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Subject: Re: [PATCH 3/4] rcutorture/nolibc: add a bit of documentation to
 explain how to use nolibc
To:     Willy Tarreau <w@1wt.eu>, 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 <rdunlap@infradead.org>
Message-ID: <c0a4b052-1dec-6923-8a1f-2c2870abc767@infradead.org>
Date:   Sat, 29 Dec 2018 14:35:34 -0800
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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 <mingo@kernel.org>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Signed-off-by: Willy Tarreau <w@1wt.eu>

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>

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 <w@1wt.eu>
>   */
>  
> +/*
> + * 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_<name>() 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_<name>() 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