1  /* Check if realpath does not consume extra stack space based on symlink
       2     existence in the path (BZ #26341)
       3     Copyright (C) 2021-2023 Free Software Foundation, Inc.
       4     This file is part of the GNU C Library.
       5  
       6     The GNU C Library is free software; you can redistribute it and/or
       7     modify it under the terms of the GNU Lesser General Public
       8     License as published by the Free Software Foundation; either
       9     version 2.1 of the License, or (at your option) any later version.
      10  
      11     The GNU C Library is distributed in the hope that it will be useful,
      12     but WITHOUT ANY WARRANTY; without even the implied warranty of
      13     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
      14     Lesser General Public License for more details.
      15  
      16     You should have received a copy of the GNU Lesser General Public
      17     License along with the GNU C Library; if not, see
      18     <https://www.gnu.org/licenses/>.  */
      19  
      20  #include <stdlib.h>
      21  #include <string.h>
      22  #include <sys/param.h>
      23  #include <unistd.h>
      24  
      25  #define __sysconf sysconf
      26  #include <eloop-threshold.h>
      27  #include <support/check.h>
      28  #include <support/support.h>
      29  #include <support/temp_file.h>
      30  #include <support/xunistd.h>
      31  #include <support/xthread.h>
      32  
      33  static char *filename;
      34  static size_t filenamelen;
      35  static char *linkname;
      36  
      37  #ifndef PATH_MAX
      38  # define PATH_MAX 1024
      39  #endif
      40  
      41  static void
      42  create_link (void)
      43  {
      44    int fd = create_temp_file ("tst-canon-bz26341", &filename);
      45    TEST_VERIFY_EXIT (fd != -1);
      46    xclose (fd);
      47  
      48    /* Make filename a canonical path.  */
      49    char *saved_filename = filename;
      50    filename = realpath (filename, NULL);
      51    free (saved_filename);
      52    TEST_VERIFY (filename != NULL);
      53  
      54    /* Create MAXLINKS symbolic links to the temporary filename.
      55       On exit, linkname has the last link created.  */
      56    char *prevlink = filename;
      57    int maxlinks = __eloop_threshold ();
      58    for (int i = 0; i < maxlinks; i++)
      59      {
      60        linkname = xasprintf ("%s%d", filename, i);
      61        xsymlink (prevlink, linkname);
      62        add_temp_file (linkname);
      63        prevlink = linkname;
      64      }
      65  
      66    filenamelen = strlen (filename);
      67  }
      68  
      69  static void *
      70  do_realpath (void *arg)
      71  {
      72    /* Old implementation of realpath allocates a PATH_MAX using alloca
      73       for each symlink in the path, leading to MAXSYMLINKS times PATH_MAX
      74       maximum stack usage.
      75       This stack allocations tries fill the thread allocated stack minus
      76       the resolved path (plus some slack), the realpath (plus some
      77       slack), and the system call usage (plus some slack).
      78       If realpath uses more than 2 * PATH_MAX plus some slack it will trigger
      79       a stackoverflow.  */
      80  
      81    const size_t syscall_usage = 1 * PATH_MAX + 1024;
      82    const size_t realpath_usage = 2 * PATH_MAX + 1024;
      83    const size_t thread_usage = 1 * PATH_MAX + 1024;
      84    size_t stack_size = support_small_thread_stack_size ()
      85  		      - syscall_usage - realpath_usage - thread_usage;
      86    char stack[stack_size];
      87    char *resolved = stack + stack_size - thread_usage + 1024;
      88  
      89    char *p = realpath (linkname, resolved);
      90    TEST_VERIFY (p != NULL);
      91    TEST_COMPARE_BLOB (resolved, filenamelen, filename, filenamelen);
      92  
      93    return NULL;
      94  }
      95  
      96  static int
      97  do_test (void)
      98  {
      99    create_link ();
     100  
     101    pthread_t th = xpthread_create (support_small_stack_thread_attribute (),
     102  				  do_realpath, NULL);
     103    xpthread_join (th);
     104  
     105    return 0;
     106  }
     107  
     108  #include <support/test-driver.c>