source3/lib/per_thread_cwd.c

   1 /*
   2    Unix SMB/Netbios implementation.
   3
   4    Copyright (C) Ralph Boehme 2019
   5    Copyright (C) Stefan Metzmacher 2019
   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
  21 #include "includes.h"
  22 #include "system/filesys.h"
  23 #include "system/threads.h"
  24 #ifdef HAVE_UNSHARE_CLONE_FS
  25 #include <sched.h>
  26 #endif /* HAVE_UNSHARE_CLONE_FS */
  27
  28 static bool _per_thread_cwd_checked;
  29 static bool _per_thread_cwd_supported;
  30 #ifdef HAVE_UNSHARE_CLONE_FS
  31 static __thread bool _per_thread_cwd_disabled;
  32 static __thread bool _per_thread_cwd_activated;
  33 #endif /* HAVE_UNSHARE_CLONE_FS */
  34
  35 /*
  36  * This is the first function to be called!
  37  * Typically in the main() function before
  38  * any threads are created.
  39  *
  40  * This can be called multiple times
  41  * as the result is cached the first time.
  42  */
  43 void per_thread_cwd_check(void)
  44 {
  45         if (_per_thread_cwd_checked) {
  46                 return;
  47         }
  48
  49 #ifdef HAVE_UNSHARE_CLONE_FS
  50         /*
  51          * While unshare(CLONE_FS) is available on
  52          * Linux for ages, unshare() is also
  53          * used to implement containers with various
  54          * per container namespaces.
  55          *
  56          * It's possible that the whole unshare()
  57          * is blocked in order to disallow nested
  58          * containers.
  59          *
  60          * That's why we sadly need a runtime check
  61          * for this.
  62          */
  63         {
  64                 int res;
  65
  66                 res = unshare(CLONE_FS);
  67                 if (res == 0) {
  68                         _per_thread_cwd_supported = true;
  69                 }
  70         }
  71
  72         /*
  73          * We're in the main thread, so we should disallow
  74          * per_thread_cwd_activate() here.
  75          */
  76         _per_thread_cwd_disabled = true;
  77 #endif /* HAVE_UNSHARE_CLONE_FS */
  78
  79         _per_thread_cwd_checked = true;
  80 }
  81
  82 /*
  83  * In order to use per_thread_cwd_supported()
  84  * per_thread_cwd_check() needs to be called first!
  85  * Otherwise an assert will be triggered!
  86  */
  87 bool per_thread_cwd_supported(void)
  88 {
  89         SMB_ASSERT(_per_thread_cwd_checked);
  90         return _per_thread_cwd_supported;
  91 }
  92
  93 /*
  94  * In order to use per_thread_cwd_disable()
  95  * should be called after any fork() in order
  96  * to mark the main thread of the process,
  97  * which should disallow per_thread_cwd_activate().
  98  *
  99  * This can be called without calling
 100  * per_thread_cwd_check() first.
 101  *
 102  * And it can't be called after calling
 103  * per_thread_cwd_activate()!
 104  * Otherwise an assert will be triggered!
 105  *
 106  * This can be called multiple times
 107  * as the result is cached the first time.
 108  */
 109 void per_thread_cwd_disable(void)
 110 {
 111 #ifdef HAVE_UNSHARE_CLONE_FS
 112         SMB_ASSERT(!_per_thread_cwd_activated);
 113         if (_per_thread_cwd_disabled) {
 114                 return;
 115         }
 116         _per_thread_cwd_disabled = true;
 117 #endif /* HAVE_UNSHARE_CLONE_FS */
 118 }
 119
 120 /*
 121  * In order to use per_thread_cwd_activate()
 122  * per_thread_cwd_supported() needs to be checked first!
 123  * Otherwise an assert will be triggered!
 124  *
 125  * This MUST only be called within helper threads!
 126  *
 127  * That means it can't be called after calling
 128  * per_thread_cwd_disable()!
 129  * Otherwise an assert will be triggered!
 130  *
 131  * This can be called multiple times
 132  * as the result is cached the first time.
 133  */
 134 void per_thread_cwd_activate(void)
 135 {
 136         SMB_ASSERT(_per_thread_cwd_checked);
 137         SMB_ASSERT(_per_thread_cwd_supported);
 138
 139 #ifdef HAVE_UNSHARE_CLONE_FS
 140         if (_per_thread_cwd_activated) {
 141                 return;
 142         }
 143
 144         SMB_ASSERT(!_per_thread_cwd_disabled);
 145
 146         {
 147                 int ret;
 148                 ret = unshare(CLONE_FS);
 149                 SMB_ASSERT(ret == 0);
 150         }
 151
 152         _per_thread_cwd_activated = true;
 153 #else /* not HAVE_UNSHARE_CLONE_FS */
 154         smb_panic(__location__);
 155 #endif /* not HAVE_UNSHARE_CLONE_FS */
 156 }