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Date: Wed, 26 Jun 2019 08:30:34 +0100
From: Radostin Stoyanov <rstoyanov1@...il.com>
To: musl@...ts.openwall.com, Rich Felker <dalias@...ifal.cx>
Cc: "Andrei Vagin (C)" <avagin@...il.com>, gorcunov@...il.com
Subject: Re: Re: seccomp causes pthread_join() to hang
On 26/06/2019 00:26, Rich Felker wrote:
> On Wed, Jun 26, 2019 at 12:18:05AM +0100, Radostin Stoyanov wrote:
>> Hello,
>>
>> In the test suite of CRIU [1] we have noticed an interesting bug
>> which is caused by commit 8f11e6127fe93093f81a52b15bb1537edc3fc8af
>> ("track all live threads in an AS-safe, fully-consistent linked
>> list") [2].
>>
>> When seccomp is used in a multithreaded application it may cause
>> pthread_join() to hang.
>>
>> This is a minimal application to reproduce the issue:
>>
>>
>> #include <errno.h>
>> #include <seccomp.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <string.h>
>> #include <pthread.h>
>> #include <unistd.h>
>>
>> static void *fn()
>> {
>> scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_KILL);
>> if (!ctx) {
>> perror("seccomp_init");
>> goto err;
>> }
>>
>> if (seccomp_load(ctx) < 0) {
>> perror("seccomp_load");
>> goto err;
>> }
>>
>> /* This should cause SIG_KILL */
>> getpid();
>> err:
>> return (void *)1;
>> }
>>
>> int main()
>> {
>> pthread_t t1;
>>
>> if (pthread_create(&t1, NULL, fn, NULL)) {
>> perror("pthread_create");
>> return -1;
>> }
>>
>> if (pthread_join(t1, NULL)) {
>> perror("pthread_join");
>> return -1;
>> }
>>
>> return 0;
>> }
>>
>>
>> Expected behaviour: Thread t1 should receive SIG_KILL and the main
>> thread should return 0.
>> Actual behaviour: pthread_join() hangs.
>> Reproducibility: Always
>> Regression: Yes
>>
>>
>> This bug can be reproduced with Alpine 3.10 ($ docker run -it
>> alpine:3.10 sh).
> A fundamental property of the pthread API, and part of why threads are
> a much better primitive than processes for lots of purposes, is that
> threads are not killable; only whole processes are.
From the man page of seccomp(2):
SECCOMP_RET_KILL_PROCESS: This value results in immediate
termination of the process, with a core dump. ...
SECCOMP_RET_KILL_THREAD (or SECCOMP_RET_KILL): This value results
in immediate termination of the thread that made the system call. The
system call is not executed. Other threads in the same thread group will
continue to execute. ...
> Any configuration
> that results in a thread being terminated out from under the process
> has all sorts of extremely dangerous conditions with memory/locks
> being left in inconsistent state, tid reuse while the application
> thinks the old thread is still alive, etc., and fundamentally can't be
> supported. What you're seeing is exposure of a serious existing
> problem with this seccomp usage, not a regression.
I wrote "Regression: Yes" because this bug was recently introduced and
it does not occur in previous versions.
IMHO causing pthread_join() to hang when a thread has been terminated is
not expected behaviour, at least because the man page for
pthread_join(3) states:
The pthread_join() function waits for the thread specified by
thread to terminate. If that thread has already terminated, then
pthread_join() returns immediately.
and indeed prior commit 8f11e612 pthread_join() returns immediately.
Radostin
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