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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 E4DF6D2E00C for ; Fri, 5 Dec 2025 06:50:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=da4bpf4Ok7phStKTzyDizM3H9xmZqm3+oD65eWUwzwM=; b=mh3cVyE6SDI1G0quUHMe6B/RRL Eb7BQvNuqLXOhyqWNvMP2oSa6w0eD95pEpObYGW4lK32mh/4UfmnEM0vfM/a/rBX34+A/WmnxOqxB IfByDtxyQN85aXfpu1iGRB6Pgfp6SqpKowDbOd7JC+Fs2krngThurh+HWeiLp58HEnfDvXGHxLQC+ 0KoN+5zGOEgzFzV/ScxaV4lER7madA2w+SwHxROO2yT31GJ1MRmJQc0Qpl074UZJjVNrQZXKbk5B/ fQt7PXguOAe1vblK4GmPDl4+tlQ8qQO2Ups5ud8J7fq/tVzROSrzqswjRyMpWK37AQYX4e8KYTeG+ 9n4w/aDQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vRPdm-0000000970u-0ez8; Fri, 05 Dec 2025 06:50:06 +0000 Received: from tor.source.kernel.org ([2600:3c04:e001:324:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1vRPdk-0000000970o-2D1P for linux-arm-kernel@lists.infradead.org; Fri, 05 Dec 2025 06:50:04 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id 5D6546000A; Fri, 5 Dec 2025 06:50:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D3D9BC4CEF1; Fri, 5 Dec 2025 06:50:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764917403; bh=8aDf/vibDSHxCUkdQ9C98faaNXbC8BlaxlUd+loxUFc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=j1wlC/C/r2z4XDKAk81naayn2F7xuj+JAxD8IRNaFn916/aFnI1qxuqaQ6sbo9gi0 N1Wb4WpgfTAVfGa0AKq4NqnPr33yB29WPlHRJaYngUt3Q6VBLHq5XiZhebKtqS78Nk 9VgeStuzidbhdTtMfX5rUjPXnwOv93GTZnnG941nI3zAMjNe0h8TJSuxCK2OlHV0ES 3hYydzxTPNSzTjMfoAQ5TAtCddqrFR/0BBOX2O4mM4hk9ZIoH9g6r/bFERktqi4lLf k78xt0DLmvZDGxHvdngMHSdbFc7ZrNIqrHUrUgVZtgbGlYDlB80NByz1yhux3qFqD+ g1KyhyuozXUGg== Date: Thu, 4 Dec 2025 22:48:09 -0800 From: Eric Biggers To: Ard Biesheuvel Cc: linux-arm-kernel@lists.infradead.org, linux-crypto@vger.kernel.org, Will Deacon , Catalin Marinas , Kees Cook , Justin Stitt Subject: Re: [PATCH] arm64/simd: Avoid pointless clearing of FP/SIMD buffer Message-ID: <20251205064809.GA26371@sol> References: <20251204162815.522879-2-ardb@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251204162815.522879-2-ardb@kernel.org> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Dec 04, 2025 at 05:28:15PM +0100, Ard Biesheuvel wrote: > The buffer provided to kernel_neon_begin() is only used if the task is > scheduled out while the FP/SIMD is in use by the kernel, or when such a > section is interrupted by a softirq that also uses the FP/SIMD. > > IOW, this happens rarely, and even if it happened often, there is still > no reason for this buffer to be cleared beforehand, which happens by > default when using a compiler that supports -ftrivial-auto-var-init. > > So mark the buffer as __uninitialized. Given that this is a variable > attribute not a type attribute, this requires that the expression is > tweaked a bit. > > Cc: Will Deacon , > Cc: Catalin Marinas , > Cc: Kees Cook > Cc: Eric Biggers > Cc: Justin Stitt > Signed-off-by: Ard Biesheuvel > --- > arch/arm64/include/asm/simd.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > The issue here is that returning a pointer to an automatic variable as > it goes out of scope is slightly dodgy, especially in the context of > __attribute__((cleanup())), on which the scoped guard API relies > heavily. However, in this case it should be safe, given that this > expression is the input to the guarded variable type's constructor. > > It is definitely not pretty, though, so hopefully here is a better way > to attach this. > > diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h > index 0941f6f58a14..825b7fe94003 100644 > --- a/arch/arm64/include/asm/simd.h > +++ b/arch/arm64/include/asm/simd.h > @@ -48,6 +48,7 @@ DEFINE_LOCK_GUARD_1(ksimd, > kernel_neon_begin(_T->lock), > kernel_neon_end(_T->lock)) > > -#define scoped_ksimd() scoped_guard(ksimd, &(struct user_fpsimd_state){}) > +#define scoped_ksimd() \ > + scoped_guard(ksimd, ({ struct user_fpsimd_state __uninitialized s; &s; })) Ick. I should have looked at the generated code more closely. It's actually worse than you describe, because the zeroing is there even without CONFIG_INIT_STACK_ALL_ZERO=y, simply because the user_fpsimd_state struct is declared using a compound literal. I'm afraid that this patch probably isn't a good idea, as it relies on undefined behavior. Before this patch, the user_fpsimd_state is declared using a compound literal, which takes on its enclosing scope, i.e. the 'for' statement generated by scoped_guard(). After this patch, it's in a new inner scope, and the pointer to it escapes from it. Unfortunately I don't think there's any way to solve this while keeping the scoped_ksimd() API as-is. Best I can come up with is to leave it to the callers to allocate the state, and then use scoped_guard() similar to a regular lock: struct user_fpsimd_state __uninitialized fpsimd_state; scoped_guard(ksimd, &fpsimd_state) foo_neon(...) Maybe wrap the state declaration with a macro: DECLARE_FPSIMD_STATE_ONSTACK(fpsimd_state); - Eric