# system_calls

**Repository Path**: cclinuxer/system_calls

## Basic Information

- **Project Name**: system_calls
- **Description**: Linux系统调用由浅入深
- **Primary Language**: Unknown
- **License**: Not specified
- **Default Branch**: master
- **Homepage**: None
- **GVP Project**: No

## Statistics

- **Stars**: 0
- **Forks**: 0
- **Created**: 2020-09-15
- **Last Updated**: 2020-12-19

## Categories & Tags

**Categories**: Uncategorized

**Tags**: None

## README

system calls
========================================

Linux的系统调用是通过软中断swi来实现的从用户态到内核态.

swi
----------------------------------------

用swi #0指令即可触发软中断，并切换到内核态(管理模式)。

https://github.com/novelinux/arch-arm-common/tree/master/swi/README.md

user --> kernel
----------------------------------------

在系统调用通过软中断swi从用户态切换到内核态的压栈过程如下所示:

```
| kernel stack |  struct pt_regs
|--------------|-----------------------
|   r0 ~ r12   |
|    sp_usr    |    sp
|    lr_usr    |    lr
|    lr_svc    |    pc
|     cpsr     |   cpsr
|      r0      |
|              |------------------------ <- sp_svc
```

**注意**:

1.lr_svc中是指向swi中断指令的下一条指令.
2.这里r0会存两个位置: 一个代表第一个参数，另一个代表返回值。

### vector_swi

软中断处理函数:

https://github.com/novelinux/linux-4.x.y/tree/master/arch/arm/kernel/entry-common.S/vector_swi.md

### struct pt_regs

https://github.com/novelinux/linux-4.x.y/tree/master/arch/arm/include/asm/ptrace.h/struct_pt_regs.md

sys_call_table
----------------------------------------

系统调用表.

https://github.com/novelinux/linux-4.x.y/tree/master/arch/arm/kernel/entry-common.S/sys_call_table.md

kernel --> user
----------------------------------------

在系统调用从内核态返回到用户态的调用过程如下所示:

```
| kernel stack |  struct pt_regs
|--------------|----------------------- <- sp_svc
|   r0 ~ r12   |         --> r1 ~ r12
|    sp_usr    |    sp   --> sp_usr
|    lr_usr    |    lr   --> lr_usr
|    lr_svc    |    pc   --> lr_svc    --> pc
|     cpsr     |   cpsr  --> spsr_cxsf
|      r0      |
|              |------------------------
```

**注意**: ret_fast_syscall不会将r0寄存器值恢复,r0要用来保存返回值.

在从内核态返回到用户态的过程是由ret_fast_syscall来完成的:

### ret_fast_syscall

https://github.com/novelinux/linux-4.x.y/tree/master/arch/arm/kernel/entry-common.S/ret_fast_syscall.md