Challenge yourself

Problem

In proc.c, there is an array initcode[] of binary code. What is its relationship with initcode.S?

Solution

Research

Testing code: -xc to make the hex dump more readable. Example:

$ od -xc test.txt
0000000      6568    6c6c    0a6f
           h   e   l   l   o  \n
0000006

The od Command

od man page

-t is the output format. x is hex. c is chars in default char set.

Note: Each Hexadecimal character represents 4 bits (0 - 15 decimal). A byte is 2 hex.

Answer

We want a memory dump of instructions in hex format, and separated by each char.

We have to load initcode, and use it to call system call exec to run init. We cannot directly load init binary as hex dump, otherwise we have to do something similar to what exec does, set up C stacks, parse ELF headers, etc.

So the simply solution is:

1. use objcopy to copy a stripped instruction only data file.

2. Use od to print the data file in hex, separated by byte (char).

3. Append 0x to each char.

4. That is the result if initcode array you see in proc.c.

References

od - dump files in various formats.

See od The UNIX School: 3 different ways of dumping hex contents of a file

Challenge!

Try to switch the hex dump of initcode to a program you wrote!

I modify initcode.S to use echo:

#include “syscall.h”

# exec(init, argv)
.globl start
start:
        la a0, echo
        la a1, argv
        li a7, SYS_exec
        ecall

# for(;;) exit();
exit:
        li a7, SYS_exit
        ecall
        jal exit

# char init[] = “/init\0”;
echo:
  .string “/echo\0”

# char *argv[] = { init, 0 };
.p2align 2
argv:
  .long echo
  .long 0

After running make, get the hex dump of the binary:

$ od -t xC initcode
0000000    17  05  00  00  13  05  05  02  97  05  00  00  93  85  05  02
0000020    9d  48  73  00  00  00  89  48  73  00  00  00  ef  f0  bf  ff
0000040    2f  65  63  68  6f  00  00  01  20  00  00  00  00  00  00  00
0000060    00  00  00
0000063

Change the array in proc.c:

// a user program that calls exec(“/echo”)
// od -t xC initcode
uchar initcode[] = {
  0x17, 0x05, 0x00, 0x00, 0x13, 0x05, 0x05, 0x02,
  0x97, 0x05, 0x00, 0x00, 0x93, 0x85, 0x05, 0x02,
  0x9d, 0x48, 0x73, 0x00, 0x00, 0x00, 0x89, 0x48,
  0x73, 0x00, 0x00, 0x00, 0xef, 0xf0, 0xbf, 0xff,
  0x2f, 0x65, 0x63, 0x68, 0x6f, 0x00, 0x00, 0x01,
  0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00
};

Make a special echo:

#include “kernel/types.h"
#include “kernel/stat.h”
#include “user/user.h”
#include “kernel/fcntl.h”

int
main(void)
{

  if(open(“console”, O_RDWR) < 0){
    mknod(“console”, 1, 1);
    open(“console”, O_RDWR);
  }
  dup(0);  // stdout
  dup(0);  // stderr

  for(;;){
    printf(“Mushroom os is the best!\n”);
  }
}

As a result, your terminal prints the following forever:

Mushroom os is the best!
Mushroom os is the best!
Mushroom os is the best!
Mushroom os is the best!
Mushroom os is the best!
Mushroom os is the best!
…

We hacked the kernel to run our program in the very first user process!