lib/opendir-safer.c

   1 /* Invoke opendir, but avoid some glitches.
   2
   3    Copyright (C) 2009-2024 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 Eric Blake.  */
  19
  20 #include <config.h>
  21
  22 #include "dirent-safer.h"
  23
  24 #include <errno.h>
  25 #include <fcntl.h>
  26 #include <unistd.h>
  27
  28 /* Like opendir, but do not clobber stdin, stdout, or stderr.  */
  29
  30 DIR *
  31 opendir_safer (char const *name)
  32 {
  33   DIR *dp = opendir (name);
  34
  35   if (dp)
  36     {
  37       int fd = dirfd (dp);
  38
  39       if (0 <= fd && fd <= STDERR_FILENO)
  40         {
  41           /* If fdopendir is native (as on Linux), then it is safe to
  42              assume dirfd(fdopendir(n))==n.  If we are using the
  43              gnulib module fdopendir, then this guarantee is not met,
  44              but fdopendir recursively calls opendir_safer up to 3
  45              times to at least get a safe fd.  If fdopendir is not
  46              present but dirfd is accurate (as on cygwin 1.5.x), then
  47              we recurse up to 3 times ourselves.  Finally, if dirfd
  48              always fails (as on mingw), then we are already safe.  */
  49           DIR *newdp;
  50           int e;
  51 #if HAVE_FDOPENDIR || GNULIB_FDOPENDIR
  52           int f = fcntl (fd, F_DUPFD_CLOEXEC, STDERR_FILENO + 1);
  53           if (f < 0)
  54             {
  55               e = errno;
  56               newdp = NULL;
  57             }
  58           else
  59             {
  60               newdp = fdopendir (f);
  61               e = errno;
  62               if (! newdp)
  63                 close (f);
  64             }
  65 #else /* !FDOPENDIR */
  66           newdp = opendir_safer (name);
  67           e = errno;
  68 #endif
  69           closedir (dp);
  70           errno = e;
  71           dp = newdp;
  72         }
  73     }
  74
  75   return dp;
  76 }