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bh=1gSp9ERj017FwLzn2f4Lfm+OB75dLSyLAQitiJsd1fk=; b=fZhH1Zm6sJIZ/uWz3AbJ2p8uASTcuee43N2y31/zwTPgTlYfTpBjX3hTC0Y16Wp3ga48H1 Jedn7zWqGggZK1Cl6XXDG0+TExabohXzPwdLh46w4om9VdoVIU5RtvL1u7I9TTyenSvnQ2 bCQ2y2m/R+am+UTmrtqT+Wuo43JE+KY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-598-gMRlrrOCMs6eRVBJbgvwqw-1; Fri, 22 Jan 2021 13:29:20 -0500 X-MC-Unique: gMRlrrOCMs6eRVBJbgvwqw-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id BE2E7A0CA2 for ; Fri, 22 Jan 2021 18:29:19 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (ovpn-113-81.ams2.redhat.com [10.36.113.81]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6B05B6EF57; Fri, 22 Jan 2021 18:29:15 +0000 (UTC) Subject: Re: [PATCH] coroutine-sigaltstack: Keep SIGUSR2 handler up To: Max Reitz , qemu-devel@nongnu.org References: <20210122102041.27031-1-mreitz@redhat.com> From: Laszlo Ersek Message-ID: <1121a803-98e7-6d41-119c-3d82717976ec@redhat.com> Date: Fri, 22 Jan 2021 19:29:14 +0100 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=lersek@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=lersek@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.182, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Paolo Bonzini , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 01/22/21 19:05, Max Reitz wrote: > On 22.01.21 18:09, Laszlo Ersek wrote: >> On 01/22/21 11:20, Max Reitz wrote: >>> Modifying signal handlers is a process-global operation.  When two >>> threads run coroutine-sigaltstack's qemu_coroutine_new() concurrently, >>> they may interfere with each other: One of them may revert the SIGUSR2 >>> handler back to the default between the other thread setting up >>> coroutine_trampoline() as the handler and raising SIGUSR2.  That SIGUSR2 >>> will then lead to the process exiting. >>> >>> Outside of coroutine-sigaltstack, qemu does not use SIGUSR2.  We can >>> thus keep the signal handler installed all the time. >>> CoroutineThreadState.tr_handler tells coroutine_trampoline() whether its >>> stack is set up so a new coroutine is to be launched (i.e., it should >>> invoke sigsetjmp()), or not (i.e., the signal came from an external >>> source and we should just perform the default action, which is to exit >>> the process). >>> >>> Note that in user-mode emulation, the guest can register signal handlers >>> for any signal but SIGSEGV and SIGBUS, so if it registers a SIGUSR2 >>> handler, sigaltstack coroutines will break from then on.  However, we do >>> not use coroutines for user-mode emulation, so that is fine. >>> >>> Suggested-by: Laszlo Ersek >>> Signed-off-by: Max Reitz >>> --- >>>   util/coroutine-sigaltstack.c | 56 +++++++++++++++++++----------------- >>>   1 file changed, 29 insertions(+), 27 deletions(-) >>> >>> diff --git a/util/coroutine-sigaltstack.c b/util/coroutine-sigaltstack.c >>> index aade82afb8..2d32afc322 100644 >>> --- a/util/coroutine-sigaltstack.c >>> +++ b/util/coroutine-sigaltstack.c >>> @@ -59,6 +59,8 @@ typedef struct { >>>     static pthread_key_t thread_state_key; >>>   +static void coroutine_trampoline(int signal); >>> + >>>   static CoroutineThreadState *coroutine_get_thread_state(void) >>>   { >>>       CoroutineThreadState *s = pthread_getspecific(thread_state_key); >>> @@ -80,6 +82,7 @@ static void qemu_coroutine_thread_cleanup(void >>> *opaque) >>>     static void __attribute__((constructor)) coroutine_init(void) >>>   { >>> +    struct sigaction sa; >>>       int ret; >>>         ret = pthread_key_create(&thread_state_key, >>> qemu_coroutine_thread_cleanup); >>> @@ -87,6 +90,20 @@ static void __attribute__((constructor)) >>> coroutine_init(void) >>>           fprintf(stderr, "unable to create leader key: %s\n", >>> strerror(errno)); >>>           abort(); >>>       } >>> + >>> +    /* >>> +     * Establish the SIGUSR2 signal handler.  This is a process-wide >>> +     * operation, and so will apply to all threads from here on. >>> +     */ >>> +    sa = (struct sigaction) { >>> +        .sa_handler = coroutine_trampoline, >>> +        .sa_flags   = SA_ONSTACK, >>> +    }; >>> + >>> +    if (sigaction(SIGUSR2, &sa, NULL) != 0) { >>> +        perror("Unable to install SIGUSR2 handler"); >>> +        abort(); >>> +    } >>>   } >>>     /* "boot" function >>> @@ -121,7 +138,17 @@ static void coroutine_trampoline(int signal) >>>       /* Get the thread specific information */ >>>       coTS = coroutine_get_thread_state(); >>>       self = coTS->tr_handler; >>> + >>> +    if (!self) { >>> +        /* >>> +         * This SIGUSR2 came from an external source, not from >>> +         * qemu_coroutine_new(), so perform the default action. >>> +         */ >>> +        exit(0); >>> +    } >>> + >>>       coTS->tr_called = 1; >>> +    coTS->tr_handler = NULL; >>>       co = &self->base; >>>         /* >> >> (8) There's a further complication here, assuming we really want to >> recognize the case when the handler is executing unexpectedly: >> >> - pthread_getspecific() is not necessarily async-signal-safe, according >> to POSIX, so calling coroutine_get_thread_state() in the "unexpected" >> case (e.g. in response to an asynchronously generated SIGUSR2) is >> problematic in its own right, > > That’s a shame. > >> - if the SIGUSR2 is delivered to a thread that has never called >> coroutine_get_thread_state() before, then we'll reach g_malloc0() inside >> coroutine_get_thread_state(), in signal handler context, which is very >> bad. > > Could be solved with a coroutine_try_get_thread_state() that will never > malloc, but return NULL then. > >> You'd have to block SIGUSR2 for the entire process (all threads) at all >> times, and only temporarily unblock it for a particular coroutine >> thread, with the sigsuspend(). The above check would suffice, that way. > > Yes, that’s what I was originally afraid of.  I feel like that may be > the complexity drop that pushes this change too far out of my comfort > zone.  (And as evidenced by your review, it already was pretty much > outside as it was.) > >> Such blocking is possible by calling pthread_sigmask() from the main >> thread, before any other thread is created (the signal mask is inherited >> across pthread_create()). I guess it could be done in coroutine_init() >> too. >> >> And *then* the pthread_sigmask() calls should indeed be removed from >> qemu_coroutine_new(). > > OTOH, that does sound rather simple... > >> (Apologies if my feedback is difficult to understand, it's my fault. I >> could propose a patch, if (and only if) you want that.) > > I can’t say I wouldn’t be happy with a patch for this code that doesn’t > bear my S-o-b. ;) > > I feel conflicted.  I can send a v2 that addresses this (probably > consisting of multiple patches then, e.g. I’d split the SIGUSR2 blocking > off the main patch), but to me, this bug is really more of a nuisance > that just blocks me from sending a pull request for my block branch... > So I’d rather not drag it out forever.  OTOH, sending a quick and bad > fix just because I can’t wait is just bad. > > I suppose I’ll have to decide over the weekend.  Though if you’re > itching to write a patch yourself, I’d definitely be grateful. OK, I'll try my hand at it; I hope I won't be eating my words. Laszlo