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 86BBE1B2181; Tue, 15 Oct 2024 13:39:59 +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=1728999601; cv=none; b=cU8Xe3L8AqrDqk5x2CqjKX51mX45OJs7A8tVFI0eRAkaKm4aH8lwC7CWcXVY3vKciHwEuzJY+cHdbqA/8n6cY3fIguR45EkW9Zp5YZPDyWAPqidI7IBAlEbjeBNA1cAa0x6xRw5u3DRHXzFhspmUe29bt1szevlp7Cae5z+lCIM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728999601; c=relaxed/simple; bh=SkNyj+3crdzW7cJachTesaxxbmUIxu0p5toJvApTMaw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=MEnDKo5RzBvFoe19n1l/Ctg0c1LQsbwN88reDxDfdZ3qEGxza61g3ZZNTvh9gxcEw82O9MxbIiY5dOaSYAD3sAQ/hryPCxTKdSjymxf2iGpmIzAYHXFmMntOhdQ/quNc2gbQm/hOqxnim1bEoy6NEW0Nr9cqbqIxQPEuNNaLezI= 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 911ABFEC; Tue, 15 Oct 2024 06:40:28 -0700 (PDT) Received: from e133380.arm.com (e133380.arm.com [10.1.197.51]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C40753F51B; Tue, 15 Oct 2024 06:39:54 -0700 (PDT) Date: Tue, 15 Oct 2024 14:39:49 +0100 From: Dave Martin To: Will Deacon Cc: Joey Gouly , Kevin Brodsky , linux-arm-kernel@lists.infradead.org, nd@arm.com, akpm@linux-foundation.org, aneesh.kumar@kernel.org, aneesh.kumar@linux.ibm.com, anshuman.khandual@arm.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, skhan@linuxfoundation.org, szabolcs.nagy@arm.com, tglx@linutronix.de, x86@kernel.org, kvmarm@lists.linux.dev, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v5 19/30] arm64: add POE signal support Message-ID: References: <20240822151113.1479789-1-joey.gouly@arm.com> <20240822151113.1479789-20-joey.gouly@arm.com> <47e1537f-5b60-4541-aed1-a20e804c137d@arm.com> <20241009144301.GA12453@willie-the-truck> <20241014171023.GA18295@willie-the-truck> <20241015095911.GA3777204@e124191.cambridge.arm.com> <20241015114116.GA19334@willie-the-truck> Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241015114116.GA19334@willie-the-truck> On Tue, Oct 15, 2024 at 12:41:16PM +0100, Will Deacon wrote: > On Tue, Oct 15, 2024 at 10:59:11AM +0100, Joey Gouly wrote: > > On Mon, Oct 14, 2024 at 06:10:23PM +0100, Will Deacon wrote: > > > Kevin, Joey, > > > > > > On Wed, Oct 09, 2024 at 03:43:01PM +0100, Will Deacon wrote: > > > > On Tue, Sep 24, 2024 at 01:27:58PM +0200, Kevin Brodsky wrote: > > > > > On 22/08/2024 17:11, Joey Gouly wrote: > > > > > > @@ -1178,6 +1237,9 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka, > > > > > > sme_smstop(); > > > > > > } > > > > > > > > > > > > + if (system_supports_poe()) > > > > > > + write_sysreg_s(POR_EL0_INIT, SYS_POR_EL0); > > > > > > > > > > At the point where setup_return() is called, the signal frame has > > > > > already been written to the user stack. In other words, we write to the > > > > > user stack first, and then reset POR_EL0. This may be problematic, > > > > > especially if we are using the alternate signal stack, which the > > > > > interrupted POR_EL0 may not grant access to. In that situation uaccess > > > > > will fail and we'll end up with a SIGSEGV. > > > > > > > > > > This issue has already been discussed on the x86 side, and as it happens > > > > > patches to reset PKRU early [1] have just landed. I don't think this is > > > > > a blocker for getting this series landed, but we should try and align > > > > > with x86. If there's no objection, I'm planning to work on a counterpart > > > > > to the x86 series (resetting POR_EL0 early during signal delivery). > > > > > > > > Did you get a chance to work on that? It would be great to land the > > > > fixes for 6.12, if possible, so that the first kernel release with POE > > > > support doesn't land with known issues. > > > > > > Looking a little more at this, I think we have quite a weird behaviour > > > on arm64 as it stands. It looks like we rely on the signal frame to hold > > > the original POR_EL0 so, if for some reason we fail to allocate space > > > for the POR context, I think we'll return back from the signal with > > > POR_EL0_INIT. That seems bad? > > > > If we don't allocate space for POR_EL0, I think the program recieves SIGSGEV? > > > > setup_sigframe_layout() > > if (system_supports_poe()) { > > err = sigframe_alloc(user, &user->poe_offset, > > sizeof(struct poe_context)); > > if (err) > > return err; > > } > > > > Through get_sigframe() and setup_rt_frame(), that eventually hets here: > > > > handle_signal() > > ret = setup_rt_frame(usig, ksig, oldset, regs); > > > > [..] > > > > signal_setup_done(ret, ksig, test_thread_flag(TIF_SINGLESTEP)); > > > > void signal_setup_done(int failed, struct ksignal *ksig, int stepping) > > { > > if (failed) > > force_sigsegv(ksig->sig); > > else > > signal_delivered(ksig, stepping); > > } > > > > So I think it's "fine"? > > Ah, yes, sorry about that. I got confused by the conditional push in > setup_sigframe(): > > if (system_supports_poe() && err == 0 && user->poe_offset) { > ... > > which gives the wrong impression that the POR is somehow optional, even > if the CPU supports POE. So we should drop that check of > 'user->poe_offset' as it cannot be NULL here. >From memory and a quick glance at the code: For other "conditionally unconditional" things, we don't have a corresponding check on user->foo. For conditional stuff, non-NULLness of user->foo is used to track whether we decided to dump the corresponding record; for consistency here, if we have system_supports_poe() && err == 0, then that's sufficient (though in prior versions of this code, POR_EL0 dumping was conditional and so the extra check did do something...) In any case, if some allocation fails then we splat out with a SIGSEGV before modifying the user task state to deliver the signal (in setup_return() etc.) If The user's POR_EL0 value is being clobbered before we get here, we would save the wrong value -- so the code would be broken anyway. So, as Joey says, this is probably fine, but the user->poe_offset check looks superfluous. The kernel will splat on us here and kill the thread if it's NULL anyway. [...] Cheers ---Dave