From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7F7CF1FF7C8 for ; Mon, 8 Dec 2025 23:24:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765236264; cv=none; b=m447m5ktlQzvTviG9Dj2xKcuVRHqv8c8kLK2uDQsB9fO1qhtJAZ8sXW+gIqr+sv7b/JmHZmoYYcx9E5LQzk5yuTYpFQH0JWJZtESm2kArMQt0OkQ30F6E7Tu+HU6MB+w5nCQaoZg3nJ0xXtcLZ6o35ItUxoluabxdKnyhC+VcMs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765236264; c=relaxed/simple; bh=daj4a+uPh2c3Xc1peJSyoolEr/C456XKe1rbcDZ5X1M=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=q+upqn1Vtdx9mMx+zLy6mzWJb0QmAPRF07JsKJbnEkgiQq+Ko8/Lz206o4DlltzavstuubvGI20g7w116w1pDCoV7YIrv8D84CMjY5naMImfjmtjE96DSmmToH7jhbt0Dj5Bq29aBum8jZ2oRDvJfTebcWQr2dSqS00FIymFAto= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=qXVkIrQD; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="qXVkIrQD" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2066C4CEF1; Mon, 8 Dec 2025 23:24:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1765236264; bh=daj4a+uPh2c3Xc1peJSyoolEr/C456XKe1rbcDZ5X1M=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=qXVkIrQDnUiIaSTr2epUv+ZU7vnvWAdFM2RqZEJBw/22zwN+jZrKlW0fFRPxXpJra RiDd5jxWnpd+3M3yIegWiH1x++iYyzhIpLb9ZYyzIMp00ppL9+RsJX/8WLncIBkNmp eJxlvyaH1MFTqnrgFy1L4jXlWdTG97zRN5vAZPzHMIA5ZDalB8EG3FtzCL4lv/bqeV fG+3fUC4hnptJDxbrKj5rmcUiAm7mHazHoyoYhTK3wTsNTXa8FIf4UopZzX0YltFYA IDUxdUuS9w8CgxPf/rh/CpwaTyxdaOa843UlrF54U70PPAqACa/1Eg/pyuasA1Q/eo kh6xgeuY2zi9A== Date: Mon, 8 Dec 2025 15:24:22 -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: <20251208232422.GD1853@quark> References: <20251204162815.522879-2-ardb@kernel.org> <20251205064809.GA26371@sol> <20251207013004.GA143349@sol> Precedence: bulk X-Mailing-List: linux-crypto@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: On Sun, Dec 07, 2025 at 10:59:29AM +0100, Ard Biesheuvel wrote: > On Sun, 7 Dec 2025 at 02:30, Eric Biggers wrote: > > > > On Fri, Dec 05, 2025 at 09:13:46AM +0100, Ard Biesheuvel wrote: > > > On Fri, 5 Dec 2025 at 07:50, Eric Biggers wrote: > > > > > > > > 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. > > > > > > > > > > How about > > > > > > --- a/arch/arm64/include/asm/simd.h > > > +++ b/arch/arm64/include/asm/simd.h > > > @@ -48,6 +48,8 @@ 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_ksimd(__UNIQUE_ID(fpsimd_state)) > > > +#define __scoped_ksimd(id) struct user_fpsimd_state __uninitialized id; \ > > > + scoped_guard(ksimd, &id) > > > > I guess that will work. It's not great that it will make scoped_ksimd() > > expand into more than one statement, which is error-prone and not > > normally allowed in macros. But it looks okay for all the current users > > of it. > > > > We could always repeat the 'for()' trick that the cleanup helpers use, e.g., > > for (struct user_fpsimd_state __uninitialized __st; true; ({ goto label; })) > if (0) { > label: break; > } else scoped_guard(ksimd, &__st) > > Would you prefer that? Hmm, I didn't consider using nested 'for' statements. It looks like that should solve the problem. It makes the implementation a bit harder to understand, but at least the 'for' statement trick isn't new... Could you send a patch? Preferably with a clear comment that documents why it's done this way. - Eric