lib/euidaccess-stat.c

   1 /* euidaccess-stat -- check if effective user id can access lstat'd file
   2    This function is probably useful only for choosing whether to issue
   3    a prompt in an implementation of POSIX-specified rm.
   4
   5    Copyright (C) 2005-2006, 2009-2010 Free Software Foundation, Inc.
   6
   7    This program is free software: you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation, either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 /* Adapted for use in GNU remove.c by Jim Meyering.  */
  21
  22 #include <config.h>
  23
  24 #include "euidaccess-stat.h"
  25
  26 #include <unistd.h>
  27
  28 #include "group-member.h"
  29 #include "stat-macros.h"
  30
  31 /* Return true if the current user has permission of type MODE
  32    on the file from which stat buffer *ST was obtained, ignoring
  33    ACLs, attributes, `read-only'ness, etc...
  34    Otherwise, return false.
  35
  36    Like the reentrant version of euidaccess, but starting with
  37    a stat buffer rather than a file name.  Hence, this function
  38    never calls access or accessx, and doesn't take into account
  39    whether the file has ACLs or other attributes, or resides on
  40    a read-only file system.  */
  41
  42 bool
  43 euidaccess_stat (struct stat const *st, int mode)
  44 {
  45   uid_t euid;
  46   unsigned int granted;
  47
  48   /* Convert the mode to traditional form, clearing any bogus bits.  */
  49   if (R_OK == 4 && W_OK == 2 && X_OK == 1 && F_OK == 0)
  50     mode &= 7;
  51   else
  52     mode = ((mode & R_OK ? 4 : 0)
  53             + (mode & W_OK ? 2 : 0)
  54             + (mode & X_OK ? 1 : 0));
  55
  56   if (mode == 0)
  57     return true;                /* The file exists.  */
  58
  59   euid = geteuid ();
  60
  61   /* The super-user can read and write any file, and execute any file
  62      that anyone can execute.  */
  63   if (euid == 0 && ((mode & X_OK) == 0
  64                     || (st->st_mode & (S_IXUSR | S_IXGRP | S_IXOTH))))
  65     return true;
  66
  67   /* Convert the file's permission bits to traditional form.  */
  68   if (   S_IRUSR == (4 << 6)
  69       && S_IWUSR == (2 << 6)
  70       && S_IXUSR == (1 << 6)
  71       && S_IRGRP == (4 << 3)
  72       && S_IWGRP == (2 << 3)
  73       && S_IXGRP == (1 << 3)
  74       && S_IROTH == (4 << 0)
  75       && S_IWOTH == (2 << 0)
  76       && S_IXOTH == (1 << 0))
  77     granted = st->st_mode;
  78   else
  79     granted = (  (st->st_mode & S_IRUSR ? 4 << 6 : 0)
  80                + (st->st_mode & S_IWUSR ? 2 << 6 : 0)
  81                + (st->st_mode & S_IXUSR ? 1 << 6 : 0)
  82                + (st->st_mode & S_IRGRP ? 4 << 3 : 0)
  83                + (st->st_mode & S_IWGRP ? 2 << 3 : 0)
  84                + (st->st_mode & S_IXGRP ? 1 << 3 : 0)
  85                + (st->st_mode & S_IROTH ? 4 << 0 : 0)
  86                + (st->st_mode & S_IWOTH ? 2 << 0 : 0)
  87                + (st->st_mode & S_IXOTH ? 1 << 0 : 0));
  88
  89   if (euid == st->st_uid)
  90     granted >>= 6;
  91   else
  92     {
  93       gid_t egid = getegid ();
  94       if (egid == st->st_gid || group_member (st->st_gid))
  95         granted >>= 3;
  96     }
  97
  98   if ((mode & ~granted) == 0)
  99     return true;
 100
 101   return false;
 102 }
 103
 104
 105 #ifdef TEST
 106 # include <errno.h>
 107 # include <stdio.h>
 108 # include <stdlib.h>
 109
 110 # include "error.h"
 111 # define _(msg) msg
 112
 113 char *program_name;
 114
 115 int
 116 main (int argc, char **argv)
 117 {
 118   char *file;
 119   int mode;
 120   bool ok;
 121   struct stat st;
 122
 123   program_name = argv[0];
 124   if (argc < 3)
 125     abort ();
 126   file = argv[1];
 127   mode = atoi (argv[2]);
 128   if (lstat (file, &st) != 0)
 129     error (EXIT_FAILURE, errno, _("cannot stat %s"), file);
 130
 131   ok = euidaccess_stat (&st, mode);
 132   printf ("%s: %s\n", file, ok ? "y" : "n");
 133   return 0;
 134 }
 135 #endif