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趣谈Linux操作系统随笔——4.0 系统调用：公司成立好了就要开始接项目

2020-11-24 12:04:42 作者：互联网

系统调用：公司成立好了就要开始接项目

软件平台：运行于VMware Workstation 12 Player下UbuntuLTS16.04_x64 系统
开发环境：Linux-4.19-rc3内核，glibc-2.9

1、系统调用的封装——glibc

以open()函数为例子，进行源码分析，由于我们是在linux基础上进行分析，所以所在的文件目录结构为：sysdeps\unix

在这个目录下有**syscalls.list，里面列着所有glibc的函数对应的系统调用**：

# File name	Caller（系统调用文件名字） Syscall name（系统调用名）	Args	Strong name	Weak names

# ...... #
open		-	open		Ci:siv	__libc_open __open open
# ...... #

同时还有一个脚本文件make-syscall.sh（文件包装器），根据当前目录下的syscalls.list文件，自动的对于每一个封装好的系统调用，生成一个文件，其中包装的规则在syscall-template.S，open()就自动生成了名为open.c的文件

#!/bin/sh

# Usage: make-syscalls.sh ../sysdeps/unix/common
# Expects $sysdirs in environment.

##############################################################################
#
# 这个脚本用于处理在各种系统调用中编码的系统调用数据。列出用于围绕适当的OS系统调用生成精简程序集系统调用包装器的文件。
# 看到syscall-template.S。更多细节在实际的包装。
#
# Syscall Signature Prefixes:
#
# E: errno and return value are not set by the call
# V: errno is not set, but errno or zero (success) is returned from the call
#
# Syscall Signature Key Letters:
#
# a: unchecked address (e.g., 1st arg to mmap)
# b: non-NULL buffer (e.g., 2nd arg to read; return value from mmap)
# B: optionally-NULL buffer (e.g., 4th arg to getsockopt)
# f: buffer of 2 ints (e.g., 4th arg to socketpair)
# F: 3rd arg to fcntl
# i: scalar (any signedness & size: int, long, long long, enum, whatever)
# I: 3rd arg to ioctl
# n: scalar buffer length (e.g., 3rd arg to read)
# N: pointer to value/return scalar buffer length (e.g., 6th arg to recvfrom)
# p: non-NULL pointer to typed object (e.g., any non-void* arg)
# P: optionally-NULL pointer to typed object (e.g., 2nd argument to gettimeofday)
# s: non-NULL string (e.g., 1st arg to open)
# S: optionally-NULL string (e.g., 1st arg to acct)
# v: vararg scalar (e.g., optional 3rd arg to open)
# V: byte-per-page vector (3rd arg to mincore)
# W: wait status, optionally-NULL pointer to int (e.g., 2nd arg of wait4)
#

##############################################################################

thisdir=$1; shift

echo ''
echo \#### DIRECTORY = $thisdir
# 检查每个比这个优先级更高的sysdep dir，并从$调用中删除在其他dir中找到的所有函数，当我们到达定义这些系统调用的目录时
sysdirs=`for dir in $sysdirs; do
	 test $dir = $thisdir && break; echo $dir; done`
echo \#### SYSDIRS = $sysdirs

# 在当前目录下获取系统调用列表
calls=`sed 's/#.*$//
/^[ 	]*$/d' $thisdir/syscalls.list`

calls=`echo "$calls" |
while read file caller rest; do
  # 删除由$dir中的文件实现的每个系统调用。
  # 如果一个系统调用指定了一个“调用者”，那么只有当调用者函数也在这个目录中实现时才编译那个系统调用。
  srcfile=-;
  for dir in $sysdirs; do
     { test -f $dir/$file.c && srcfile=$dir/$file.c; } ||
     { test -f $dir/$file.S && srcfile=$dir/$file.S; } ||
     { test x$caller != x- &&
	{ { test -f $dir/$caller.c && srcfile=$dir/$caller.c; } ||
	  { test -f $dir/$caller.S && srcfile=$dir/$caller.S; }; }; } && break;
  done;
  echo $file $srcfile $caller $rest;
done`

# Any calls left?
test -n "$calls" || exit 0

# This uses variables $weak, $strong, and $any_versioned.
emit_weak_aliases()
{
  # A shortcoming in the current gas is that it will only allow one
  # version-alias per symbol.  So we create new strong aliases as needed.
  vcount=""

  # We use the <shlib-compat.h> macros to generate the versioned aliases
  # so that the version sets can be mapped to the configuration's
  # minimum version set as per shlib-versions DEFAULT lines.  If an
  # entry point is specified in the form NAME@VERSION:OBSOLETED, a
  # SHLIB_COMPAT conditional is generated.
  if [ $any_versioned = t ]; then
    echo "	 echo '#include <shlib-compat.h>'; \\"
  fi

  for name in $weak; do
    case $name in
      *@@*)
	base=`echo $name | sed 's/@@.*//'`
	ver=`echo $name | sed 's/.*@@//;s/\./_/g'`
	echo "	 echo '#if IS_IN (libc)'; \\"
	if test -z "$vcount" ; then
	  source=$strong
	  vcount=1
	else
	  source="${strong}_${vcount}"
	  vcount=`expr $vcount + 1`
	  echo "	 echo 'strong_alias ($strong, $source)'; \\"
	fi
	echo "	 echo 'versioned_symbol (libc, $source, $base, $ver)'; \\"
	echo "	 echo '#else'; \\"
	echo "	 echo 'weak_alias ($strong, $base)'; \\"
	echo "	 echo '#endif'; \\"
	;;
      *@*)
	base=`echo $name | sed 's/@.*//'`
	ver=`echo $name | sed 's/.*@//;s/\./_/g'`
	case $ver in
	  *:*)
	    compat_ver=${ver#*:}
	    ver=${ver%%:*}
	    compat_cond=" && SHLIB_COMPAT (libc, $ver, $compat_ver)"
	    ;;
	  *)
	    compat_cond=
	    ;;
	esac
	echo "	 echo '#if defined SHARED && IS_IN (libc)$compat_cond'; \\"
	if test -z "$vcount" ; then
	  source=$strong
	  vcount=1
	else
	  source="${strong}_${vcount}"
	  vcount=`expr $vcount + 1`
	  echo "	 echo 'strong_alias ($strong, $source)'; \\"
	fi
	echo "	 echo 'compat_symbol (libc, $source, $base, $ver)'; \\"
	echo "	 echo '#endif'; \\"
	;;
      !*)
	name=`echo $name | sed 's/.//'`
	echo "	 echo 'strong_alias ($strong, $name)'; \\"
	echo "	 echo 'hidden_def ($name)'; \\"
	;;
      *)
	echo "	 echo 'weak_alias ($strong, $name)'; \\"
	echo "	 echo 'hidden_weak ($name)'; \\"
	;;
    esac
  done
}


# 发出规则来编译剩余的系统调用$calls.
echo "$calls" |
while read file srcfile caller syscall args strong weak; do

  vdso_syscall=
  case x"$syscall" in
  *:*@*)
    vdso_syscall="${syscall#*:}"
    syscall="${syscall%:*}"
    ;;
  esac

  case x"$syscall" in
  x-) callnum=_ ;;
  *)
  # 判断$syscall是否在syscall.h中定义了一个数字
  callnum=-
  eval `{ echo "#include <sysdep.h>";
	echo "callnum=SYS_ify ($syscall)"; } |
	  $asm_CPP -D__OPTIMIZE__ - |
	  sed -n -e "/^callnum=.*$syscall/d" \
		 -e "/^\(callnum=\)[ 	]*\(.*\)/s//\1'\2'/p"`
  ;;
  esac

  noerrno=0
  errval=0
  case $args in
  E*) noerrno=1; args=`echo $args | sed 's/E:\?//'`;;
  V*) errval=1; args=`echo $args | sed 's/V:\?//'`;;
  esac

  # 根据信息派生出参数的数目
  case $args in
  [0-9]) nargs=$args;;
  ?:) nargs=0;;
  ?:?) nargs=1;;
  ?:??) nargs=2;;
  ?:???) nargs=3;;
  ?:????) nargs=4;;
  ?:?????) nargs=5;;
  ?:??????) nargs=6;;
  ?:???????) nargs=7;;
  ?:????????) nargs=8;;
  ?:?????????) nargs=9;;
  esac

  # Make sure only the first syscall rule is used, if multiple dirs
  # define the same syscall.
  echo ''
  echo "#### CALL=$file NUMBER=$callnum ARGS=$args SOURCE=$srcfile"

  # 如果多个dirs定义相同的系统调用，请确保只使用第一个系统调用规则
  any_versioned=f
  shared_only=f
  case $weak in
    *@@*) any_versioned=t ;;
    *@*) any_versioned=t shared_only=t ;;
  esac

 case x$srcfile"$callnum" in
 x--)
  # 额外系统调用的未定义调用。
  if [ x$caller != x- ]; then
    if [ $noerrno != 0 ]; then
      echo >&2 "$0: no number for $fileno, no-error syscall ($strong $weak)"
      exit 2
    fi
    echo "unix-stub-syscalls += $strong $weak"
  fi
  ;;
 x*-) ;; ### 对于未定义的调用不做任何事情
 x-*)
  echo "ifeq (,\$(filter $file,\$(unix-syscalls)))"

  if test $shared_only = t; then
    # The versioned symbols are only in the shared library.
    echo "ifneq (,\$(filter .os,\$(object-suffixes)))"
  fi
  # Accumulate the list of syscall files for this directory.
  echo "unix-syscalls += $file"
  test x$caller = x- || echo "unix-extra-syscalls += $file"

  # Emit a compilation rule for this syscall.
  if test $shared_only = t; then
    # The versioned symbols are only in the shared library.
    echo "shared-only-routines += $file"
    test -n "$vdso_syscall" || echo "\$(objpfx)${file}.os: \\"
  else
    object_suffixes='$(object-suffixes)'
    test -z "$vdso_syscall" || object_suffixes='$(object-suffixes-noshared)'
    echo "\
\$(foreach p,\$(sysd-rules-targets),\
\$(foreach o,${object_suffixes},\$(objpfx)\$(patsubst %,\$p,$file)\$o)): \\"
  fi

  echo "		\$(..)sysdeps/unix/make-syscalls.sh"
  case x"$callnum" in
  x_)
  echo "\
	\$(make-target-directory)
	(echo '/* Dummy module requested by syscalls.list */'; \\"
  ;;
  x*)
  echo "\
	\$(make-target-directory)
	(echo '#define SYSCALL_NAME $syscall'; \\
	 echo '#define SYSCALL_NARGS $nargs'; \\
	 echo '#define SYSCALL_SYMBOL $strong'; \\
	 echo '#define SYSCALL_NOERRNO $noerrno'; \\
	 echo '#define SYSCALL_ERRVAL $errval'; \\
	 echo '#include <syscall-template.S>'; \\"
  ;;
  esac

  # Append any weak aliases or versions defined for this syscall function.
  emit_weak_aliases

  # And finally, pipe this all into the compiler.
  echo '	) | $(compile-syscall) '"\
\$(foreach p,\$(patsubst %$file,%,\$(basename \$(@F))),\$(\$(p)CPPFLAGS))"

  if test -n "$vdso_syscall"; then
    # In the shared library, we're going to emit an IFUNC using a vDSO function.
    # $vdso_syscall looks like "name@KERNEL_X.Y" where "name" is the symbol
    # name in the vDSO and KERNEL_X.Y is its symbol version.
    vdso_symbol="${vdso_syscall%@*}"
    vdso_symver="${vdso_syscall#*@}"
    vdso_symver=`echo "$vdso_symver" | sed 's/\./_/g'`
    cat <<EOF

\$(foreach p,\$(sysd-rules-targets),\$(objpfx)\$(patsubst %,\$p,$file).os): \\
		\$(..)sysdeps/unix/make-syscalls.sh
	\$(make-target-directory)
	(echo '#define ${strong} __redirect_${strong}'; \\
	 echo '#include <dl-vdso.h>'; \\
	 echo '#undef ${strong}'; \\
	 echo '#define vdso_ifunc_init() \\'; \\
	 echo '  PREPARE_VERSION_KNOWN (symver, ${vdso_symver})'; \\
	 echo '__ifunc (__redirect_${strong}, ${strong},'; \\
	 echo '         _dl_vdso_vsym ("${vdso_symbol}", &symver), void,'; \\
	 echo '         vdso_ifunc_init)'; \\
EOF
    # This is doing "hidden_def (${strong})", but the compiler
    # doesn't know that we've defined ${strong} in the same file, so
    # we can't do it the normal way.
    cat <<EOF
	 echo 'asm (".globl __GI_${strong}");'; \\
	 echo 'asm ("__GI_${strong} = ${strong}");'; \\
EOF
    emit_weak_aliases
    cat <<EOF
	) | \$(compile-stdin.c) \
\$(foreach p,\$(patsubst %$file,%,\$(basename \$(@F))),\$(\$(p)CPPFLAGS))
EOF
  fi

  if test $shared_only = t; then
    # The versioned symbols are only in the shared library.
    echo endif
  fi

  echo endif
 ;;
 esac

done

具体的包装规则syscall-template.S

/* Assembly code template for system call stubs.
   Copyright (C) 2009-2019 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <http://www.gnu.org/licenses/>.  */

/* The real guts of this work are in the macros defined in the
   machine- and kernel-specific sysdep.h header file.  Cancellable syscalls
   should be implemented using C implementation with SYSCALL_CANCEL macro.

   Each system call's object is built by a rule in sysd-syscalls
   generated by make-syscalls.sh that #include's this file after
   defining a few macros:
	SYSCALL_NAME		syscall name
	SYSCALL_NARGS		number of arguments this call takes
	SYSCALL_SYMBOL		primary symbol name
	SYSCALL_NOERRNO		1 to define a no-errno version (see below)
	SYSCALL_ERRVAL		1 to define an error-value version (see below)

   We used to simply pipe the correct three lines below through cpp into
   the assembler.  The main reason to have this file instead is so that
   stub objects can be assembled with -g and get source line information
   that leads a user back to a source file and these fine comments.  The
   average user otherwise has a hard time knowing which "syscall-like"
   functions in libc are plain stubs and which have nontrivial C wrappers.
   Some versions of the "plain" stub generation macros are more than a few
   instructions long and the untrained eye might not distinguish them from
   some compiled code that inexplicably lacks source line information.  */

#include <sysdep.h>

/* This indirection is needed so that SYMBOL gets macro-expanded.  */
#define syscall_hidden_def(SYMBOL)		hidden_def (SYMBOL)

#define T_PSEUDO(SYMBOL, NAME, N)		PSEUDO (SYMBOL, NAME, N)
#define T_PSEUDO_NOERRNO(SYMBOL, NAME, N)	PSEUDO_NOERRNO (SYMBOL, NAME, N)
#define T_PSEUDO_ERRVAL(SYMBOL, NAME, N)	PSEUDO_ERRVAL (SYMBOL, NAME, N)
#define T_PSEUDO_END(SYMBOL)			PSEUDO_END (SYMBOL)
#define T_PSEUDO_END_NOERRNO(SYMBOL)		PSEUDO_END_NOERRNO (SYMBOL)
#define T_PSEUDO_END_ERRVAL(SYMBOL)		PSEUDO_END_ERRVAL (SYMBOL)

#if SYSCALL_NOERRNO

/* This kind of system call stub never returns an error.
   We return the return value register to the caller unexamined.  */

T_PSEUDO_NOERRNO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
	ret_NOERRNO
T_PSEUDO_END_NOERRNO (SYSCALL_SYMBOL)

#elif SYSCALL_ERRVAL

/* This kind of system call stub returns the errno code as its return
   value, or zero for success.  We may massage the kernel's return value
   to meet that ABI, but we never set errno here.  */

T_PSEUDO_ERRVAL (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
	ret_ERRVAL
T_PSEUDO_END_ERRVAL (SYSCALL_SYMBOL)


#else

/* This is a "normal" system call stub: if there is an error,
   it returns -1 and sets errno.  */

T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
	ret
T_PSEUDO_END (SYSCALL_SYMBOL)

#endif

syscall_hidden_def (SYSCALL_SYMBOL)

通过查看源码可知，常见的系统调用使用的是如下代码：，其宏定义为#define T_PSEUDO(SYMBOL, NAME, N) PSEUDO (SYMBOL, NAME, N)

/* This is a "normal" system call stub: if there is an error,
   it returns -1 and sets errno.  */

T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
	ret
T_PSEUDO_END (SYSCALL_SYMBOL)

#endif

syscall_hidden_def (SYSCALL_SYMBOL)

在sysdeps\unix\sysv\linux\sh\sysdep.h目录下的sysdep.h文件下可以找到宏定义PSEUDO的具体实现逻辑

#define	PSEUDO(name, syscall_name, args) \
 .text; \
 ENTRY (name); \
    DO_CALL (syscall_name, args); \
    mov r0,r1; \
    mov _IMM12,r2; \
    shad r2,r1; \
    not r1,r1; \
    tst r1,r1; \
    bf .Lpseudo_end; \
    SYSCALL_ERROR_HANDLER; \
 .Lpseudo_end:

通过观察可知，几乎所有的系统调用最终都会调用DO_CALL这个宏，但是这个宏在32位和64位的定义是不一样的。

2、32位系统调用过程

在sysdeps\unix\sysv\linux\i386目录下的sysdep.h文件，有DO_CALL的定义：

2.1 执行32位对应的`DO_CALL`

可以看到，系统调用中的每个参数所存放的对应寄存器中如下所示（系统调用号保存在eax寄存器中），最终会执行ENTER_KERNEL陷入内核

# define ENTER_KERNEL int $0x80

/* Linux takes system call arguments in registers:

	syscall number	%eax	     call-clobbered
	arg 1		%ebx	     call-saved
	arg 2		%ecx	     call-clobbered
	arg 3		%edx	     call-clobbered
	arg 4		%esi	     call-saved
	arg 5		%edi	     call-saved
	arg 6		%ebp	     call-saved

   ......
 */

#undef	DO_CALL
#define DO_CALL(syscall_name, args)			      		      \
    PUSHARGS_##args							      \
    DOARGS_##args							      \
    movl $SYS_ify (syscall_name), %eax;					      \
    ENTER_KERNEL							      \
    POPARGS_##args

2.2 在DO_CALL中陷入内核ENTER_KERNEL

通过宏定义可以看到，ENTER_KERNEL的实质定义为int $0x80，即触发一个软中断，通过它就可以陷入（trap）内核

即调用ENTRY(entry_INT80_32)汇编函数，这个需要结合linux-4.19的源码来看，在linux-4.19-rc3\arch\x86\entry\entry_32.S文件中

通过注释可以看到，其寄存器与glibc中的DO_CALL定义中的注释相同

/* Arguments:
 * eax  system call number
 * ebx  arg1
 * ecx  arg2
 * edx  arg3
 * esi  arg4
 * edi  arg5
 * ebp  arg6
 */
ENTRY(entry_INT80_32)
	ASM_CLAC
	pushl	%eax			/* pt_regs->orig_ax */

	SAVE_ALL pt_regs_ax=$-ENOSYS switch_stacks=1	/* save rest */

	/*
	 * User mode is traced as though IRQs are on, and the interrupt gate
	 * turned them off.
	 */
	TRACE_IRQS_OFF

	movl	%esp, %eax
	call	do_int80_syscall_32
.Lsyscall_32_done:

	/*......*/
	INTERRUPT_RETURN

其调用流程为：

通过push和SAVE_ALL将当前用户态的寄存器，保存在pt_regs结构里面；

然后调用do_int80_syscall_32函数 ，进行系统调用；

调用INTERRUPT_RETURN返回；

pt_regs结构体定义：

struct pt_regs {
	/*
	 * NB: 32-bit x86 CPUs are inconsistent as what happens in the
	 * following cases (where %seg represents a segment register):
	 *
	 * - pushl %seg: some do a 16-bit write and leave the high
	 *   bits alone
	 * - movl %seg, [mem]: some do a 16-bit write despite the movl
	 * - IDT entry: some (e.g. 486) will leave the high bits of CS
	 *   and (if applicable) SS undefined.
	 *
	 * Fortunately, x86-32 doesn't read the high bits on POP or IRET,
	 * so we can just treat all of the segment registers as 16-bit
	 * values.
	 */
	unsigned long bx;
	unsigned long cx;
	unsigned long dx;
	unsigned long si;
	unsigned long di;
	unsigned long bp;
	unsigned long ax;
	unsigned short ds;
	unsigned short __dsh;
	unsigned short es;
	unsigned short __esh;
	unsigned short fs;
	unsigned short __fsh;
	unsigned short gs;
	unsigned short __gsh;
	unsigned long orig_ax;
	unsigned long ip;
	unsigned short cs;
	unsigned short __csh;
	unsigned long flags;
	unsigned long sp;
	unsigned short ss;
	unsigned short __ssh;
};

do_int80_syscall_32()实现：

将系统调用号从eax里面取出来
根据系统调用号，在#define ia32_sys_call_table sys_call_table系统调用表中找到相应的函数进行调用
寄存器中保存的参数取出来，作为函数参数。

/* Handles int $0x80 */
__visible void do_int80_syscall_32(struct pt_regs *regs)
{
	enter_from_user_mode();
	local_irq_enable();
	do_syscall_32_irqs_on(regs);
}

/* ...... */
static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
{
	struct thread_info *ti = current_thread_info();
	unsigned int nr = (unsigned int)regs->orig_ax;

	/* ...... */
		/*
		 * It's possible that a 32-bit syscall implementation
		 * takes a 64-bit parameter but nonetheless assumes that
		 * the high bits are zero.  Make sure we zero-extend all
		 * of the args.
		 */
		regs->ax = ia32_sys_call_table[nr](
			(unsigned int)regs->bx, (unsigned int)regs->cx,
			(unsigned int)regs->dx, (unsigned int)regs->si,
			(unsigned int)regs->di, (unsigned int)regs->bp);

	syscall_return_slowpath(regs);
}

INTERRUPT_RETURN定义：

iret指令将原来用户态保存的现场恢复回来，包含代码段、指令指针寄存器等，这时候用户态进程恢复执行。
```
#define INTERRUPT_RETURN		iret
```

2.3 小结

在32位中实现系统调用，以open为例子，过程如下：

在用户层执行open(char *pathname, int flags, mode_t mode);
进入glibc库：

2.1 最终会调用DO_CALL(syscall_name, args)，在其中进行：

2.1.1 把系统调用号与参数保存到寄存器；

2.1.2 调用ENTER_KERNEL陷入内核：

2.1.2.1 通过push和SAVE_ALL将当前用户态的寄存器，保存在pt_regs结构里面；

2.1.2.2 然后调用do_int80_syscall_32函数 ，进行系统调用：（进入内核）

2.1.2.2.1 将系统调用号从eax里面取出来；

2.1.2.2.2 根据系统调用号，在#define ia32_sys_call_table sys_call_table系统调用表中找到相应的函数进行调用；

2.1.2.2.3 寄存器中保存的参数取出来，作为函数参数；

2.1.3 调用INTERRUPT_RETURN返回，恢复用户态；

3、64位系统调用过程

3.1 执行64位对应的`DO_CALL`

在sysdeps\unix\sysv\linux\x86_64\sysdep.h文件下

可以看到，系统调用中的每个参数所存放的对应寄存器中如下所示（系统调用号保存在rax寄存器中），最终会执行syscall陷入内核

/* The Linux/x86-64 kernel expects the system call parameters in
   registers according to the following table:

    syscall number	rax
    arg 1		rdi
    arg 2		rsi
    arg 3		rdx
    arg 4		r10
    arg 5		r8
    arg 6		r9

    */

# undef	DO_CALL
# define DO_CALL(syscall_name, args)		\
    DOARGS_##args				\
    movl $SYS_ify (syscall_name), %eax;		\
    syscall;

3.2 在`DO_CALL`中陷入内核`syscall`

syscall指令还使用了一种特殊的寄存器，我们叫特殊模块寄存器（Model Specific Registers，简称MSR）。

这种寄存器是CPU为了完成某些特殊控制功能为目的的寄存器，其中就有系统调用。
rdmsr和wrmsr是用来读写特殊模块寄存器的。
```
wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64);
```

MSR_LSTAR就是这样一个特殊的寄存器，当syscall指令调用的时候，会从这个寄存器里面拿出函数地址来调用，也就是调用entry_SYSCALL_64。

在arch/x86/entry/entry_64.S中定义了entry_SYSCALL_64。

ENTRY(entry_SYSCALL_64)
	/* ....... */

	/* Construct struct pt_regs on stack */
	pushq	$__USER_DS			/* pt_regs->ss */
	pushq	PER_CPU_VAR(rsp_scratch)	/* pt_regs->sp */
	pushq	%r11				/* pt_regs->flags */
	pushq	$__USER_CS			/* pt_regs->cs */
	pushq	%rcx				/* pt_regs->ip */
	
GLOBAL(entry_SYSCALL_64_after_hwframe)
	pushq	%rax				/* pt_regs->orig_ax */

	PUSH_AND_CLEAR_REGS rax=$-ENOSYS

	TRACE_IRQS_OFF

	/* IRQs are off. */
	movq	%rax, %rdi
	movq	%rsp, %rsi
	call	do_syscall_64		/* returns with IRQs disabled */

	/* ...... */
syscall_return_via_sysret:
	/* rcx and r11 are already restored (see code above) */
	UNWIND_HINT_EMPTY
	POP_REGS pop_rdi=0 skip_r11rcx=1

	/*
	 * Now all regs are restored except RSP and RDI.
	 * Save old stack pointer and switch to trampoline stack.
	 */
	movq	%rsp, %rdi
	movq	PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp

	pushq	RSP-RDI(%rdi)	/* RSP */
	pushq	(%rdi)		/* RDI */

	/*
	 * We are on the trampoline stack.  All regs except RDI are live.
	 * We can do future final exit work right here.
	 */
	SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi

	popq	%rdi
	popq	%rsp
	USERGS_SYSRET64
END(entry_SYSCALL_64)

其调用流程为：

通过pushq将当前用户态的寄存器，保存在pt_regs结构里面；

然后调用do_syscall_64函数 ，进行系统调用；

调用USERGS_SYSRET64返回；

do_syscall_64函数实现：

过程为：

从rax里面拿出系统调用号;
根据系统调用号，在系统调用表sys_call_table中找到相应的函数进行调用;
将寄存器中保存的参数取出来，作为函数参数。

__visible void do_syscall_64(struct pt_regs *regs)
{
        struct thread_info *ti = current_thread_info();
        unsigned long nr = regs->orig_ax;
......
        if (likely((nr & __SYSCALL_MASK) < NR_syscalls)) {
                regs->ax = sys_call_table[nr & __SYSCALL_MASK](
                        regs->di, regs->si, regs->dx,
                        regs->r10, regs->r8, regs->r9);
        }
        syscall_return_slowpath(regs);
}

所以，无论是32位，还是64位，都会到系统调用表sys_call_table这里来。
USERGS_SYSRET64：

sysretq指令将原来用户态保存的现场恢复回来，包含代码段、指令指针寄存器等，这时候用户态进程恢复执行。
```
#define USERGS_SYSRET64				\
	swapgs;					\
	sysretq;
```

3.3 小结

在64位中实现系统调用，以open为例子，过程如下：

在用户层执行open(char *pathname, int flags, mode_t mode);
进入glibc库：

2.1 最终会调用DO_CALL(syscall_name, args)，在其中进行：

2.1.1 把系统调用号与参数保存到寄存器；

2.1.2 调用syscall陷入内核：

2.1.2.1 通过pushq将当前用户态的寄存器，保存在pt_regs结构里面；

2.1.2.2 然后调用do_syscall_64函数 ，进行系统调用：（进入内核）

2.1.2.2.1 将系统调用号从rax里面取出来；

2.1.2.2.2 根据系统调用号，在系统调用表sys_call_table中找到相应的函数进行调用;

2.1.2.2.3 寄存器中保存的参数取出来，作为函数参数；

2.1.3 调用USERGS_SYSRET64返回，恢复用户态；

4、系统调用表的生成(在Linux内核中)

4.1 32位系统调用表的生成

4.1.1 系统调用表的定义

位置：arch\x86\entry\syscalls\syscall_32.tbl

源码：

#
# 32-bit system call numbers and entry vectors
#
# The format is:
#   <number>     <abi>         <name>        <entry point>        <compat entry point>
# <系统调用号> <寄存器保护规则> <系统调用名称> <系统调用在内核的实现函数（入口点）> <兼容入口点>
#
# The __ia32_sys and __ia32_compat_sys stubs are created on-the-fly for
# sys_*() system calls and compat_sys_*() compat system calls if
# IA32_EMULATION is defined, and expect struct pt_regs *regs as their only
# parameter.
#
# The abi is always "i386" for this file.
#
0	i386	restart_syscall		sys_restart_syscall		__ia32_sys_restart_syscall
1	i386	exit			sys_exit			__ia32_sys_exit
2	i386	fork			sys_fork			__ia32_sys_fork
3	i386	read			sys_read			__ia32_sys_read
4	i386	write			sys_write			__ia32_sys_write
5	i386	open			sys_open			__ia32_compat_sys_open
6	i386	close			sys_close			__ia32_sys_close
7	i386	waitpid			sys_waitpid			__ia32_sys_waitpid
8	i386	creat			sys_creat			__ia32_sys_creat

4.1.2 系统调用表的声明

位置：include\linux\syscalls.h

源码：

/* __ARCH_WANT_SYSCALL_NO_AT */
asmlinkage long sys_open(const char __user *filename,
				int flags, umode_t mode);
asmlinkage long sys_link(const char __user *oldname,
				const char __user *newname);
asmlinkage long sys_unlink(const char __user *pathname);

4.1.3 系统调用表的实现

以open函数为例子：

位置：fs\open.c

源码：

/* ...... */

SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
{
	if (force_o_largefile())
		flags |= O_LARGEFILE;

	return do_sys_open(AT_FDCWD, filename, flags, mode);
}

SYSCALL_DEFINE4(openat, int, dfd, const char __user *, filename, int, flags,
		umode_t, mode)
{
	if (force_o_largefile())
		flags |= O_LARGEFILE;

	return do_sys_open(dfd, filename, flags, mode);
}

/* ...... */

可以看到其形式十分奇怪，通过查看syscalls.h声明，可以知道是根据参数的数目选择对应的宏

#define SYSCALL_DEFINE1(name, ...) SYSCALL_DEFINEx(1, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE2(name, ...) SYSCALL_DEFINEx(2, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE3(name, ...) SYSCALL_DEFINEx(3, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE4(name, ...) SYSCALL_DEFINEx(4, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE5(name, ...) SYSCALL_DEFINEx(5, _##name, __VA_ARGS__)
#define SYSCALL_DEFINE6(name, ...) SYSCALL_DEFINEx(6, _##name, __VA_ARGS__)

#define SYSCALL_DEFINE_MAXARGS	6

#define SYSCALL_DEFINEx(x, sname, ...)				\
	SYSCALL_METADATA(sname, x, __VA_ARGS__)			\
	__SYSCALL_DEFINEx(x, sname, __VA_ARGS__)

#define __PROTECT(...) asmlinkage_protect(__VA_ARGS__)

/*
 * The asmlinkage stub is aliased to a function named __se_sys_*() which
 * sign-extends 32-bit ints to longs whenever needed. The actual work is
 * done within __do_sys_*().
 */
#ifndef __SYSCALL_DEFINEx
#define __SYSCALL_DEFINEx(x, name, ...)					\
	__diag_push();							\
	__diag_ignore(GCC, 8, "-Wattribute-alias",			\
		      "Type aliasing is used to sanitize syscall arguments");\
	asmlinkage long sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))	\
		__attribute__((alias(__stringify(__se_sys##name))));	\
	ALLOW_ERROR_INJECTION(sys##name, ERRNO);			\
	static inline long __do_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__));\
	asmlinkage long __se_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__));	\
	asmlinkage long __se_sys##name(__MAP(x,__SC_LONG,__VA_ARGS__))	\
	{								\
		long ret = __do_sys##name(__MAP(x,__SC_CAST,__VA_ARGS__));\
		__MAP(x,__SC_TEST,__VA_ARGS__);				\
		__PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__));	\
		return ret;						\
	}								\
	__diag_pop();							\
	static inline long __do_sys##name(__MAP(x,__SC_DECL,__VA_ARGS__))
#endif /* __SYSCALL_DEFINEx */

宏展开后，SYSCALL_DEFINE3得到其具体实现如下：

asmlinkage long sys_open(const char __user * filename, int flags, int mode)
{
    long ret;


    if (force_o_largefile())
        flags |= O_LARGEFILE;


    ret = do_sys_open(AT_FDCWD, filename, flags, mode);
    asmlinkage_protect(3, ret, filename, flags, mode);
    return ret;
}

4.1.4 建立联系

利用arch/x86/entry/syscalls/Makefile

# SPDX-License-Identifier: GPL-2.0
out := arch/$(SRCARCH)/include/generated/asm   #输出文件地址
uapi := arch/$(SRCARCH)/include/generated/uapi/asm

# 如果当前没有创建，则建立输出文件
_dummy := $(shell [ -d '$(out)' ] || mkdir -p '$(out)') \
	  $(shell [ -d '$(uapi)' ] || mkdir -p '$(uapi)')

# 所需要的定义文件
syscall32 := $(srctree)/$(src)/syscall_32.tbl		# 32位系统使用指定目录下syscall_32.tbl
syscall64 := $(srctree)/$(src)/syscall_64.tbl		# 64位系统使用指定目录下syscall_64.tbl

# 所需要脚本文件的地址
syshdr := $(srctree)/$(src)/syscallhdr.sh
systbl := $(srctree)/$(src)/syscalltbl.sh

quiet_cmd_syshdr = SYSHDR  $@
      cmd_syshdr = $(CONFIG_SHELL) '$(syshdr)' '$<' '$@' \
		   '$(syshdr_abi_$(basetarget))' \
		   '$(syshdr_pfx_$(basetarget))' \
		   '$(syshdr_offset_$(basetarget))'
quiet_cmd_systbl = SYSTBL  $@
      cmd_systbl = $(CONFIG_SHELL) '$(systbl)' $< $@

quiet_cmd_hypercalls = HYPERCALLS $@
      cmd_hypercalls = $(CONFIG_SHELL) '$<' $@ $(filter-out $<,$^)

# 所需要依赖与指定的协议
syshdr_abi_unistd_32 := i386
$(uapi)/unistd_32.h: $(syscall32) $(syshdr)
	$(call if_changed,syshdr)

syshdr_abi_unistd_32_ia32 := i386
syshdr_pfx_unistd_32_ia32 := ia32_
$(out)/unistd_32_ia32.h: $(syscall32) $(syshdr)
	$(call if_changed,syshdr)

syshdr_abi_unistd_x32 := common,x32
syshdr_offset_unistd_x32 := __X32_SYSCALL_BIT
$(uapi)/unistd_x32.h: $(syscall64) $(syshdr)
	$(call if_changed,syshdr)

syshdr_abi_unistd_64 := common,64
$(uapi)/unistd_64.h: $(syscall64) $(syshdr)
	$(call if_changed,syshdr)

syshdr_abi_unistd_64_x32 := x32
syshdr_pfx_unistd_64_x32 := x32_

# 输出文件名与地址
$(out)/unistd_64_x32.h: $(syscall64) $(syshdr)
	$(call if_changed,syshdr)

$(out)/syscalls_32.h: $(syscall32) $(systbl)
	$(call if_changed,systbl)
$(out)/syscalls_64.h: $(syscall64) $(systbl)
	$(call if_changed,systbl)

$(out)/xen-hypercalls.h: $(srctree)/scripts/xen-hypercalls.sh
	$(call if_changed,hypercalls)

$(out)/xen-hypercalls.h: $(srctree)/include/xen/interface/xen*.h

# 建立联系并生成输出文件
uapisyshdr-y			+= unistd_32.h unistd_64.h unistd_x32.h
syshdr-y			+= syscalls_32.h
syshdr-$(CONFIG_X86_64)		+= unistd_32_ia32.h unistd_64_x32.h
syshdr-$(CONFIG_X86_64)		+= syscalls_64.h
syshdr-$(CONFIG_XEN)		+= xen-hypercalls.h

targets	+= $(uapisyshdr-y) $(syshdr-y)

PHONY += all
all: $(addprefix $(uapi)/,$(uapisyshdr-y))
all: $(addprefix $(out)/,$(syshdr-y))
	@:

依赖两个脚本

第一个脚本arch/x86/entry/syscalls/syscallhdr.sh，会在文件中生成#define NR_open

#!/bin/sh
# SPDX-License-Identifier: GPL-2.0

in="$1"
out="$2"
my_abis=`echo "($3)" | tr ',' '|'`
prefix="$4"
offset="$5"

fileguard=_ASM_X86_`basename "$out" | sed \
    -e 'y/abcdefghijklmnopqrstuvwxyz/ABCDEFGHIJKLMNOPQRSTUVWXYZ/' \
    -e 's/[^A-Z0-9_]/_/g' -e 's/__/_/g'`
grep -E "^[0-9A-Fa-fXx]+[[:space:]]+${my_abis}" "$in" | sort -n | (
    echo "#ifndef ${fileguard}"
    echo "#define ${fileguard} 1"
    echo ""

# 生成 #define NR_open
    while read nr abi name entry ; do
	if [ -z "$offset" ]; then
	    echo "#define __NR_${prefix}${name} $nr"
	else
	    echo "#define __NR_${prefix}${name} ($offset + $nr)"
        fi
    done

    echo ""
    echo "#endif /* ${fileguard} */"
) > "$out"

第二个脚本arch/x86/entry/syscalls/syscalltbl.sh，会在文件中生成SYSCALL(NR_open, sys_open)

#!/bin/sh
# SPDX-License-Identifier: GPL-2.0

in="$1"
out="$2"

syscall_macro() {
    abi="$1"
    nr="$2"
    entry="$3"

    # Entry can be either just a function name or "function/qualifier"
    real_entry="${entry%%/*}"
    if [ "$entry" = "$real_entry" ]; then
        qualifier=
    else
        qualifier=${entry#*/}
    fi

    # 生成SYSCALL(NR_open, sys_open)
    echo "__SYSCALL_${abi}($nr, $real_entry, $qualifier)"
}

emit() {
    abi="$1"
    nr="$2"
    entry="$3"
    compat="$4"
    umlentry=""

    if [ "$abi" = "64" -a -n "$compat" ]; then
	echo "a compat entry for a 64-bit syscall makes no sense" >&2
	exit 1
    fi

    # For CONFIG_UML, we need to strip the __x64_sys prefix
    if [ "$abi" = "64" -a "${entry}" != "${entry#__x64_sys}" ]; then
	    umlentry="sys${entry#__x64_sys}"
    fi

    if [ -z "$compat" ]; then
	if [ -n "$entry" -a -z "$umlentry" ]; then
	    syscall_macro "$abi" "$nr" "$entry"
	elif [ -n "$umlentry" ]; then # implies -n "$entry"
	    echo "#ifdef CONFIG_X86"
	    syscall_macro "$abi" "$nr" "$entry"
	    echo "#else /* CONFIG_UML */"
	    syscall_macro "$abi" "$nr" "$umlentry"
	    echo "#endif"
	fi
    else
	echo "#ifdef CONFIG_X86_32"
	if [ -n "$entry" ]; then
	    syscall_macro "$abi" "$nr" "$entry"
	fi
	echo "#else"
	syscall_macro "$abi" "$nr" "$compat"
	echo "#endif"
    fi
}

grep '^[0-9]' "$in" | sort -n | (
    while read nr abi name entry compat; do
	abi=`echo "$abi" | tr '[a-z]' '[A-Z]'`
	if [ "$abi" = "COMMON" -o "$abi" = "64" ]; then
	    # COMMON is the same as 64, except that we don't expect X32
	    # programs to use it.  Our expectation has nothing to do with
	    # any generated code, so treat them the same.
	    emit 64 "$nr" "$entry" "$compat"
	elif [ "$abi" = "X32" ]; then
	    # X32 is equivalent to 64 on an X32-compatible kernel.
	    echo "#ifdef CONFIG_X86_X32_ABI"
	    emit 64 "$nr" "$entry" "$compat"
	    echo "#endif"
	elif [ "$abi" = "I386" ]; then
	    emit "$abi" "$nr" "$entry" "$compat"
	else
	    echo "Unknown abi $abi" >&2
	    exit 1
	fi
    done
) > "$out"

生成输出文件，建立系统调用号和系统调用实现函数之间的对应关系。

根据syscall_32.tbl生成unistd_32.h，位置：arch\sh\include\uapi\asm\unistd_32.h

/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
#ifndef __ASM_SH_UNISTD_32_H
#define __ASM_SH_UNISTD_32_H

/*
 * Copyright (C) 1999  Niibe Yutaka
 */

/*
 * This file contains the system call numbers.
 */

#define __NR_restart_syscall	  0
#define __NR_exit		  1
#define __NR_fork		  2
#define __NR_read		  3
#define __NR_write		  4
#define __NR_open		  5
#define __NR_close		  6
#define __NR_waitpid		  7
#define __NR_creat		  8
#define __NR_link		  9
#define __NR_unlink		 10
#define __NR_execve		 11
#define __NR_chdir		 12
#define __NR_time		 13
#define __NR_mknod		 14
#define __NR_chmod		 15
#define __NR_lchown		 16

/* ...... */

根据syscall_64.tbl生成unistd_64.h，位置：arch\sh\include\uapi\asm\unistd_64.h

/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
#ifndef __ASM_SH_UNISTD_64_H
#define __ASM_SH_UNISTD_64_H

/*
 * include/asm-sh/unistd_64.h
 *
 * This file contains the system call numbers.
 *
 * Copyright (C) 2000, 2001  Paolo Alberelli
 * Copyright (C) 2003 - 2007 Paul Mundt
 * Copyright (C) 2004  Sean McGoogan
 *
 * This file is subject to the terms and conditions of the GNU General Public
 * License.  See the file "COPYING" in the main directory of this archive
 * for more details.
 */
#define __NR_restart_syscall	  0
#define __NR_exit		  1
#define __NR_fork		  2
#define __NR_read		  3
#define __NR_write		  4
#define __NR_open		  5
#define __NR_close		  6
#define __NR_waitpid		  7
#define __NR_creat		  8
#define __NR_link		  9
#define __NR_unlink		 10
#define __NR_execve		 11
#define __NR_chdir		 12
#define __NR_time		 13
#define __NR_mknod		 14
#define __NR_chmod		 15
#define __NR_lchown		 16

/* ...... */

4.1.5 系统调用表的最终

位置：arch\x86\entry\syscall_32.c

源码：

// SPDX-License-Identifier: GPL-2.0
/* System call table for i386. */

/* ...... */

__visible const sys_call_ptr_t ia32_sys_call_table[__NR_syscall_compat_max+1] = {
	/*
	 * Smells like a compiler bug -- it doesn't work
	 * when the & below is removed.
	 */
	[0 ... __NR_syscall_compat_max] = &sys_ni_syscall,
#include <asm/syscalls_32.h>

4.2 64位系统调用表的生成

4.2.1 系统调用表的定义

位置：arch\x86\entry\syscalls\syscall_64.tbl

源码：与32位对比，其系统调用号、abi协议是不一样的，且没有兼容入口点

#
# 64-bit system call numbers and entry vectors
#
# The format is:
# <number> <abi> <name> <entry point>
#
# The __x64_sys_*() stubs are created on-the-fly for sys_*() system calls
#
# The abi is "common", "64" or "x32" for this file.
#
0	common	read			__x64_sys_read
1	common	write			__x64_sys_write
2	common	open			__x64_sys_open
3	common	close			__x64_sys_close
4	common	stat			__x64_sys_newstat
5	common	fstat			__x64_sys_newfstat
6	common	lstat			__x64_sys_newlstat
7	common	poll			__x64_sys_poll
8	common	lseek			__x64_sys_lseek
9	common	mmap			__x64_sys_mmap
10	common	mprotect		__x64_sys_mprotect
11	common	munmap			__x64_sys_munmap

## ......

4.2.2 系统调用表的声明

位置：include\linux\syscalls.h

源码：32位与64位一致

/* __ARCH_WANT_SYSCALL_NO_AT */
asmlinkage long sys_open(const char __user *filename,
				int flags, umode_t mode);
asmlinkage long sys_link(const char __user *oldname,
				const char __user *newname);
asmlinkage long sys_unlink(const char __user *pathname);

4.2.3 系统调用表的实现