From: Willy Tarreau Date: Sat, 29 Dec 2018 18:02:18 +0000 (+0100) Subject: rcutorture/nolibc: Add a bit of documentation to explain how to use nolibc X-Git-Tag: v5.1-rc1~166^2^2^6~3 X-Git-Url: https://asedeno.scripts.mit.edu/gitweb/?a=commitdiff_plain;h=cc72a50994b4910e36445d750b2749b86c37d32b;p=linux.git rcutorture/nolibc: Add a bit of documentation to explain how to use nolibc 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 Signed-off-by: Paul E. McKenney Reviewed-by: Joey Pabalinas Reviewed-by: Randy Dunlap --- diff --git a/tools/testing/selftests/rcutorture/bin/nolibc.h b/tools/testing/selftests/rcutorture/bin/nolibc.h index cfbbbad4bca4..1708e9f9f8aa 100644 --- a/tools/testing/selftests/rcutorture/bin/nolibc.h +++ b/tools/testing/selftests/rcutorture/bin/nolibc.h @@ -3,7 +3,85 @@ * Copyright (C) 2017-2018 Willy Tarreau */ -/* some archs (at least aarch64) don't expose the regular syscalls anymore by +/* + * 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;