From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on inbox.vuxu.org X-Spam-Level: X-Spam-Status: No, score=-3.4 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED,RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL autolearn=ham autolearn_force=no version=3.4.4 Received: (qmail 25458 invoked from network); 10 Aug 2020 17:04:51 -0000 Received: from mother.openwall.net (195.42.179.200) by inbox.vuxu.org with ESMTPUTF8; 10 Aug 2020 17:04:51 -0000 Received: (qmail 25873 invoked by uid 550); 10 Aug 2020 17:04:48 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Reply-To: musl@lists.openwall.com Received: (qmail 25855 invoked from network); 10 Aug 2020 17:04:48 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; t=1597079076; s=strato-dkim-0002; d=oflebbe.de; h=Message-Id:In-Reply-To:To:References:Date:Subject:From: X-RZG-CLASS-ID:X-RZG-AUTH:From:Subject:Sender; bh=uIg+KHxN6lcAPYoPaQtszsp53zrWLxvZv1fAjmsJEoo=; b=ScTQ00s9z+VDMVZeRpQd8frxEumatIzkNhuKACdoO3C7XEWR9eg+S558wUUyi9qXoQ Ekj8ybSyNB21lvu6zwXNlSzZPKrtQqMagZqJToQfG8jCMKjpY7cWIyhpat6jrtFelqAp GxTjB3z4rwrB2vzsjoQXFKvh9MdhooUUoqXpbS1KOndKYuxAkHxTKg1hijY4Y4OHVbdz ViTymts5xRZDYr4vVQMgGxabOadCy757g8acQA6IdCZrpzf7F+RMY8bWRJUaTxUKiRPt qsTi6ebk/uRMZKd1rTEw/phhkiAnmgFO9B0i5gioOCQKrAD/MeSNfduk9r9KoXmb/7AL 7PnQ== X-RZG-AUTH: ":I2okekakfv3mKNs8YSFayssNXg+upDn7jErwclkPntL+Rbxoy1FyfmVPIbDECH82zNYb4zXt9RFJJgQXvGBS959f7VPVP3v3qQ==" X-RZG-CLASS-ID: mo00 From: Olaf Flebbe Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Mime-Version: 1.0 (Mac OS X Mail 13.4 \(3608.120.23.2.1\)) Date: Mon, 10 Aug 2020 19:04:36 +0200 References: <20200809003958.GE3265@brightrain.aerifal.cx> <2142D551-13BE-4033-94F7-80A7B2C01890@oflebbe.de> <20200810163647.GI3265@brightrain.aerifal.cx> <20200810170053.GK3265@brightrain.aerifal.cx> To: musl@lists.openwall.com In-Reply-To: <20200810170053.GK3265@brightrain.aerifal.cx> Message-Id: <3F1D0BA8-6B88-4B3C-A6F0-DF02DAA90212@oflebbe.de> X-Mailer: Apple Mail (2.3608.120.23.2.1) Subject: Re: [musl] Revisiting sigaltstack and implementation-internal signals Hi Rick, Thanks for explanation, indeed: This might be a problem, if the business = logic of the handler is under application control. But I was assuming that the handler context of __synccall is under musl = control . Olaf > Am 10.08.2020 um 19:00 schrieb Rich Felker : >=20 > On Mon, Aug 10, 2020 at 06:57:21PM +0200, Olaf Flebbe wrote: >> Hi Rick , >>=20 >> While the alternate stack is in use on cannot change the alternate = stack. >>=20 >> See https://pubs.opengroup.org/onlinepubs/9699919799/=20 >> EPERM Error. >=20 > No change of the alternate stack is described here. The minimal > example of the scenario only has one call to sigaltstack in the whole > program. >=20 >=20 >>> Am 10.08.2020 um 18:36 schrieb Rich Felker : >>>=20 >>> On Mon, Aug 10, 2020 at 10:15:13AM +0200, Olaf Flebbe wrote: >>>> Hi,=20 >>>>=20 >>>> I have some problems to follow the discussion here. >>>>=20 >>>> It is not about musl to create an alternate stack, it is to *honor* = the alternate stack, if the application installed one, for a reason. >>>>=20 >>>> I am proposing smthg like >>>>=20 >>>> --- /oss/musl-1.2.1/src/thread/synccall.c >>>> +++ /work/musl/src/thread/synccall.c >>>> @@ -45,7 +45,7 @@ >>>> { >>>> sigset_t oldmask; >>>> int cs, i, r; >>>> - struct sigaction sa =3D { .sa_flags =3D SA_RESTART, .sa_handler = =3D handler }; >>>> + struct sigaction sa =3D { .sa_flags =3D SA_RESTART|SA_ONSTACK, = ...sa_handler =3D handler }; >>>> pthread_t self =3D __pthread_self(), td; >>>> int count =3D 0; >>>>=20 >>>> This will fix the problem with dynamic stacks, like go implements = it.=20 >>>> If the application does not install one, kernel will ignore >>>> SA_ONSTACK. (This is even specified by POSIX, since there is no >>>> error condition mentioned in man page specifically for this). >>>=20 >>> It's fundamental, since presence and identity of an alternate stack >>> are thread-local properties and SA_ONSTACK is global to the signal >>> disposition. >>>=20 >>> The behavior we're concerned about this alterring is not the case >>> where an application does not install an alternate stack; of course >>> that's unaffected. The interesting case is where an application does >>> install one, but expects (albeit IMO wrongly; that's what we're = trying >>> to establish) that the stack memory is not touched/clobbered unless >>> there's actually an SA_ONSTACK signal handler present to run on it = and >>> such a signal arrives. With the proposed change, the memory for the >>> alternate stack can be clobbered asynchronously with no such signal >>> handler existing. (In case it's not clear, the above code is *not a >>> signal handler* from the perspective that's relevant; it's an >>> implementation detail internal to the implementation.) >>>=20 >>> One way such clobbering could manifest is when a signal handler >>> running on the alternate stack temporarily moves the stack pointer = to >>> somewhere else (not on the alternate stack), via swapcontext or some >>> other method. In this case, if a signal for cancellation or synccall >>> arrives, the kernel will consider the alt stack not in use, and will >>> start using it again from the beginning, clobbering the = still-running >>> frames. >>>=20 >>> Rich >>=20