[PATCH v3 2/2] nptl: Fix race between pthread_kill and thread exit (bug 12889)
Adhemerval Zanella
adhemerval.zanella@linaro.org
Tue Sep 7 13:21:13 GMT 2021
On 06/09/2021 09:34, Florian Weimer via Libc-alpha wrote:
> A new thread exit lock and flag are introduced. They are used to
> detect that the thread is about to exit or has exited in
> __pthread_kill_internal, and the signal is not sent in this case.
>
> The test sysdeps/pthread/tst-pthread_cancel-select-loop.c is derived
> from a downstream test originally written by Marek Polacek.
LGTM, with just a suggestion to use C11 atomics instead of atomic
builtins on the tests.
Reviewed-by: Adhemerval Zanela <adhemerval.zanella@linaro.org>
> ---
> nptl/allocatestack.c | 3 +
> nptl/descr.h | 6 +
> nptl/pthread_create.c | 14 ++
> nptl/pthread_kill.c | 65 +++++----
> sysdeps/pthread/Makefile | 2 +
> .../pthread/tst-pthread_cancel-select-loop.c | 87 +++++++++++++
> sysdeps/pthread/tst-pthread_kill-exiting.c | 123 ++++++++++++++++++
> 7 files changed, 275 insertions(+), 25 deletions(-)
> 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 0356993c26..fa8100719f 100644
> --- a/nptl/allocatestack.c
> +++ b/nptl/allocatestack.c
> @@ -31,6 +31,7 @@
> #include <futex-internal.h>
> #include <kernel-features.h>
> #include <nptl-stack.h>
> +#include <libc-lock.h>
>
> /* Default alignment of stack. */
> #ifndef STACK_ALIGN
> @@ -126,6 +127,8 @@ get_cached_stack (size_t *sizep, void **memp)
> /* No pending event. */
> result->nextevent = NULL;
>
> + result->exiting = false;
> + __libc_lock_init (result->exit_lock);
> result->tls_state = (struct tls_internal_t) { 0 };
>
> /* Clear the DTV. */
Ok.
> diff --git a/nptl/descr.h b/nptl/descr.h
> index e1c8831f09..41ee56feb2 100644
> --- a/nptl/descr.h
> +++ b/nptl/descr.h
> @@ -395,6 +395,12 @@ struct pthread
> PTHREAD_CANCEL_ASYNCHRONOUS). */
> unsigned char canceltype;
>
> + /* Used in __pthread_kill_internal to detected a thread that has
> + exited or is about to exit. exit_lock must only be acquired
> + after blocking signals. */
> + bool exiting;
> + int exit_lock; /* A low-level lock (for use with __libc_lock_init etc). */
> +
> /* Used on strsignal. */
> struct tls_internal_t tls_state;
>
> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
> index 7607f36e26..a559f86cc2 100644
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c
> @@ -36,6 +36,7 @@
> #include <sys/single_threaded.h>
> #include <version.h>
> #include <clone_internal.h>
> +#include <futex-internal.h>
>
> #include <shlib-compat.h>
>
> @@ -484,6 +485,19 @@ start_thread (void *arg)
> /* This was the last thread. */
> exit (0);
>
> + /* This 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);
> +
> + /* Tell __pthread_kill_internal that this thread is about to exit.
> + If there is a __pthread_kill_internal in progress, this delays
> + the thread exit until the signal has been queued by the kernel
> + (so that the TID used to send it remains valid). */
> + __libc_lock_lock (pd->exit_lock);
> + pd->exiting = true;
> + __libc_lock_unlock (pd->exit_lock);
> +
> #ifndef __ASSUME_SET_ROBUST_LIST
> /* If this thread has any robust mutexes locked, handle them now. */
> # if __PTHREAD_MUTEX_HAVE_PREV
Ok.
> diff --git a/nptl/pthread_kill.c b/nptl/pthread_kill.c
> index 5d4c86f920..fb7862eff7 100644
> --- a/nptl/pthread_kill.c
> +++ b/nptl/pthread_kill.c
> @@ -16,6 +16,7 @@
> License along with the GNU C Library; if not, see
> <https://www.gnu.org/licenses/>. */
>
> +#include <libc-lock.h>
> #include <unistd.h>
> #include <pthreadP.h>
> #include <shlib-compat.h>
> @@ -23,37 +24,51 @@
> int
> __pthread_kill_internal (pthread_t threadid, int signo)
> {
> - pid_t tid;
> struct pthread *pd = (struct pthread *) threadid;
> -
> if (pd == THREAD_SELF)
> - /* It is a special case to handle raise() implementation after a vfork
> - call (which does not update the PD tid field). */
> - tid = INLINE_SYSCALL_CALL (gettid);
> - else
> - /* Force load of pd->tid into local variable or register. Otherwise
> - if a thread exits between ESRCH test and tgkill, we might return
> - EINVAL, because pd->tid would be cleared by the kernel. */
> - tid = atomic_forced_read (pd->tid);
> -
> - int val;
> - if (__glibc_likely (tid > 0))
> {
> - pid_t pid = __getpid ();
> -
> - val = INTERNAL_SYSCALL_CALL (tgkill, pid, tid, signo);
> - val = (INTERNAL_SYSCALL_ERROR_P (val)
> - ? INTERNAL_SYSCALL_ERRNO (val) : 0);
> + /* Use the actual TID from the kernel, so that it refers to the
> + current thread even if called after vfork. There is no
> + signal blocking in this case, so that the signal is delivered
> + immediately, before __pthread_kill_internal returns: a signal
> + sent to the thread itself needs to be delivered
> + synchronously. (It is unclear if Linux guarantees the
> + delivery of all pending signals after unblocking in the code
> + below. POSIX only guarantees delivery of a single signal,
> + which may not be the right one.) */
> + pid_t tid = INTERNAL_SYSCALL_CALL (gettid);
> + int ret = INTERNAL_SYSCALL_CALL (kill, tid, signo);
> + return INTERNAL_SYSCALL_ERROR_P (ret) ? INTERNAL_SYSCALL_ERRNO (ret) : 0;
> }
Ok.
> +
> + /* Block all signals, as required by pd->exit_lock. */
> + sigset_t old_mask;
> + __libc_signal_block_all (&old_mask);
> + __libc_lock_lock (pd->exit_lock);
> +
> + int ret;
> + if (pd->exiting)
> + /* The thread is about to exit (or has exited). Sending the
> + signal is either not observable (the target thread has already
> + blocked signals at this point), or it will fail, or it might be
> + delivered to a new, unrelated thread that has reused the TID.
> + So do not actually send the signal. Do not report an error
> + because the threadid argument is still valid (the thread ID
> + lifetime has not ended), and ESRCH (for example) would be
> + misleading. */
> + ret = 0;
> else
> - /* The kernel reports that the thread has exited. POSIX specifies
> - the ESRCH error only for the case when the lifetime of a thread
> - ID has ended, but calling pthread_kill on such a thread ID is
> - undefined in glibc. Therefore, do not treat kernel thread exit
> - as an error. */
> - val = 0;
> + {
> + /* Using tgkill is a safety measure. pd->exit_lock ensures that
> + the target thread cannot exit. */
> + ret = INTERNAL_SYSCALL_CALL (tgkill, __getpid (), pd->tid, signo);
> + ret = INTERNAL_SYSCALL_ERROR_P (ret) ? INTERNAL_SYSCALL_ERRNO (ret) : 0;
> + }
> +
> + __libc_lock_unlock (pd->exit_lock);
> + __libc_signal_restore_set (&old_mask);
>
> - return val;
> + return ret;
> }
>
> int
Ok.
> 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 := \
Ok.
> 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);
Why not use C11 atomics here? Same for other atomic builts usage.
> + 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>
Ok.
> diff --git a/sysdeps/pthread/tst-pthread_kill-exiting.c b/sysdeps/pthread/tst-pthread_kill-exiting.c
> new file mode 100644
> index 0000000000..f803e94f11
> --- /dev/null
> +++ b/sysdeps/pthread/tst-pthread_kill-exiting.c
> @@ -0,0 +1,123 @@
> +/* 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);
Why not use C11 atomics here? Same for other atomic builts usage.
> + 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;
> +
> +/* Set by the main thread to true after timeout has been set to
> + true. */
> +static bool exiting;
> +
> +static void *
> +sender_thread_function (void *unused)
> +{
> + while (true)
> + {
> + /* Wait until target_thread has been initialized. The target
> + thread and main thread participate in this barrier. */
> + xpthread_barrier_wait (&barrier_1);
> +
> + if (exiting)
> + break;
> +
> + 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);
> + }
> +
> + exiting = true;
> +
> + /* Signal the sending threads to exit. */
> + xpthread_create (NULL, target_thread_function, NULL);
> + xpthread_barrier_wait (&barrier_1);
> +
> + for (int i = 0; i < array_length (threads); ++i)
> + xpthread_join (threads[i]);
> + xpthread_join (thr_timeout);
> +
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
>
Ok.
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