From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793748374; b=Xs43gKNEbygYc4vT5U9p2XdT7dxsvLrMlQ1IIU60IUKxSDhGGIkmZ2pkulCAmm3kHKAQ 73bENGiixjVkROWOI7ix4G6PZ+/+MK2n+FzxhbxRCD8S5kPqSbz2hxG5k9DoP9uTuM8bl z4IlFUovHt97n61B+xi1h0eZi3q3JiQy/0yy5O1DkIhJti2iljSzjcBbWvttLm+BAujVs Rd8sZz2rQ/W2s53Fs3YOS/f5n/Ae+eWL/Lcvjg7L0q3eGmNG13Nx1zvvbf4GSaj361BN4 yVSaDMLB7wb8Lhlq6Ut6DGgAV3/mCahSiCmZJt/J94nM/i9hhsHlYW03zHJukeGZy3Q== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793748374; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=a2HSfSPgf9NaCTt4G+ji84x3KpqIbwOOtGbhHXN9F1g=; b=qFnU0P36nDSzppf0PT3NoJBUgZ8GKgIxdMOLHqhFAi2gTVy0WpTOHqGVl6urndnZUcFe JIQsGygT9dvk8+O6nCjlpwfVtfaW1/HFljQRtLJQJ/qdCLL7AC5bNowbwkEIoOVGqnv3j rK0DB3gooeU/q1ARFlOvq/W4PeD7+E7WI1uZuES5xkVCQy9bwuZAPuA/svRBpUOjC4bfv XCLx3AvTlLme+xUbRE2Uazkym1N+Pg1uipspbNrAbi0bkhvy15cJJOSZgWnTaMCusvgWO X79G2cpYEiPQ7iQm2SPVEOYyfwKg0snYrlqr6WY0eVfnyrvZBsI1mC9qz1Zerqn+IbA== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=VYyM4G53; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B2FC3606A32C for ; Tue, 3 Nov 2026 23:26:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=VYyM4G53 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 257BF1F0093A; Tue, 3 Nov 2026 23:26:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793748373; bh=a2HSfSPgf9NaCTt4G+ji84x3KpqIbwOOtGbhHXN9F1g=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=VYyM4G53lWze9hqCTBDUCnJIHR9VPZtKOzTE/7mhLozJkCoRtJWd06ykfXGja7INv dOCRIojLJRJlooI+3Y9jQZgXFbADGKUqRTvGooGObTvyyXCGj12y8lSuCMiZtruVfZ Q63NkS/ydcW6Ayd0QuVwSZI7o66GQPTMGZmhTlM6pnfjtDKzM0eM+TMLHmBSOCORXC GYTXxr//Pwbor1MXWlMssoh4d8wQLjSH51MLdwbLtrMqA6S3l8i2QBNSzDTKf1YVCg 7/uH+W5v59/al8ZRASTRJ4f4mjcjRoJg3b+qUmFGIggsEPJzb3ICOPjj7wXIVj8Ew7 xcXBEhXiunvTQ== Date: Tue, 3 Nov 2026 15:26:11 -0800 From: Eric Biggers To: linux-crypto@vger.kernel.org Cc: Ard Biesheuvel , "Jason A . Donenfeld" , Herbert Xu Subject: Re: [PATCH] lib/crypto: poly1305: Hide the mask value from compilers Message-ID: <20261103232611.GG2281@quark> References: <20261031042637.68815-1-ebiggers@kernel.org> X-Mailing-List: linux-crypto@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261031042637.68815-1-ebiggers@kernel.org> On Fri, Oct 30, 2026 at 09:26:37PM -0700, Eric Biggers wrote: > poly1305_core_emit() does a constant-time select between h and h - p, > where h is the accumulator and p is the Poly1305 prime of 2^130 - 5. > > The mask used in this select isn't hidden from the compiler, though, so > the compiler can infer that it's either all 0's or all 1's. > > In theory, the compiler could then use that to recognize the select, and > "optimize" it with a branch. > > Fortunately, in practice, I couldn't find any gcc or clang options that > made that happen. > > Regardless, add OPTIMIZER_HIDE_VAR() to "hide" the value of the mask for > safety against any future compiler optimizations. > > Signed-off-by: Eric Biggers > --- > lib/crypto/poly1305-donna32.c | 1 + > lib/crypto/poly1305-donna64.c | 1 + > 2 files changed, 2 insertions(+) Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-next - Eric