[PATCH 3/3] nptl: Fix race between pthread_kill and thread exit (bug 12889)
Adhemerval Zanella
adhemerval.zanella@linaro.org
Thu Aug 19 16:12:46 GMT 2021
On 17/08/2021 10:51, Florian Weimer via Libc-alpha wrote:
> Thread exit is delayed until all pending pthread_kill operations
> on the thread have completed. This avoids sending signals to the
> wrong thread or a spurious ESRCH error.
>
> The test sysdeps/pthread/tst-pthread_cancel-select-loop.c is derived
> from a downstream test originally written by Marek Polacek.
And I really hope we could first sort out the BZ#19951 and move away
of using pthread::tid as the synchronization member to check thread
termination. I sent a possible fix [1], but it does not fully handle
the pthread::tid access, it would require to a proper lock to
synchronize between thread exit and pthread_kill (as Rich has suggested
on BZ#12889).
I will need to send an updated version of my pthread fixes patchset,
since clone3 changes broke it and INVALID_TD_P needs fixing as well.
[1] https://patchwork.sourceware.org/project/glibc/patch/20210610193639.3650754-5-adhemerval.zanella@linaro.org/
> ---
> nptl/allocatestack.c | 1 +
> nptl/descr.h | 12 ++
> nptl/pthread_create.c | 20 ++++
> nptl/pthread_kill.c | 23 +++-
> support/Makefile | 3 +-
> sysdeps/pthread/Makefile | 2 +
> sysdeps/pthread/tst-kill4.c | 90 --------------
> .../pthread/tst-pthread_cancel-select-loop.c | 87 ++++++++++++++
> sysdeps/pthread/tst-pthread_kill-exiting.c | 110 ++++++++++++++++++
> 9 files changed, 254 insertions(+), 94 deletions(-)
> delete mode 100644 sysdeps/pthread/tst-kill4.c
> create mode 100644 sysdeps/pthread/tst-pthread_cancel-select-loop.c
> create mode 100644 sysdeps/pthread/tst-pthread_kill-exiting.c
>
> diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c
> index cfe37a3443..344f7f8ec5 100644
> --- a/nptl/allocatestack.c
> +++ b/nptl/allocatestack.c
> @@ -127,6 +127,7 @@ get_cached_stack (size_t *sizep, void **memp)
> /* No pending event. */
> result->nextevent = NULL;
>
> + result->exit_signal_futex = 0;
> result->tls_state = (struct tls_internal_t) { 0 };
>
> /* Clear the DTV. */
> diff --git a/nptl/descr.h b/nptl/descr.h
> index c85778d449..7255d1dc77 100644
> --- a/nptl/descr.h
> +++ b/nptl/descr.h
> @@ -396,6 +396,18 @@ struct pthread
> PTHREAD_CANCEL_ASYNCHRONOUS). */
> unsigned char canceltype;
>
> + /* Futex to prevent thread exit during concurrent pthread_kill. The
> + least-significant bit is an exit flag. The remaining bits count
> + the number of pthread_kill operations in progress.
> +
> + Upon exit, the thread bit sets the LSB, and waits until the futex
> + variable is 1.
> +
> + pthread_kill atomically adds 2 to the futex variable before the
> + signal-sending operation, subtracts 2 after it, and skips sending
> + the signal if the least-significant bit is set. */
> + unsigned int exit_signal_futex;
> +
> /* Used on strsignal. */
> struct tls_internal_t tls_state;
>
> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
> index d8ec299cb1..7e1a3cfcdd 100644
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c
> @@ -37,6 +37,7 @@
> #include <sys/single_threaded.h>
> #include <version.h>
> #include <clone_internal.h>
> +#include <futex-internal.h>
>
> #include <shlib-compat.h>
>
> @@ -485,6 +486,25 @@ start_thread (void *arg)
> /* This was the last thread. */
> exit (0);
>
> + /* Blocking signals is required to make pthread_kill
> + async-signal-safe. It prevents sending a signal from this thread
> + to itself during its final stages. This must come after the exit
> + call above because atexit handlers must not run with signals
> + blocked. */
> + __libc_signal_block_all (NULL);
> +
> + /* Delay exiting this thread until there are no concurrent
> + pthread_kill operations in progress. */
> + {
> + /* Indicate that no further signals should be sent. */
> + atomic_fetch_or_release (&pd->exit_signal_futex, 1);
> +
> + /* Wait until there are no pthread_kill operations left. */
> + unsigned int val;
> + while ((val = atomic_load_acquire (&pd->exit_signal_futex)) != 1)
> + futex_wait (&pd->exit_signal_futex, val, FUTEX_PRIVATE);
> + }
> +
> #ifndef __ASSUME_SET_ROBUST_LIST
> /* If this thread has any robust mutexes locked, handle them now. */
> # if __PTHREAD_MUTEX_HAVE_PREV
> diff --git a/nptl/pthread_kill.c b/nptl/pthread_kill.c
> index 5d4c86f920..b67bd4b769 100644
> --- a/nptl/pthread_kill.c
> +++ b/nptl/pthread_kill.c
> @@ -18,6 +18,7 @@
>
> #include <unistd.h>
> #include <pthreadP.h>
> +#include <futex-internal.h>
> #include <shlib-compat.h>
>
> int
> @@ -41,9 +42,25 @@ __pthread_kill_internal (pthread_t threadid, int signo)
> {
> pid_t pid = __getpid ();
>
> - val = INTERNAL_SYSCALL_CALL (tgkill, pid, tid, signo);
> - val = (INTERNAL_SYSCALL_ERROR_P (val)
> - ? INTERNAL_SYSCALL_ERRNO (val) : 0);
> + if (atomic_fetch_add_acq_rel (&pd->exit_signal_futex, 2) & 1)
> + /* If the thread is exiting, it has already blocked all
> + signals, so sending the signal does not have any effect.
> + Not sending the signal avoids a spurious ESRCH from tgkill
> + if the thread manages to complete its exit before the
> + signal can be sent. */
> + val = 0;
> + else
> + {
> + val = INTERNAL_SYSCALL_CALL (tgkill, pid, tid, signo);
> + val = (INTERNAL_SYSCALL_ERROR_P (val)
> + ? INTERNAL_SYSCALL_ERRNO (val) : 0);
> + }
> +
> + /* A value 3 before the update indicates that the thread is
> + exiting, and this is the last remaining concurrent
> + pthread_kill operation. */
> + if (atomic_fetch_add_acq_rel (&pd->exit_signal_futex, -2) == 3)
> + futex_wake (&pd->exit_signal_futex, 1, FUTEX_PRIVATE);
> }
> else
> /* The kernel reports that the thread has exited. POSIX specifies
> diff --git a/support/Makefile b/support/Makefile
> index a462781718..ef2b1a980a 100644
> --- a/support/Makefile
> +++ b/support/Makefile
> @@ -82,9 +82,10 @@ libsupport-routines = \
> support_test_compare_blob \
> support_test_compare_failure \
> support_test_compare_string \
> - support_write_file_string \
> support_test_main \
> support_test_verify_impl \
> + support_wait_for_thread_exit \
> + support_write_file_string \
> temp_file \
> timespec \
> timespec-time64 \
> diff --git a/sysdeps/pthread/Makefile b/sysdeps/pthread/Makefile
> index dedfa0d290..48dba717a1 100644
> --- a/sysdeps/pthread/Makefile
> +++ b/sysdeps/pthread/Makefile
> @@ -119,7 +119,9 @@ tests += tst-cnd-basic tst-mtx-trylock tst-cnd-broadcast \
> tst-unwind-thread \
> tst-pt-vfork1 tst-pt-vfork2 tst-vfork1x tst-vfork2x \
> tst-pthread_cancel-exited \
> + tst-pthread_cancel-select-loop \
> tst-pthread_kill-exited \
> + tst-pthread_kill-exiting \
> # tests
>
> tests-time64 := \
> diff --git a/sysdeps/pthread/tst-kill4.c b/sysdeps/pthread/tst-kill4.c
> deleted file mode 100644
> index 9563939792..0000000000
> --- a/sysdeps/pthread/tst-kill4.c
> +++ /dev/null
> @@ -1,90 +0,0 @@
> -/* Copyright (C) 2003-2021 Free Software Foundation, Inc.
> - This file is part of the GNU C Library.
> - Contributed by Ulrich Drepper <drepper@redhat.com>, 2003.
> -
> - The GNU C Library is free software; you can redistribute it and/or
> - modify it under the terms of the GNU Lesser General Public
> - License as published by the Free Software Foundation; either
> - version 2.1 of the License, or (at your option) any later version.
> -
> - The GNU C Library is distributed in the hope that it will be useful,
> - but WITHOUT ANY WARRANTY; without even the implied warranty of
> - MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> - Lesser General Public License for more details.
> -
> - You should have received a copy of the GNU Lesser General Public
> - License along with the GNU C Library; if not, see
> - <https://www.gnu.org/licenses/>. */
> -
> -#include <errno.h>
> -#include <pthread.h>
> -#include <signal.h>
> -#include <stdio.h>
> -#include <stdlib.h>
> -#include <unistd.h>
> -
> -
> -static void *
> -tf (void *a)
> -{
> - return NULL;
> -}
> -
> -
> -int
> -do_test (void)
> -{
> - pthread_attr_t at;
> - if (pthread_attr_init (&at) != 0)
> - {
> - puts ("attr_create failed");
> - exit (1);
> - }
> -
> - /* Limit thread stack size, because if it is too large, pthread_join
> - will free it immediately rather than put it into stack cache. */
> - if (pthread_attr_setstacksize (&at, 2 * 1024 * 1024) != 0)
> - {
> - puts ("setstacksize failed");
> - exit (1);
> - }
> -
> - pthread_t th;
> - if (pthread_create (&th, &at, tf, NULL) != 0)
> - {
> - puts ("create failed");
> - exit (1);
> - }
> -
> - pthread_attr_destroy (&at);
> -
> - if (pthread_join (th, NULL) != 0)
> - {
> - puts ("join failed");
> - exit (1);
> - }
> -
> - /* The following only works because we assume here something about
> - the implementation. Namely, that the memory allocated for the
> - thread descriptor is not going away, that the TID field is
> - cleared and therefore the signal is sent to process 0, and that
> - we can savely assume there is no other process with this ID at
> - that time. */
> - int e = pthread_kill (th, 0);
> - if (e == 0)
> - {
> - puts ("pthread_kill succeeded");
> - exit (1);
> - }
> - if (e != ESRCH)
> - {
> - puts ("pthread_kill didn't return ESRCH");
> - exit (1);
> - }
> -
> - return 0;
> -}
> -
> -
> -#define TEST_FUNCTION do_test ()
> -#include "../test-skeleton.c"
> diff --git a/sysdeps/pthread/tst-pthread_cancel-select-loop.c b/sysdeps/pthread/tst-pthread_cancel-select-loop.c
> new file mode 100644
> index 0000000000..a62087589c
> --- /dev/null
> +++ b/sysdeps/pthread/tst-pthread_cancel-select-loop.c
> @@ -0,0 +1,87 @@
> +/* Test that pthread_cancel succeeds during thread exit.
> + Copyright (C) 2021 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <https://www.gnu.org/licenses/>. */
> +
> +/* This test tries to trigger an internal race condition in
> + pthread_cancel, where the cancellation signal is sent after the
> + thread has begun the cancellation process. This can result in a
> + spurious ESRCH error. For the original bug 12889, the window is
> + quite small, so the bug was not reproduced in every run. */
> +
> +#include <stdbool.h>
> +#include <stddef.h>
> +#include <support/check.h>
> +#include <support/xthread.h>
> +#include <support/xunistd.h>
> +#include <sys/select.h>
> +#include <unistd.h>
> +
> +/* Set to true by timeout_thread_function when the test should
> + terminate. */
> +static bool timeout;
> +
> +static void *
> +timeout_thread_function (void *unused)
> +{
> + usleep (5 * 1000 * 1000);
> + __atomic_store_n (&timeout, true, __ATOMIC_RELAXED);
> + return NULL;
> +}
> +
> +/* Used for blocking the select function below. */
> +static int pipe_fds[2];
> +
> +static void *
> +canceled_thread_function (void *unused)
> +{
> + while (true)
> + {
> + fd_set rfs;
> + fd_set wfs;
> + fd_set efs;
> + FD_ZERO (&rfs);
> + FD_ZERO (&wfs);
> + FD_ZERO (&efs);
> + FD_SET (pipe_fds[0], &rfs);
> +
> + /* If the cancellation request is recognized early, the thread
> + begins exiting while the cancellation signal arrives. */
> + select (FD_SETSIZE, &rfs, &wfs, &efs, NULL);
> + }
> + return NULL;
> +}
> +
> +static int
> +do_test (void)
> +{
> + xpipe (pipe_fds);
> + pthread_t thr_timeout = xpthread_create (NULL, timeout_thread_function, NULL);
> +
> + while (!__atomic_load_n (&timeout, __ATOMIC_RELAXED))
> + {
> + pthread_t thr = xpthread_create (NULL, canceled_thread_function, NULL);
> + xpthread_cancel (thr);
> + TEST_VERIFY (xpthread_join (thr) == PTHREAD_CANCELED);
> + }
> +
> + xpthread_join (thr_timeout);
> + xclose (pipe_fds[0]);
> + xclose (pipe_fds[1]);
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
> diff --git a/sysdeps/pthread/tst-pthread_kill-exiting.c b/sysdeps/pthread/tst-pthread_kill-exiting.c
> new file mode 100644
> index 0000000000..15550d2310
> --- /dev/null
> +++ b/sysdeps/pthread/tst-pthread_kill-exiting.c
> @@ -0,0 +1,110 @@
> +/* Test that pthread_kill succeeds during thread exit.
> + Copyright (C) 2021 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <https://www.gnu.org/licenses/>. */
> +
> +/* This test verifies that pthread_kill for a thread that is exiting
> + succeeds (with or without actually delivering the signal). */
> +
> +#include <array_length.h>
> +#include <stdbool.h>
> +#include <stddef.h>
> +#include <support/xsignal.h>
> +#include <support/xthread.h>
> +#include <unistd.h>
> +
> +/* Set to true by timeout_thread_function when the test should
> + terminate. */
> +static bool timeout;
> +
> +static void *
> +timeout_thread_function (void *unused)
> +{
> + usleep (1000 * 1000);
> + __atomic_store_n (&timeout, true, __ATOMIC_RELAXED);
> + return NULL;
> +}
> +
> +/* Used to synchronize the sending threads with the target thread and
> + main thread. */
> +static pthread_barrier_t barrier_1;
> +static pthread_barrier_t barrier_2;
> +
> +/* The target thread to which signals are to be sent. */
> +static pthread_t target_thread;
> +
> +static void *
> +sender_thread_function (void *unused)
> +{
> + while (!__atomic_load_n (&timeout, __ATOMIC_RELAXED))
> + {
> + /* Wait until target_thread has been initialized. The target
> + thread and main thread participate in this barrier. */
> + xpthread_barrier_wait (&barrier_1);
> +
> + xpthread_kill (target_thread, SIGUSR1);
> +
> + /* Communicate that the signal has been sent. The main thread
> + participates in this barrier. */
> + xpthread_barrier_wait (&barrier_2);
> + }
> + return NULL;
> +}
> +
> +static void *
> +target_thread_function (void *unused)
> +{
> + target_thread = pthread_self ();
> + xpthread_barrier_wait (&barrier_1);
> + return NULL;
> +}
> +
> +static int
> +do_test (void)
> +{
> + xsignal (SIGUSR1, SIG_IGN);
> +
> + pthread_t thr_timeout = xpthread_create (NULL, timeout_thread_function, NULL);
> +
> + pthread_t threads[4];
> + xpthread_barrier_init (&barrier_1, NULL, array_length (threads) + 2);
> + xpthread_barrier_init (&barrier_2, NULL, array_length (threads) + 1);
> +
> + for (int i = 0; i < array_length (threads); ++i)
> + threads[i] = xpthread_create (NULL, sender_thread_function, NULL);
> +
> + while (!__atomic_load_n (&timeout, __ATOMIC_RELAXED))
> + {
> + xpthread_create (NULL, target_thread_function, NULL);
> +
> + /* Wait for the target thread to be set up and signal sending to
> + start. */
> + xpthread_barrier_wait (&barrier_1);
> +
> + /* Wait for signal sending to complete. */
> + xpthread_barrier_wait (&barrier_2);
> +
> + xpthread_join (target_thread);
> + }
> +
> + for (int i = 0; i < array_length (threads); ++i)
> + xpthread_join (threads[i]);
> + xpthread_join (thr_timeout);
> +
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
>
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