sysdeps/nacl/pthread-pids.h

   1 /* Initialize pid and tid fields of struct pthread.  NaCl version.
   2    Copyright (C) 2015 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library 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 GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <pthreadP.h>
  20
  21
  22 /* NaCl has no concept of PID or TID, nor even any notion of an
  23    identifier for a thread within the process.  But various places in
  24    the NPTL implementation rely on using the 'tid' field of the TCB
  25    (struct pthread) as an identifier that is unique at least among all
  26    live threads in the process.  So we must synthesize some number to
  27    use.  Conveniently, the 'pthread_t' value itself is already unique
  28    in exactly this way (because it's the 'struct pthread' pointer).
  29
  30    The only wrinkle is that 'tid' is a (32-bit) 'int' and its high
  31    (sign) bit is used for special purposes, so we must be absolutely
  32    sure that we never use a pointer value with the high bit set.  (It
  33    also cannot be zero, but zero is never a valid pointer anyway.)
  34    The NaCl sandbox models for 32-bit machines limit the address space
  35    to less than 3GB (in fact, to 1GB), so it's already impossible that
  36    a valid pointer will have its high bit set.  But the NaCl x86-64
  37    sandbox model allows a full 4GB of address space, so we cannot
  38    assume that an arbitrary pointer value will not have the high bit
  39    set.  Conveniently, there are always unused bits in the pointer
  40    value for a 'struct pthread', because it is always aligned to at
  41    least 32 bits and so the low bits are always zero.  Hence, we can
  42    safely avoid the danger of a nonzero high bit just by shifting the
  43    pointer value right.  */
  44
  45 static inline int
  46 __nacl_get_tid (struct pthread *pd)
  47 {
  48   uintptr_t id = (uintptr_t) pd;
  49   int tid = id >> 1;
  50   assert ((id & 1) == 0);
  51   assert (sizeof id == sizeof tid);
  52   assert (tid > 0);
  53   /* This ensures that NACL_EXITING_TID (lowlevellock.h) can never
  54      be a valid TID value.  */
  55   assert ((tid & 1) == 0);
  56   return tid;
  57 }
  58
  59
  60 /* Initialize PD->pid and PD->tid for the initial thread.  If there is
  61    setup required to arrange that __exit_thread causes PD->tid to be
  62    cleared and futex-woken, then this function should do that as well.  */
  63 static inline void
  64 __pthread_initialize_pids (struct pthread *pd)
  65 {
  66   pd->tid = __nacl_get_tid (pd);
  67   pd->pid = -1;
  68 }