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=-6.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_PASS 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 30772C43387 for ; Sat, 29 Dec 2018 22:35:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E5BCB20869 for ; Sat, 29 Dec 2018 22:35:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="AOIYtw7m" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727798AbeL2Wfi (ORCPT ); Sat, 29 Dec 2018 17:35:38 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:42490 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725997AbeL2Wfi (ORCPT ); Sat, 29 Dec 2018 17:35:38 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=Content-Transfer-Encoding: Content-Type:In-Reply-To:MIME-Version:Date:Message-ID:From:References:Cc:To: Subject:Sender:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=1ebKAZTzEZQGRt1qeZY4niOz6ef633S3rOfC5kpzNC8=; b=AOIYtw7mun+E5n8WSRs06HZza xUMX9FwlA4ZJRViTHasNhVchc9huJ2H8WwJrACxQSnxafhckPzRd0/FXdwuBdwO25Oc1/DeN3JJDh hYHboUeUlR3Esb8F9e4Yd+di2haXtPrwWGR78otLqLpXwIdzi3shVK7ADCM2BaNcLjFB7GWj9Y7d8 voMA137Zcl8xPXMPMxRxdUHH8Un9oYr58SoZnKYpTLpCi1+faaP8n/BbIUJWKtiW0vSbshuEmHOq9 2/5oEc2YDIlIXX0q7yFjxDMwS1vlXbzckTdhtF1CvqSEU9SUHbCyRpMQoGIuj5vgone9NLiCaI4K2 vj4v9wGeQ==; Received: from static-50-53-52-16.bvtn.or.frontiernet.net ([50.53.52.16] helo=midway.dunlab) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1gdNCj-0003DT-6i; Sat, 29 Dec 2018 22:35:37 +0000 Subject: Re: [PATCH 3/4] rcutorture/nolibc: add a bit of documentation to explain how to use nolibc To: Willy Tarreau , mingo@kernel.org, paulmck@linux.vnet.ibm.com Cc: linux-kernel@vger.kernel.org References: <20181229180218.15798-1-w@1wt.eu> <20181229180218.15798-4-w@1wt.eu> From: Randy Dunlap Message-ID: Date: Sat, 29 Dec 2018 14:35:34 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.0 MIME-Version: 1.0 In-Reply-To: <20181229180218.15798-4-w@1wt.eu> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/29/18 10:02 AM, Willy Tarreau wrote: > Ingo rightfully asked for a bit more documentation in the nolibc header, > so this patch adds some explanation about its purpose, how it's made, and > how to use it. > > Cc: Ingo Molnar > Cc: Paul E. McKenney > Cc: Randy Dunlap > Signed-off-by: Willy Tarreau Reviewed-by: Randy Dunlap Thanks. > --- > tools/testing/selftests/rcutorture/bin/nolibc.h | 90 +++++++++++++++++++++---- > 1 file changed, 78 insertions(+), 12 deletions(-) > > diff --git a/tools/testing/selftests/rcutorture/bin/nolibc.h b/tools/testing/selftests/rcutorture/bin/nolibc.h > index 985364c..6643ba9 100644 > --- a/tools/testing/selftests/rcutorture/bin/nolibc.h > +++ b/tools/testing/selftests/rcutorture/bin/nolibc.h > @@ -3,6 +3,84 @@ > * Copyright (C) 2017-2018 Willy Tarreau > */ > > +/* > + * This file is designed to be used as a libc alternative for minimal programs > + * with very limited requirements. It consists of a small number of syscall and > + * type definitions, and the minimal startup code needed to call main(). > + * All syscalls are declared as static functions so that they can be optimized > + * away by the compiler when not used. > + * > + * Syscalls are split into 3 levels: > + * - the lower level is the arch-specific syscall() definition, consisting in > + * assembly code in compound expressions. These are called my_syscall0() to > + * my_syscall6() depending on the number of arguments. The MIPS > + * implementation is limited to 5 arguments. All input arguments are cast > + * to a long stored in a register. These expressions always return the > + * syscall's return value as a signed long value which is often either a > + * pointer or the negated errno value. > + * > + * - the second level is mostly architecture-independent. It is made of > + * static functions called sys_() which rely on my_syscallN() > + * depending on the syscall definition. These functions are responsible > + * for exposing the appropriate types for the syscall arguments (int, > + * pointers, etc) and for setting the appropriate return type (often int). > + * A few of them are architecture-specific because the syscalls are not all > + * mapped exactly the same among architectures. For example, some archs do > + * not implement select() and need pselect6() instead, so the sys_select() > + * function will have to abstract this. > + * > + * - the third level is the libc call definition. It exposes the lower raw > + * sys_() calls in a way that looks like what a libc usually does, > + * takes care of specific input values, and of setting errno upon error. > + * There can be minor variations compared to standard libc calls. For > + * example the open() call always takes 3 args here. > + * > + * The errno variable is declared static and unused. This way it can be > + * optimized away if not used. However this means that a program made of > + * multiple C files may observe different errno values (one per C file). For > + * the type of programs this project targets it usually is not a problem. The > + * resulting program may even be reduced by defining the NOLIBC_IGNORE_ERRNO > + * macro, in which case the errno value will never be assigned. > + * > + * Some stdint-like integer types are defined. These are valid on all currently > + * supported architectures, because signs are enforced, ints are assumed to be > + * 32 bits, longs the size of a pointer and long long 64 bits. If more > + * architectures have to be supported, this may need to be adapted. > + * > + * Some macro definitions like the O_* values passed to open(), and some > + * structures like the sys_stat struct depend on the architecture. > + * > + * The definitions start with the architecture-specific parts, which are picked > + * based on what the compiler knows about the target architecture, and are > + * completed with the generic code. Since it is the compiler which sets the > + * target architecture, cross-compiling normally works out of the box without > + * having to specify anything. > + * > + * Finally some very common libc-level functions are provided. It is the case > + * for a few functions usually found in string.h, ctype.h, or stdlib.h. Nothing > + * is currently provided regarding stdio emulation. > + * > + * The macro NOLIBC is always defined, so that it is possible for a program to > + * check this macro to know if it is being built against and decide to disable > + * some features or simply not to include some standard libc files. > + * > + * Ideally this file should be split in multiple files for easier long term > + * maintenance, but provided as a single file as it is now, it's quite > + * convenient to use. Maybe some variations involving a set of includes at the > + * top could work. > + * > + * A simple static executable may be built this way : > + * $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \ > + * -static -include nolibc.h -lgcc -o hello hello.c > + * > + * A very useful calling convention table may be found here : > + * http://man7.org/linux/man-pages/man2/syscall.2.html > + * > + * This doc is quite convenient though not necessarily up to date : > + * https://w3challs.com/syscalls/ > + * > + */ > + > /* some archs (at least aarch64) don't expose the regular syscalls anymore by > * default, either because they have an "_at" replacement, or because there are > * more modern alternatives. For now we'd rather still use them. > @@ -19,18 +97,6 @@ > > #define NOLIBC > > -/* Build a static executable this way : > - * $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \ > - * -static -include nolibc.h -lgcc -o hello hello.c > - * > - * Useful calling convention table found here : > - * http://man7.org/linux/man-pages/man2/syscall.2.html > - * > - * This doc is even better : > - * https://w3challs.com/syscalls/ > - */ > - > - > /* this way it will be removed if unused */ > static int errno; > > -- ~Randy