GNU/Linux Tips

1 Mounting partition from VDI image

$ od -j344 -N4 -td4 /work/FreeDos.vdi | awk 'NR==1{print $2;}'
8192

$ sudo losetup -f --show -o 8192 /work/FreeDos.vdi
/dev/loop0

$ sudo fdisk  -lu /dev/loop0

Disk /dev/loop0: 113 MB, 113246208 bytes
16 heads, 63 sectors/track, 219 cylinders, total 221184 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

      Device Boot      Start         End      Blocks   Id  System
/dev/loop0p1   *          63     1023119      511528+   6  FAT16

$ sudo mount -o loop,offset=$((512*63)) /dev/loop0 /mnt

$ ls /mnt/
AUTOEXEC.BAT  BOOTSECT.BIN  COMMAND.COM  FDCONFIG.SYS  FDOS  KERNEL.SYS


$ sudo umount /mnt
$ sudo losetup -d /dev/loop0

$ VBoxManage clonehd /work/FreeDos.vdi /work/freedos.vdi --format VDI --variant Fixe

2 GDB Tips

1. 查看寄存器

(gdb) i r
(gdb) i r a                     # 查看所有寄存器（包括浮点、多媒体）
(gdb) i r esp
(gdb) i r pc

2. 查看内存

(gdb) x /wx 0x80040000    # 以16进制显示指定地址处的数据
(gdb) x /8x $esp
(gdb) x /16x $esp+12
(gdb) x /16s 0x86468700   # 以字符串形式显示指定地址处的数据
(gdb) x /24i 0x8048a51      # 以指令形式显示指定地址处的数据（24条）

3. 修改寄存器的值

(gdb) set $v0 = 0x004000000
(gdb) set $epc = 0xbfc00000

4. 修改内存的值

(gdb) set {unsigned int}0x8048a51=0x0
(gdb) set *(unsigned int*)0x8048a54=0x55aa55aa

5. 内存搜索

Usage: find <start> <end> <count> <value>

(gdb) define find                            
set $ptr = $arg0
set $cnt = 0
while ( ($ptr<=$arg1) && ($cnt<$arg2) )
    if ( *(unsigned int *)$ptr == $arg3 )
        x /wx $ptr
        set $cnt = $cnt + 1
    end
    set $ptr = $ptr + 4
end
end

6. 断点、监测点

(gdb) b *0x80400000
(gdb) watch *(unsigned int *)0xbffff400==0x90909090