Re: [Qemu-devel] [PULL 02/46] qemu-thread: fix races on threads that exit very quickly

Re: [Qemu-devel] [PULL 02/46] qemu-thread: fix races on threads that exit very quickly

[linzhecheng]

> -----邮件原件-----
> 发件人: Eric Blake [mailto:eblake@redhat.com]
> 发送时间: 2017年12月21日 11:36
> 收件人: linzhecheng <linzhecheng@huawei.com>; Paolo Bonzini
> <pbonzini@redhat.com>; qemu-devel@nongnu.org; famz@redhat.com
> 抄送: wangxin (U) <wangxinxin.wang@huawei.com>
> 主题: Re: [Qemu-devel] [PULL 02/46] qemu-thread: fix races on threads that
> exit very quickly
> 
> On 12/20/2017 09:29 PM, linzhecheng wrote:
> 
> >> +} QemuThreadArgs;
> >> +
> >> +static void *qemu_thread_start(void *args) {
> >> +    QemuThreadArgs *qemu_thread_args = args;
> >> +    void *(*start_routine)(void *) = qemu_thread_args->start_routine;
> >> +    void *arg = qemu_thread_args->arg;
> >> +
> >> +    /* Attempt to set the threads name; note that this is for debug, so
> >> +     * we're not going to fail if we can't set it.
> >> +     */
> >> +    pthread_setname_np(pthread_self(), qemu_thread_args->name);
> >> +    g_free(qemu_thread_args->name);
> >> +    g_free(qemu_thread_args);
> > If qemu_thread_args is freed here, start_routine(arg) will lead to use
> > after free because arg equals to qemu_thread_args
> 
> No, we explicitly copied qemu_thread_args->arg into a local variable prior to
> freeing qemu_thread_args, so that we do not have to dereference the freed
> variable.
OK, that's true. 
> 
> --
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3266
> Virtualization:  qemu.org | libvirt.org

Re: [Qemu-devel] [PULL 02/46] qemu-thread: fix races on threads that exit very quickly

[Eric Blake]

On 12/20/2017 09:40 PM, linzhecheng wrote:

>>> If qemu_thread_args is freed here, start_routine(arg) will lead to use
>>> after free because arg equals to qemu_thread_args
>>
>> No, we explicitly copied qemu_thread_args->arg into a local variable prior to
>> freeing qemu_thread_args, so that we do not have to dereference the freed
>> variable.
> OK, that's true.

By the way, your mailer is breaking threading; it is omitting 
'In-Reply-To:' and 'References:' headers, which makes every mail from 
you show up as a new top-level thread, rather than properly threaded to 
what you are responding to.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Re: [Qemu-devel] [PULL 02/46] qemu-thread: fix races on threads that exit very quickly

[Eric Blake]

On 12/20/2017 09:29 PM, linzhecheng wrote:

>> +} QemuThreadArgs;
>> +
>> +static void *qemu_thread_start(void *args) {
>> +    QemuThreadArgs *qemu_thread_args = args;
>> +    void *(*start_routine)(void *) = qemu_thread_args->start_routine;
>> +    void *arg = qemu_thread_args->arg;
>> +
>> +    /* Attempt to set the threads name; note that this is for debug, so
>> +     * we're not going to fail if we can't set it.
>> +     */
>> +    pthread_setname_np(pthread_self(), qemu_thread_args->name);
>> +    g_free(qemu_thread_args->name);
>> +    g_free(qemu_thread_args);
> If qemu_thread_args is freed here, start_routine(arg) will lead to use after free because arg equals to qemu_thread_args

No, we explicitly copied qemu_thread_args->arg into a local variable 
prior to freeing qemu_thread_args, so that we do not have to dereference 
the freed variable.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Re: [Qemu-devel] [PULL 02/46] qemu-thread: fix races on threads that exit very quickly

[linzhecheng]

> -----邮件原件-----
> 发件人: Paolo Bonzini [mailto:paolo.bonzini@gmail.com] 代表 Paolo Bonzini
> 发送时间: 2017年12月21日 1:14
> 收件人: qemu-devel@nongnu.org
> 抄送: linzhecheng <linzhecheng@huawei.com>
> 主题: [PULL 02/46] qemu-thread: fix races on threads that exit very quickly
> 
> From: linzhecheng <linzhecheng@huawei.com>
> 
> If we create a thread with QEMU_THREAD_DETACHED mode, QEMU may get a
> segfault with low probability.
> 
> The backtrace is:
>    #0  0x00007f46c60291d7 in __GI_raise (sig=sig@entry=6)
> at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
>    #1  0x00007f46c602a8c8 in __GI_abort () at abort.c:90
>    #2  0x00000000008543c9 in PAT_abort ()
>    #3  0x000000000085140d in patchIllInsHandler ()
>    #4  <signal handler called>
>    #5  pthread_detach (th=139933037614848) at pthread_detach.c:50
>    #6  0x0000000000829759 in qemu_thread_create
> (thread=thread@entry=0x7ffdaa8205e0, name=name@entry=0x94d94a "io-
> task-worker", start_routine=start_routine@entry=0x7eb9a0
> <qio_task_thread_worker>,
>        arg=arg@entry=0x3f5cf70, mode=mode@entry=1) at
> util/qemu_thread_posix.c:512
>    #7  0x00000000007ebc96 in qio_task_run_in_thread (task=0x31db2c0,
> worker=worker@entry=0x7e7e40 <qio_channel_socket_connect_worker>,
> opaque=0xcd23380, destroy=0x7f1180 <qapi_free_SocketAddress>)
>        at io/task.c:141
>    #8  0x00000000007e7f33 in qio_channel_socket_connect_async
> (ioc=ioc@entry=0x626c0b0, addr=<optimized out>,
> callback=callback@entry=0x55e080 <qemu_chr_socket_connected>,
> opaque=opaque@entry=0x42862c0,
>        destroy=destroy@entry=0x0) at io/channel_socket.c:194
>    #9  0x000000000055bdd1 in socket_reconnect_timeout (opaque=0x42862c0)
> at qemu_char.c:4744
>    #10 0x00007f46c72483b3 in g_timeout_dispatch () from /usr/lib64/libglib-
> 2.0.so.0
>    #11 0x00007f46c724799a in g_main_context_dispatch () from
> /usr/lib64/libglib-2.0.so.0
>    #12 0x000000000076c646 in glib_pollfds_poll () at main_loop.c:228
>    #13 0x000000000076c6eb in os_host_main_loop_wait (timeout=348000000)
> at main_loop.c:273
>    #14 0x000000000076c815 in main_loop_wait
> (nonblocking=nonblocking@entry=0) at main_loop.c:521
>    #15 0x000000000056a511 in main_loop () at vl.c:2076
>    #16 0x0000000000420705 in main (argc=<optimized out>, argv=<optimized
> out>, envp=<optimized out>) at vl.c:4940
> 
> The cause of this problem is a glibc bug; for more information, see
> https://sourceware.org/bugzilla/show_bug.cgi?id=19951.
> The solution for this bug is to use pthread_attr_setdetachstate.
> 
> There is a similar issue with pthread_setname_np, which is moved from
> creating thread to created thread.
> 
> Signed-off-by: linzhecheng <linzhecheng@huawei.com>
> Message-Id: <20171128044656.10592-1-linzhecheng@huawei.com>
> Reviewed-by: Fam Zheng <famz@redhat.com> [Simplify the code by removing
> qemu_thread_set_name, and free the arguments  before invoking the start
> routine. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  util/qemu-thread-posix.c | 59 ++++++++++++++++++++++++++++++++++---------
> -----
>  1 file changed, 42 insertions(+), 17 deletions(-)
> 
> diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c index
> 7306475..fcd369b 100644
> --- a/util/qemu-thread-posix.c
> +++ b/util/qemu-thread-posix.c
> @@ -479,15 +479,29 @@ static void __attribute__((constructor))
> qemu_thread_atexit_init(void)  }
> 
> 
> -/* Attempt to set the threads name; note that this is for debug, so
> - * we're not going to fail if we can't set it.
> - */
> -static void qemu_thread_set_name(QemuThread *thread, const char *name) -
> {  #ifdef CONFIG_PTHREAD_SETNAME_NP
> -    pthread_setname_np(thread->thread, name);
> -#endif
> +typedef struct {
> +    void *(*start_routine)(void *);
> +    void *arg;
> +    char *name;
> +} QemuThreadArgs;
> +
> +static void *qemu_thread_start(void *args) {
> +    QemuThreadArgs *qemu_thread_args = args;
> +    void *(*start_routine)(void *) = qemu_thread_args->start_routine;
> +    void *arg = qemu_thread_args->arg;
> +
> +    /* Attempt to set the threads name; note that this is for debug, so
> +     * we're not going to fail if we can't set it.
> +     */
> +    pthread_setname_np(pthread_self(), qemu_thread_args->name);
> +    g_free(qemu_thread_args->name);
> +    g_free(qemu_thread_args);
If qemu_thread_args is freed here, start_routine(arg) will lead to use after free because arg equals to qemu_thread_args
> +    return start_routine(arg);
>  }
> +#endif
> +
> 
>  void qemu_thread_create(QemuThread *thread, const char *name,
>                         void *(*start_routine)(void*), @@ -496,29 +510,40 @@ void
> qemu_thread_create(QemuThread *thread, const char *name,
>      sigset_t set, oldset;
>      int err;
>      pthread_attr_t attr;
> +    QemuThreadArgs *qemu_thread_args;
> 
>      err = pthread_attr_init(&attr);
>      if (err) {
>          error_exit(err, __func__);
>      }
> 
> +    if (mode == QEMU_THREAD_DETACHED) {
> +        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
> +    }
> +
>      /* Leave signal handling to the iothread.  */
>      sigfillset(&set);
>      pthread_sigmask(SIG_SETMASK, &set, &oldset);
> -    err = pthread_create(&thread->thread, &attr, start_routine, arg);
> -    if (err)
> -        error_exit(err, __func__);
> 
> +#ifdef CONFIG_PTHREAD_SETNAME_NP
>      if (name_threads) {
> -        qemu_thread_set_name(thread, name);
> +        qemu_thread_args = g_new0(QemuThreadArgs, 1);
> +        qemu_thread_args->name = g_strdup(name);
> +        qemu_thread_args->start_routine = start_routine;
> +        qemu_thread_args->arg = arg;
> +
> +        err = pthread_create(&thread->thread, &attr,
> +                             qemu_thread_start, qemu_thread_args);
> +    } else
> +#endif
> +    {
> +        err = pthread_create(&thread->thread, &attr,
> +                             start_routine, arg);
>      }
> 
> -    if (mode == QEMU_THREAD_DETACHED) {
> -        err = pthread_detach(thread->thread);
> -        if (err) {
> -            error_exit(err, __func__);
> -        }
> -    }
> +    if (err)
> +        error_exit(err, __func__);
> +
>      pthread_sigmask(SIG_SETMASK, &oldset, NULL);
> 
>      pthread_attr_destroy(&attr);
> --
> 1.8.3.1
> 

[Qemu-devel] [PULL 02/46] qemu-thread: fix races on threads that exit very quickly

[Paolo Bonzini]

From: linzhecheng <linzhecheng@huawei.com>

If we create a thread with QEMU_THREAD_DETACHED mode, QEMU may get a segfault with low probability.

The backtrace is:
   #0  0x00007f46c60291d7 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
   #1  0x00007f46c602a8c8 in __GI_abort () at abort.c:90
   #2  0x00000000008543c9 in PAT_abort ()
   #3  0x000000000085140d in patchIllInsHandler ()
   #4  <signal handler called>
   #5  pthread_detach (th=139933037614848) at pthread_detach.c:50
   #6  0x0000000000829759 in qemu_thread_create (thread=thread@entry=0x7ffdaa8205e0, name=name@entry=0x94d94a "io-task-worker", start_routine=start_routine@entry=0x7eb9a0 <qio_task_thread_worker>,
       arg=arg@entry=0x3f5cf70, mode=mode@entry=1) at util/qemu_thread_posix.c:512
   #7  0x00000000007ebc96 in qio_task_run_in_thread (task=0x31db2c0, worker=worker@entry=0x7e7e40 <qio_channel_socket_connect_worker>, opaque=0xcd23380, destroy=0x7f1180 <qapi_free_SocketAddress>)
       at io/task.c:141
   #8  0x00000000007e7f33 in qio_channel_socket_connect_async (ioc=ioc@entry=0x626c0b0, addr=<optimized out>, callback=callback@entry=0x55e080 <qemu_chr_socket_connected>, opaque=opaque@entry=0x42862c0,
       destroy=destroy@entry=0x0) at io/channel_socket.c:194
   #9  0x000000000055bdd1 in socket_reconnect_timeout (opaque=0x42862c0) at qemu_char.c:4744
   #10 0x00007f46c72483b3 in g_timeout_dispatch () from /usr/lib64/libglib-2.0.so.0
   #11 0x00007f46c724799a in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
   #12 0x000000000076c646 in glib_pollfds_poll () at main_loop.c:228
   #13 0x000000000076c6eb in os_host_main_loop_wait (timeout=348000000) at main_loop.c:273
   #14 0x000000000076c815 in main_loop_wait (nonblocking=nonblocking@entry=0) at main_loop.c:521
   #15 0x000000000056a511 in main_loop () at vl.c:2076
   #16 0x0000000000420705 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4940

The cause of this problem is a glibc bug; for more information, see
https://sourceware.org/bugzilla/show_bug.cgi?id=19951.
The solution for this bug is to use pthread_attr_setdetachstate.

There is a similar issue with pthread_setname_np, which is moved
from creating thread to created thread.

Signed-off-by: linzhecheng <linzhecheng@huawei.com>
Message-Id: <20171128044656.10592-1-linzhecheng@huawei.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
[Simplify the code by removing qemu_thread_set_name, and free the arguments
 before invoking the start routine. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 util/qemu-thread-posix.c | 59 ++++++++++++++++++++++++++++++++++--------------
 1 file changed, 42 insertions(+), 17 deletions(-)

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index 7306475..fcd369b 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -479,15 +479,29 @@ static void __attribute__((constructor)) qemu_thread_atexit_init(void)
 }
 
 
-/* Attempt to set the threads name; note that this is for debug, so
- * we're not going to fail if we can't set it.
- */
-static void qemu_thread_set_name(QemuThread *thread, const char *name)
-{
 #ifdef CONFIG_PTHREAD_SETNAME_NP
-    pthread_setname_np(thread->thread, name);
-#endif
+typedef struct {
+    void *(*start_routine)(void *);
+    void *arg;
+    char *name;
+} QemuThreadArgs;
+
+static void *qemu_thread_start(void *args)
+{
+    QemuThreadArgs *qemu_thread_args = args;
+    void *(*start_routine)(void *) = qemu_thread_args->start_routine;
+    void *arg = qemu_thread_args->arg;
+
+    /* Attempt to set the threads name; note that this is for debug, so
+     * we're not going to fail if we can't set it.
+     */
+    pthread_setname_np(pthread_self(), qemu_thread_args->name);
+    g_free(qemu_thread_args->name);
+    g_free(qemu_thread_args);
+    return start_routine(arg);
 }
+#endif
+
 
 void qemu_thread_create(QemuThread *thread, const char *name,
                        void *(*start_routine)(void*),
@@ -496,29 +510,40 @@ void qemu_thread_create(QemuThread *thread, const char *name,
     sigset_t set, oldset;
     int err;
     pthread_attr_t attr;
+    QemuThreadArgs *qemu_thread_args;
 
     err = pthread_attr_init(&attr);
     if (err) {
         error_exit(err, __func__);
     }
 
+    if (mode == QEMU_THREAD_DETACHED) {
+        pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
+    }
+
     /* Leave signal handling to the iothread.  */
     sigfillset(&set);
     pthread_sigmask(SIG_SETMASK, &set, &oldset);
-    err = pthread_create(&thread->thread, &attr, start_routine, arg);
-    if (err)
-        error_exit(err, __func__);
 
+#ifdef CONFIG_PTHREAD_SETNAME_NP
     if (name_threads) {
-        qemu_thread_set_name(thread, name);
+        qemu_thread_args = g_new0(QemuThreadArgs, 1);
+        qemu_thread_args->name = g_strdup(name);
+        qemu_thread_args->start_routine = start_routine;
+        qemu_thread_args->arg = arg;
+
+        err = pthread_create(&thread->thread, &attr,
+                             qemu_thread_start, qemu_thread_args);
+    } else
+#endif
+    {
+        err = pthread_create(&thread->thread, &attr,
+                             start_routine, arg);
     }
 
-    if (mode == QEMU_THREAD_DETACHED) {
-        err = pthread_detach(thread->thread);
-        if (err) {
-            error_exit(err, __func__);
-        }
-    }
+    if (err)
+        error_exit(err, __func__);
+
     pthread_sigmask(SIG_SETMASK, &oldset, NULL);
 
     pthread_attr_destroy(&attr);
-- 
1.8.3.1