112 lines
5.1 KiB
C
112 lines
5.1 KiB
C
/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
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/* nolibc.h
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* Copyright (C) 2017-2018 Willy Tarreau <w@1wt.eu>
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*/
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/*
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* This file is designed to be used as a libc alternative for minimal programs
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* with very limited requirements. It consists of a small number of syscall and
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* type definitions, and the minimal startup code needed to call main().
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* All syscalls are declared as static functions so that they can be optimized
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* away by the compiler when not used.
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*
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* Syscalls are split into 3 levels:
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* - The lower level is the arch-specific syscall() definition, consisting in
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* assembly code in compound expressions. These are called my_syscall0() to
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* my_syscall6() depending on the number of arguments. The MIPS
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* implementation is limited to 5 arguments. All input arguments are cast
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* to a long stored in a register. These expressions always return the
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* syscall's return value as a signed long value which is often either a
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* pointer or the negated errno value.
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*
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* - The second level is mostly architecture-independent. It is made of
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* static functions called sys_<name>() which rely on my_syscallN()
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* depending on the syscall definition. These functions are responsible
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* for exposing the appropriate types for the syscall arguments (int,
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* pointers, etc) and for setting the appropriate return type (often int).
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* A few of them are architecture-specific because the syscalls are not all
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* mapped exactly the same among architectures. For example, some archs do
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* not implement select() and need pselect6() instead, so the sys_select()
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* function will have to abstract this.
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*
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* - The third level is the libc call definition. It exposes the lower raw
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* sys_<name>() calls in a way that looks like what a libc usually does,
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* takes care of specific input values, and of setting errno upon error.
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* There can be minor variations compared to standard libc calls. For
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* example the open() call always takes 3 args here.
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*
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* The errno variable is declared static and unused. This way it can be
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* optimized away if not used. However this means that a program made of
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* multiple C files may observe different errno values (one per C file). For
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* the type of programs this project targets it usually is not a problem. The
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* resulting program may even be reduced by defining the NOLIBC_IGNORE_ERRNO
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* macro, in which case the errno value will never be assigned.
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*
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* Some stdint-like integer types are defined. These are valid on all currently
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* supported architectures, because signs are enforced, ints are assumed to be
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* 32 bits, longs the size of a pointer and long long 64 bits. If more
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* architectures have to be supported, this may need to be adapted.
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*
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* Some macro definitions like the O_* values passed to open(), and some
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* structures like the sys_stat struct depend on the architecture.
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*
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* The definitions start with the architecture-specific parts, which are picked
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* based on what the compiler knows about the target architecture, and are
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* completed with the generic code. Since it is the compiler which sets the
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* target architecture, cross-compiling normally works out of the box without
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* having to specify anything.
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*
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* Finally some very common libc-level functions are provided. It is the case
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* for a few functions usually found in string.h, ctype.h, or stdlib.h.
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*
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* The nolibc.h file is only a convenient entry point which includes all other
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* files. It also defines the NOLIBC macro, so that it is possible for a
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* program to check this macro to know if it is being built against and decide
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* to disable some features or simply not to include some standard libc files.
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*
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* A simple static executable may be built this way :
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* $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
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* -static -include nolibc.h -o hello hello.c -lgcc
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*
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* Simple programs meant to be reasonably portable to various libc and using
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* only a few common includes, may also be built by simply making the include
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* path point to the nolibc directory:
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* $ gcc -fno-asynchronous-unwind-tables -fno-ident -s -Os -nostdlib \
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* -I../nolibc -o hello hello.c -lgcc
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*
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* The available standard (but limited) include files are:
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* ctype.h, errno.h, signal.h, stdio.h, stdlib.h, string.h, time.h
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*
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* In addition, the following ones are expected to be provided by the compiler:
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* float.h, stdarg.h, stddef.h
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*
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* The following ones which are part to the C standard are not provided:
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* assert.h, locale.h, math.h, setjmp.h, limits.h
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*
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* A very useful calling convention table may be found here :
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* http://man7.org/linux/man-pages/man2/syscall.2.html
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*
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* This doc is quite convenient though not necessarily up to date :
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* https://w3challs.com/syscalls/
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*
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*/
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#ifndef _NOLIBC_H
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#define _NOLIBC_H
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#include "std.h"
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#include "arch.h"
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#include "types.h"
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#include "sys.h"
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#include "ctype.h"
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#include "signal.h"
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#include "stdio.h"
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#include "stdlib.h"
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#include "string.h"
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#include "time.h"
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#include "unistd.h"
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/* Used by programs to avoid std includes */
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#define NOLIBC
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#endif /* _NOLIBC_H */
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