From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 947DF1A0B15; Sun, 20 Sep 2026 03:46:42 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789876003; cv=none; b=JozroZQhK+jNCY7AbZ/Xw3QYkcVChe79P54u+1xtvg5IvytOjtTzmGGfc13l+6dYPBavQaDCm5M63/7ziAeZKKtWsr2r2Btd06eFgOiG+w2E3lYZ3D4nSP/ZksmgOAutot94KtGBeVwpD1pPOlefQNFzbIez572/6+l7gb0l1h4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789876003; c=relaxed/simple; bh=ZbWAqJRTY3VosimGz01P04wQqhvs31GplZgQSEgfbFw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=pFQCSaknNLTw92BKf+9EYPPu8NPFsrDG4Rg75OjW8VyWPgf6N3Gv1jpBEo5PPHRl1UCE9128nhralT6yjjYTLHmLLOPoa2DZSlSwq+3+4fn2cqXtVqXajaCBuzDBsT/GHc7tioHzcLR0szWuScLtkx2IMTMYvvDVLO1pYVYHjj4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Pq1c/x2e; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Pq1c/x2e" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 499CE1F00893; Sun, 20 Sep 2026 03:46:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789876002; bh=RAgjdNuY0GHwXBswA5NShTSJab25cAeZ2wGxG0I/vUo=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=Pq1c/x2eNDaaA3FhHkOhGuCnsEwXECeh01hVEHOpDv8HFPU3D8PLxh2wVU0nylDaJ 7EaPp5slBBZUiOqsjhzEs7IgscKJPgdM3bXnpCs8IVuHJRaG5hD0rrtfwkhInaUah2 UEwXWAbH8m3JDS27foafZhT7dYncShEItD34AqEMdXClbdFXPJauYFWvvxe4hWYd96 N8ZnwsnLY9Xqn+sIDhK7K9lW5wcmPeM+9vJTKsXnZtJMmVVGCpp7vNL+wlt5H2W4zk L6Y0Pud3rjcJlvf9NtFTivJRz4pQVpslkC4pi8p+/DINuR9TeguqodaZGqQrIxp572 Ic8aE8TCrC2FA== Date: Sat, 19 Sep 2026 20:46:42 -0700 From: Kees Cook To: "Lorenzo Stoakes (ARM)" Cc: Linus Torvalds , Nathan Chancellor , Nicolas Schier , Nick Desaulniers , Bill Wendling , Justin Stitt , Masahiro Yamada , Alexey Gladkov , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Arnd Bergmann , Catalin Marinas , Will Deacon , Mark Rutland , Ard Biesheuvel , Ilias Apalodimas , Josh Poimboeuf , Peter Zijlstra , Miguel Ojeda , Boqun Feng , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Daniel Almeida , Tamir Duberstein , Alexandre Courbot , Onur =?iso-8859-1?Q?=D6zkan?= , Jonathan Corbet , Randy Dunlap , "Gustavo A. R. Silva" , linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, linux-riscv@lists.infradead.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-efi@vger.kernel.org, rust-for-linux@vger.kernel.org, linux-doc@vger.kernel.org, Jens Axboe , linux-hardening@vger.kernel.org Subject: Re: [PATCH v3 09/20] kbuild: implement and use depcheck to check dependency timestamps Message-ID: <202609192045.24A5A4E876@keescook> References: <20260917-build-speedup-v3-0-9ecf4163ff36@kernel.org> <20260917-build-speedup-v3-9-9ecf4163ff36@kernel.org> <202609171117.35BFDCC13@keescook> <202609171229.31BB336E1@keescook> Precedence: bulk X-Mailing-List: linux-kernel@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 Sat, Sep 19, 2026 at 06:02:46PM +0100, Lorenzo Stoakes (ARM) wrote: > On Thu, Sep 17, 2026 at 01:59:26PM -0700, Kees Cook wrote: > > So, yeah, it makes me nervous, and it may make incremental builds less > > idempotent. But I can't really find extant problems and the demo is > > slightly contrived, but not exactly an impossible situation. > > Ack thanks for this! > > There is definitely an issue here, your example is fine in the stock kernel > but one run late in v3. > > Solution is to have depcheck treat any dependency that is itself a target > of the same run as never being 'fresh' (it already reads every .cmd file in > the directory so has all the dependency information it needs), so that's > fixed in v4. > > Confirmed that it fixes the demo'd bug here, that touching a header > rebuilds exactly the same objects as it did before and that noop build perf > was not harmed by the change. Oh very nice! Thank you for finding a solution there. :) -- Kees Cook