From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 30BAF1D47A2 for ; Tue, 6 Aug 2024 14:31:09 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722954672; cv=none; b=THnECg1eCNpMuriuC2lf+UIklXiA1BbYInDFFQ3b/m0hYKMhf908Qo8IPBT9jLWuZhTjRnAr39/+od0JsaFXVz4+fo0xP8EiBHgVCePZkBrKYBcTTwBFwApuw1TYKBFAl2Gv1AL2szIY9XR1Ro74hoA0k7T+tKjPoRJnFVJJkZU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1722954672; c=relaxed/simple; bh=UbGntWwak7debnKS0YGwFMxuMJ0yzkcZRC6KPYpDBw4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=XuyHheqskrWvjfranvdvOIkScEGqNTiYP3ExJ7O9arV25EoZIQ3Vle6vM1rxBPIQ7Kun73RfKSeLRAqotAJwwY50RQuDh/CzLKt3aVYcQdh/3Wbb2JHSvTN8/WKSJENWAQFk3mPeCF+58PrqbFSmAg/3xydWlFoKAj9qfYgs5lo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3C5DEFEC; Tue, 6 Aug 2024 07:31:35 -0700 (PDT) Received: from e124191.cambridge.arm.com (e124191.cambridge.arm.com [10.1.197.45]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E82AC3F6A8; Tue, 6 Aug 2024 07:31:05 -0700 (PDT) Date: Tue, 6 Aug 2024 15:31:03 +0100 From: Joey Gouly To: Dave Martin Cc: linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, aneesh.kumar@kernel.org, aneesh.kumar@linux.ibm.com, bp@alien8.de, broonie@kernel.org, catalin.marinas@arm.com, christophe.leroy@csgroup.eu, dave.hansen@linux.intel.com, hpa@zytor.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, maz@kernel.org, mingo@redhat.com, mpe@ellerman.id.au, naveen.n.rao@linux.ibm.com, npiggin@gmail.com, oliver.upton@linux.dev, shuah@kernel.org, szabolcs.nagy@arm.com, tglx@linutronix.de, will@kernel.org, x86@kernel.org, kvmarm@lists.linux.dev Subject: Re: [PATCH v4 18/29] arm64: add POE signal support Message-ID: <20240806143103.GB2017741@e124191.cambridge.arm.com> References: <20240503130147.1154804-1-joey.gouly@arm.com> <20240503130147.1154804-19-joey.gouly@arm.com> <20240801155441.GB841837@e124191.cambridge.arm.com> <20240806103532.GA1986436@e124191.cambridge.arm.com> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240806103532.GA1986436@e124191.cambridge.arm.com> On Tue, Aug 06, 2024 at 11:35:32AM +0100, Joey Gouly wrote: > On Thu, Aug 01, 2024 at 05:22:45PM +0100, Dave Martin wrote: > > On Thu, Aug 01, 2024 at 04:54:41PM +0100, Joey Gouly wrote: > > > On Thu, Jul 25, 2024 at 05:00:18PM +0100, Dave Martin wrote: > > > > Hi, > > > > > > > > On Fri, May 03, 2024 at 02:01:36PM +0100, Joey Gouly wrote: > > > > > Add PKEY support to signals, by saving and restoring POR_EL0 from the stackframe. > > > > > > > > > > Signed-off-by: Joey Gouly > > > > > Cc: Catalin Marinas > > > > > Cc: Will Deacon > > > > > Reviewed-by: Mark Brown > > > > > Acked-by: Szabolcs Nagy > > > > > --- > > > > > arch/arm64/include/uapi/asm/sigcontext.h | 7 ++++ > > > > > arch/arm64/kernel/signal.c | 52 ++++++++++++++++++++++++ > > > > > 2 files changed, 59 insertions(+) > > > > > > > > > > diff --git a/arch/arm64/include/uapi/asm/sigcontext.h b/arch/arm64/include/uapi/asm/sigcontext.h > > > > > index 8a45b7a411e0..e4cba8a6c9a2 100644 > > > > > --- a/arch/arm64/include/uapi/asm/sigcontext.h > > > > > +++ b/arch/arm64/include/uapi/asm/sigcontext.h > > > > > > > > [...] > > > > > > > > > @@ -980,6 +1013,13 @@ static int setup_sigframe_layout(struct rt_sigframe_user_layout *user, > > > > > return err; > > > > > } > > > > > > > > > > + if (system_supports_poe()) { > > > > > + err = sigframe_alloc(user, &user->poe_offset, > > > > > + sizeof(struct poe_context)); > > > > > + if (err) > > > > > + return err; > > > > > + } > > > > > + > > > > > return sigframe_alloc_end(user); > > > > > } > > > > > > > > > > @@ -1020,6 +1060,15 @@ static int setup_sigframe(struct rt_sigframe_user_layout *user, > > > > > __put_user_error(current->thread.fault_code, &esr_ctx->esr, err); > > > > > } > > > > > > > > > > + if (system_supports_poe() && err == 0 && user->poe_offset) { > > > > > + struct poe_context __user *poe_ctx = > > > > > + apply_user_offset(user, user->poe_offset); > > > > > + > > > > > + __put_user_error(POE_MAGIC, &poe_ctx->head.magic, err); > > > > > + __put_user_error(sizeof(*poe_ctx), &poe_ctx->head.size, err); > > > > > + __put_user_error(read_sysreg_s(SYS_POR_EL0), &poe_ctx->por_el0, err); > > > > > + } > > > > > + > > > > > > > > Does the AArch64 procedure call standard say anything about whether > > > > POR_EL0 is caller-saved? > > > > > > I asked about this, and it doesn't say anything and they don't plan on it, > > > since it's very application specific. > > > > Right. I think that confirms that we don't absolutely need to preserve > > POR_EL0, because if compiler-generated code was allowed to fiddle with > > this and not clean up after itself, the PCS would have to document this. > > > > > > > > > > > > > > > > > > In theory we could skip saving this register if it is already > > > > POR_EL0_INIT (which it often will be), and if the signal handler is not > > > > supposed to modify and leave the modified value in the register when > > > > returning. > > > > > > > > The complexity of the additional check my be a bit pointless though, > > > > and the the handler might theoretically want to change the interrupted > > > > code's POR_EL0 explicitly, which would be complicated if POE_MAGIC is > > > > sometimes there and sometimes not. > > > > > > > > > > > > > > > I think trying to skip/optimise something here would be more effort than any > > > possible benefits! > > > > Actually, having thought about this some more I think that only dumping > > this register if != POR_EL0_INIT may be right right thing to do. > > > > This would mean that old binary would stacks never see poe_context in > > the signal frame, and so will never experience unexpected stack > > overruns (at least, not due solely to the presence of this feature). > > They can already see things they don't expect, like FPMR that was added > recently. > > > > > POE-aware signal handlers have to do something fiddly and nonportable > > to obtain the original value of POR_EL0 regardless, so requiring them > > do handle both cases (present in sigframe and absent) doesn't seem too > > onerous to me. > > If the signal handler wanted to modify the value, from the default, wouldn't it > need to mess around with the sig context stuff, to allocate some space for > POR_EL0, such that the kernel would restore it properly? (If that's even > possible). > > > > > > > Do you think this approach would break any known use cases? > > Not sure. > We talked about this offline, helped me understand it more, and I think something like this makes sense: diff --git arch/arm64/kernel/signal.c arch/arm64/kernel/signal.c index 561986947530..ca7d4e0be275 100644 --- arch/arm64/kernel/signal.c +++ arch/arm64/kernel/signal.c @@ -1024,7 +1025,10 @@ static int setup_sigframe_layout(struct rt_sigframe_user_layout *user, return err; } - if (system_supports_poe()) { + if (system_supports_poe() && + (add_all || + mm_pkey_allocation_map(current->mm) != 0x1 || + read_sysreg_s(SYS_POR_EL0) != POR_EL0_INIT)) { err = sigframe_alloc(user, &user->poe_offset, sizeof(struct poe_context)); if (err) That is, we only save the POR_EL0 value if any pkeys have been allocated (other than pkey 0) *or* if POR_EL0 is a non-default value. The latter case is a corner case, where a userspace would have changed POR_EL0 before allocating any extra pkeys. That could be: - pkey 0, if it restricts pkey 0 without allocating other pkeys, it's unlikely the program can do anything useful anyway - Another pkey, which userspace probably shouldn't do anyway. The man pages say: The kernel guarantees that the contents of the hardware rights register (PKRU) will be preserved only for allocated protection keys. Any time a key is unallocated (either before the first call returning that key from pkey_alloc() or after it is freed via pkey_free()), the kernel may make arbitrary changes to the parts of the rights register affecting access to that key. So userspace shouldn't be changing POR_EL0 before allocating pkeys anyway.. Thanks, Joey From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 93891C3DA64 for ; Tue, 6 Aug 2024 14:32:06 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4WdbNj05rXz3dHj for ; Wed, 7 Aug 2024 00:32:05 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=arm.com (client-ip=217.140.110.172; helo=foss.arm.com; envelope-from=joey.gouly@arm.com; receiver=lists.ozlabs.org) Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lists.ozlabs.org (Postfix) with ESMTP id 4WdbNF6MNmz3cY1 for ; Wed, 7 Aug 2024 00:31:40 +1000 (AEST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3C5DEFEC; Tue, 6 Aug 2024 07:31:35 -0700 (PDT) Received: from e124191.cambridge.arm.com (e124191.cambridge.arm.com [10.1.197.45]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E82AC3F6A8; Tue, 6 Aug 2024 07:31:05 -0700 (PDT) Date: Tue, 6 Aug 2024 15:31:03 +0100 From: Joey Gouly To: Dave Martin Subject: Re: [PATCH v4 18/29] arm64: add POE signal support Message-ID: <20240806143103.GB2017741@e124191.cambridge.arm.com> References: <20240503130147.1154804-1-joey.gouly@arm.com> <20240503130147.1154804-19-joey.gouly@arm.com> <20240801155441.GB841837@e124191.cambridge.arm.com> <20240806103532.GA1986436@e124191.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240806103532.GA1986436@e124191.cambridge.arm.com> X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: szabolcs.nagy@arm.com, catalin.marinas@arm.com, dave.hansen@linux.intel.com, linux-mm@kvack.org, hpa@zytor.com, shuah@kernel.org, maz@kernel.org, x86@kernel.org, christophe.leroy@csgroup.eu, aneesh.kumar@kernel.org, mingo@redhat.com, naveen.n.rao@linux.ibm.com, will@kernel.org, npiggin@gmail.com, broonie@kernel.org, bp@alien8.de, kvmarm@lists.linux.dev, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, oliver.upton@linux.dev, aneesh.kumar@linux.ibm.com, linux-fsdevel@vger.kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Aug 06, 2024 at 11:35:32AM +0100, Joey Gouly wrote: > On Thu, Aug 01, 2024 at 05:22:45PM +0100, Dave Martin wrote: > > On Thu, Aug 01, 2024 at 04:54:41PM +0100, Joey Gouly wrote: > > > On Thu, Jul 25, 2024 at 05:00:18PM +0100, Dave Martin wrote: > > > > Hi, > > > > > > > > On Fri, May 03, 2024 at 02:01:36PM +0100, Joey Gouly wrote: > > > > > Add PKEY support to signals, by saving and restoring POR_EL0 from the stackframe. > > > > > > > > > > Signed-off-by: Joey Gouly > > > > > Cc: Catalin Marinas > > > > > Cc: Will Deacon > > > > > Reviewed-by: Mark Brown > > > > > Acked-by: Szabolcs Nagy > > > > > --- > > > > > arch/arm64/include/uapi/asm/sigcontext.h | 7 ++++ > > > > > arch/arm64/kernel/signal.c | 52 ++++++++++++++++++++++++ > > > > > 2 files changed, 59 insertions(+) > > > > > > > > > > diff --git a/arch/arm64/include/uapi/asm/sigcontext.h b/arch/arm64/include/uapi/asm/sigcontext.h > > > > > index 8a45b7a411e0..e4cba8a6c9a2 100644 > > > > > --- a/arch/arm64/include/uapi/asm/sigcontext.h > > > > > +++ b/arch/arm64/include/uapi/asm/sigcontext.h > > > > > > > > [...] > > > > > > > > > @@ -980,6 +1013,13 @@ static int setup_sigframe_layout(struct rt_sigframe_user_layout *user, > > > > > return err; > > > > > } > > > > > > > > > > + if (system_supports_poe()) { > > > > > + err = sigframe_alloc(user, &user->poe_offset, > > > > > + sizeof(struct poe_context)); > > > > > + if (err) > > > > > + return err; > > > > > + } > > > > > + > > > > > return sigframe_alloc_end(user); > > > > > } > > > > > > > > > > @@ -1020,6 +1060,15 @@ static int setup_sigframe(struct rt_sigframe_user_layout *user, > > > > > __put_user_error(current->thread.fault_code, &esr_ctx->esr, err); > > > > > } > > > > > > > > > > + if (system_supports_poe() && err == 0 && user->poe_offset) { > > > > > + struct poe_context __user *poe_ctx = > > > > > + apply_user_offset(user, user->poe_offset); > > > > > + > > > > > + __put_user_error(POE_MAGIC, &poe_ctx->head.magic, err); > > > > > + __put_user_error(sizeof(*poe_ctx), &poe_ctx->head.size, err); > > > > > + __put_user_error(read_sysreg_s(SYS_POR_EL0), &poe_ctx->por_el0, err); > > > > > + } > > > > > + > > > > > > > > Does the AArch64 procedure call standard say anything about whether > > > > POR_EL0 is caller-saved? > > > > > > I asked about this, and it doesn't say anything and they don't plan on it, > > > since it's very application specific. > > > > Right. I think that confirms that we don't absolutely need to preserve > > POR_EL0, because if compiler-generated code was allowed to fiddle with > > this and not clean up after itself, the PCS would have to document this. > > > > > > > > > > > > > > > > > > In theory we could skip saving this register if it is already > > > > POR_EL0_INIT (which it often will be), and if the signal handler is not > > > > supposed to modify and leave the modified value in the register when > > > > returning. > > > > > > > > The complexity of the additional check my be a bit pointless though, > > > > and the the handler might theoretically want to change the interrupted > > > > code's POR_EL0 explicitly, which would be complicated if POE_MAGIC is > > > > sometimes there and sometimes not. > > > > > > > > > > > > > > > I think trying to skip/optimise something here would be more effort than any > > > possible benefits! > > > > Actually, having thought about this some more I think that only dumping > > this register if != POR_EL0_INIT may be right right thing to do. > > > > This would mean that old binary would stacks never see poe_context in > > the signal frame, and so will never experience unexpected stack > > overruns (at least, not due solely to the presence of this feature). > > They can already see things they don't expect, like FPMR that was added > recently. > > > > > POE-aware signal handlers have to do something fiddly and nonportable > > to obtain the original value of POR_EL0 regardless, so requiring them > > do handle both cases (present in sigframe and absent) doesn't seem too > > onerous to me. > > If the signal handler wanted to modify the value, from the default, wouldn't it > need to mess around with the sig context stuff, to allocate some space for > POR_EL0, such that the kernel would restore it properly? (If that's even > possible). > > > > > > > Do you think this approach would break any known use cases? > > Not sure. > We talked about this offline, helped me understand it more, and I think something like this makes sense: diff --git arch/arm64/kernel/signal.c arch/arm64/kernel/signal.c index 561986947530..ca7d4e0be275 100644 --- arch/arm64/kernel/signal.c +++ arch/arm64/kernel/signal.c @@ -1024,7 +1025,10 @@ static int setup_sigframe_layout(struct rt_sigframe_user_layout *user, return err; } - if (system_supports_poe()) { + if (system_supports_poe() && + (add_all || + mm_pkey_allocation_map(current->mm) != 0x1 || + read_sysreg_s(SYS_POR_EL0) != POR_EL0_INIT)) { err = sigframe_alloc(user, &user->poe_offset, sizeof(struct poe_context)); if (err) That is, we only save the POR_EL0 value if any pkeys have been allocated (other than pkey 0) *or* if POR_EL0 is a non-default value. The latter case is a corner case, where a userspace would have changed POR_EL0 before allocating any extra pkeys. That could be: - pkey 0, if it restricts pkey 0 without allocating other pkeys, it's unlikely the program can do anything useful anyway - Another pkey, which userspace probably shouldn't do anyway. The man pages say: The kernel guarantees that the contents of the hardware rights register (PKRU) will be preserved only for allocated protection keys. Any time a key is unallocated (either before the first call returning that key from pkey_alloc() or after it is freed via pkey_free()), the kernel may make arbitrary changes to the parts of the rights register affecting access to that key. So userspace shouldn't be changing POR_EL0 before allocating pkeys anyway.. Thanks, Joey