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e4585134ca
The use after free described in BZ#19951 is due the use of two different
PD fields, 'joinid' and 'cancelhandling', to describe the thread state
and to synchronize the calls of pthread_join, pthread_detach,
pthread_exit, and normal thread exit.
Any state change potentially requires to check for both field
atomically to handle partial state (such as pthread_join() with a
cancellation handler to issue a 'joinstate' field rollback).
This patch uses a different PD member with 4 possible states (JOINABLE,
DETACHED, EXITING, and EXITED) instead of pthread 'tid' field, with
the following logic:
1. On pthread_create the inital state is set either to JOINABLE or
DETACHED depending of the pthread attribute used.
2. On pthread_detach, a CAS is issued on the state. If the CAS
fails it means that thread is already detached (DETACHED) or is
being terminated (EXITING). For former an EINVAL is returned,
while for latter pthread_detach should be reponsible to join the
thread (and deallocate any internal resource).
3. In the exit phase of the wrapper function for the thread start
routine (reached either if the thread function has returned,
pthread_exit has being called, or cancellation handled has been
acted upon) we issue a CAS on state to set to EXITING mode. If the
thread is previously on DETACHED mode the thread itself is
responsible for arranging the deallocation of any resource,
otherwise the thread needs to be joined (detached threads cannot
immediately deallocate themselves).
4. The clear_tid_field on 'clone' call is changed to set the new
'state' field on thread exit (EXITED). This state is only
reached at thread termination.
5. The pthread_join implementation is now simpler: the futex wait
is done directly on thread state and there is no need to reset it
in case of timeout since the state is now set either by
pthread_detach() or by the kernel on process termination.
The race condition on pthread_detach is avoided with only one atomic
operation on PD state: once the mode is set to THREAD_STATE_DETACHED
it is up to thread itself to deallocate its memory (done on the exit
phase at pthread_create()).
Also, the INVALID_NOT_TERMINATED_TD_P is removed since a a negative
tid is not possible and the macro is not used anywhere.
This change trigger an invalid C11 thread tests: it crates a thread,
which detaches itself, and after a timeout the creating thread checks
if the join fails. The issue is once thrd_join() is called the thread
lifetime is not defined.
Checked on x86_64-linux-gnu, i686-linux-gnu, aarch64-linux-gnu,
arm-linux-gnueabihf, and powerpc64-linux-gnu.
102 lines
3.3 KiB
C
102 lines
3.3 KiB
C
/* Common definition for pthread_{timed,try}join{_np}.
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Copyright (C) 2017-2025 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include "pthreadP.h"
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#include <atomic.h>
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#include <stap-probe.h>
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#include <time.h>
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#include <futex-internal.h>
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/* Check for a possible deadlock situation where the threads are waiting for
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each other to finish. Note that this is a "may" error. To be 100% sure we
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catch this error we would have to lock the data structures but it is not
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necessary. In the unlikely case that two threads are really caught in this
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situation they will deadlock. It is the programmer's problem to figure
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this out. */
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static inline bool
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check_for_deadlock (struct pthread *pd)
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{
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struct pthread *self = THREAD_SELF;
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return ((pd == self
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|| (atomic_load_acquire (&self->joinstate) == THREAD_STATE_DETACHED
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&& (pd->cancelhandling
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& (CANCELING_BITMASK | CANCELED_BITMASK | EXITING_BITMASK
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| TERMINATED_BITMASK)) == 0))
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&& !cancel_enabled_and_canceled (self->cancelhandling));
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}
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int
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__pthread_clockjoin_ex (pthread_t threadid, void **thread_return,
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clockid_t clockid,
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const struct __timespec64 *abstime)
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{
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struct pthread *pd = (struct pthread *) threadid;
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/* Make sure the clock and time specified are valid. */
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if (abstime
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&& __glibc_unlikely (!futex_abstimed_supported_clockid (clockid)
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|| ! valid_nanoseconds (abstime->tv_nsec)))
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return EINVAL;
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LIBC_PROBE (pthread_join, 1, threadid);
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int result = 0;
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unsigned int state;
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while ((state = atomic_load_acquire (&pd->joinstate))
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!= THREAD_STATE_EXITED)
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{
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if (check_for_deadlock (pd))
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return EDEADLK;
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/* POSIX states calling pthread_join on a non joinable thread is
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undefined. However, if PD is still in the cache we can warn
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the caller. */
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if (state == THREAD_STATE_DETACHED)
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return EINVAL;
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/* pthread_join is a cancellation entrypoint and we use the same
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rationale for pthread_timedjoin_np.
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The kernel notifies a process which uses CLONE_CHILD_CLEARTID via
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a memory zeroing and futex wake-up when the process terminates.
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The futex operation is not private. */
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int ret = __futex_abstimed_wait_cancelable64 (&pd->joinstate, state,
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clockid, abstime,
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LLL_SHARED);
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if (ret == ETIMEDOUT || ret == EOVERFLOW)
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{
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result = ret;
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break;
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}
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}
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void *pd_result = pd->result;
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if (__glibc_likely (result == 0))
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{
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if (thread_return != NULL)
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*thread_return = pd_result;
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/* Free the TCB. */
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__nptl_free_tcb (pd);
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}
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LIBC_PROBE (pthread_join_ret, 3, threadid, result, pd_result);
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return result;
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}
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