_fd_dotdot_st;
	  struct stat proc_self_st;
	  char dotdot_buf[PROC_SELF_FD_NAME_SIZE_BOUND (sizeof ".." - 1)];
	  sprintf (dotdot_buf, PROC_SELF_FD_FORMAT, proc_self_fd, "..");
	  proc_status =
	    ((stat (dotdot_buf, &proc_self_fd_dotdot_st) == 0
	      && stat ("/proc/self", &proc_self_st) == 0
	      && SAME_INODE (proc_self_fd_dotdot_st, proc_self_st))
	     ? 1 : -1);
	  close (proc_self_fd);
	}
    }

  if (proc_status < 0)
    return NULL;
  else
    {
      size_t bufsize = PROC_SELF_FD_NAME_SIZE_BOUND (strlen (file));
      char *result = (bufsize < OPENAT_BUFFER_SIZE ? buf : xmalloc (bufsize));
      sprintf (result, PROC_SELF_FD_FORMAT, fd, file);
      return result;
    }
}