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 X-Spam-Level: X-Spam-Status: No, score=-13.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E51C5C61DD8 for ; Mon, 16 Nov 2020 16:23:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8E79822265 for ; Mon, 16 Nov 2020 16:23:08 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="RnBgIZuP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732018AbgKPQXB (ORCPT ); Mon, 16 Nov 2020 11:23:01 -0500 Received: from mail.kernel.org ([198.145.29.99]:52426 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731859AbgKPQXA (ORCPT ); Mon, 16 Nov 2020 11:23:00 -0500 Received: from localhost.localdomain (NE2965lan1.rev.em-net.ne.jp [210.141.244.193]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 5FC9722263; Mon, 16 Nov 2020 16:22:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1605543779; bh=Te2tFxY1BOS4uysqvA7qp8kR0pfzUS27egjsYkCWDd4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=RnBgIZuPot9Cb2GQ0+l/sN1bbNyreTLkgw0ifz88NRTZ5C6c9oH4aYzGXCVpYnfXz UKIDjDZBU7M5BBi5IW0Zl1GYVrtrOvmVbuP9TMazXP3OfHATD61H4+n4nR/T1H2kbE Hf33rOgEZjrcA6onW5f5BGClDUn1LvBjtdbK1Jfo= From: Masami Hiramatsu To: Steven Rostedt , Linus Torvalds Cc: Chen Yu , Chen Yu , Masami Hiramatsu , LKML , Ingo Molnar Subject: [PATCH v2 2/2] Documentation: bootconfig: Update file format on initrd image Date: Tue, 17 Nov 2020 01:22:55 +0900 Message-Id: <160554377552.96595.9872059645320329905.stgit@devnote2> X-Mailer: git-send-email 2.25.1 In-Reply-To: <160554375807.96595.16142008590130221699.stgit@devnote2> References: <160554375807.96595.16142008590130221699.stgit@devnote2> User-Agent: StGit/0.19 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org To align the total file size, add padding null character when appending the bootconfig to initrd image. Signed-off-by: Masami Hiramatsu --- 0 files changed diff --git a/Documentation/admin-guide/bootconfig.rst b/Documentation/admin-guide/bootconfig.rst index a22024f9175e..363599683784 100644 --- a/Documentation/admin-guide/bootconfig.rst +++ b/Documentation/admin-guide/bootconfig.rst @@ -137,15 +137,22 @@ Boot Kernel With a Boot Config ============================== Since the boot configuration file is loaded with initrd, it will be added -to the end of the initrd (initramfs) image file with size, checksum and -12-byte magic word as below. +to the end of the initrd (initramfs) image file with padding, size, +checksum and 12-byte magic word as below. -[initrd][bootconfig][size(u32)][checksum(u32)][#BOOTCONFIG\n] +[initrd][bootconfig][padding][size(u32)][checksum(u32)][#BOOTCONFIG\n] + +When the boot configuration is added to the initrd image, the total +file size is aligned to 4 bytes. To fill the gap, null characters +(``\0``) will be added. Thus the ``size`` is the length of the bootconfig +file + padding bytes. The Linux kernel decodes the last part of the initrd image in memory to get the boot configuration data. Because of this "piggyback" method, there is no need to change or -update the boot loader and the kernel image itself. +update the boot loader and the kernel image itself as long as the boot +loader passes the correct initrd file size. If by any chance, the boot +loader passes a longer size, the kernel feils to find the bootconfig data. To do this operation, Linux kernel provides "bootconfig" command under tools/bootconfig, which allows admin to apply or delete the config file @@ -176,7 +183,8 @@ up to 512 key-value pairs. If keys contains 3 words in average, it can contain 256 key-value pairs. In most cases, the number of config items will be under 100 entries and smaller than 8KB, so it would be enough. If the node number exceeds 1024, parser returns an error even if the file -size is smaller than 32KB. +size is smaller than 32KB. (Note that this maximum size is not including +the padding null characters.) Anyway, since bootconfig command verifies it when appending a boot config to initrd image, user can notice it before boot.