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Merge pull request #762 from Mutated1994/patch-8

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feat: Get a memory dump in real mode
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0xAX 2020-11-29 19:00:10 +06:00 committed by GitHub
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Booting/linux-bootstrap-1.md
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@ -253,6 +253,84 @@ where `X` is the address of the kernel boot sector being loaded. In my case, `X`
![kernel first address](images/kernel_first_address.png)
How to get this memory dump in real mode?
--------------------------------------------------------------------------------
```
root@parallels-vm:/usr/src/linux# more arch/x86/kernel/vmlinux.lds
...
SECTIONS
{
. = (0xffffffff80000000 + ALIGN(0x1000000, 0x200000));
phys_startup_64 = ABSOLUTE(startup_64 - 0xffffffff80000000);
.text : AT(ADDR(.text) - 0xffffffff80000000) {
_text = .;
_stext = .;
....
```
```
root@parallels-vm:/usr/src/linux# nm vmlinux|grep startup_64
0000000001000000 A phys_startup_64
ffffffff81000030 T secondary_startup_64
ffffffff810001f0 T __startup_64
ffffffff81000000 T startup_64
```
Here we can see the memory address of the entry point, which is `0x0000000001000000`. Let's go ahead.
Before trying to debug the kernel, please see [Booting a Custom Linux Kernel in QEMU and Debugging It With GDB](http://nickdesaulniers.github.io/blog/2018/10/24/booting-a-custom-linux-kernel-in-qemu-and-debugging-it-with-gdb/)
#### Step 1
Booting in QEMU
```
qemu-system-x86_64 -kernel /usr/src/linux-4.14.207/arch/x86_64/boot/bzImage -nographic -append "console=ttyS0 nokaslr" -initrd /data/busybox/busybox-1.28.0/initramfs.cpio.gz -S -s
```
#### Step 2
Attaching GDB to QEMU
```
gdb vmlinux
(gdb) target remote :1234
(gdb) hbreak *0x0000000001000000
(gdb) c
(gdb) dump binary memory /tmp/dump 0x0000 0x20000
```
#### Step 3
```
root@parallels-vm:/# hd /tmp/dump |grep -A 31 MZ
00010000 4d 5a ea 07 00 c0 07 8c c8 8e d8 8e c0 8e d0 31 |MZ.............1|
00010010 e4 fb fc be 40 00 ac 20 c0 74 09 b4 0e bb 07 00 |....@.. .t......|
00010020 cd 10 eb f2 31 c0 cd 16 cd 19 ea f0 ff 00 f0 00 |....1...........|
00010030 00 00 00 00 00 00 00 00 00 00 00 00 82 00 00 00 |................|
00010040 55 73 65 20 61 20 62 6f 6f 74 20 6c 6f 61 64 65 |Use a boot loade|
00010050 72 2e 0d 0a 0a 52 65 6d 6f 76 65 20 64 69 73 6b |r....Remove disk|
00010060 20 61 6e 64 20 70 72 65 73 73 20 61 6e 79 20 6b | and press any k|
00010070 65 79 20 74 6f 20 72 65 62 6f 6f 74 2e 2e 2e 0d |ey to reboot....|
00010080 0a 00 50 45 00 00 64 86 04 00 00 00 00 00 00 00 |..PE..d.........|
00010090 00 00 01 00 00 00 a0 00 06 02 0b 02 02 14 20 d5 |.............. .|
000100a0 80 00 00 00 00 00 e0 b8 79 01 80 46 00 00 00 02 |........y..F....|
000100b0 00 00 00 00 00 00 00 00 00 00 20 00 00 00 20 00 |.......... ... .|
000100c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000100d0 00 00 00 90 fa 01 00 02 00 00 00 00 00 00 0a 00 |................|
000100e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00010100 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 00 |................|
00010110 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
00010130 00 00 00 00 00 00 00 00 00 00 2e 73 65 74 75 70 |...........setup|
00010140 00 00 e0 41 00 00 00 02 00 00 e0 41 00 00 00 02 |...A.......A....|
00010150 00 00 00 00 00 00 00 00 00 00 00 00 00 00 20 00 |.............. .|
00010160 50 60 2e 72 65 6c 6f 63 00 00 20 00 00 00 e0 43 |P`.reloc.. ....C|
00010170 00 00 20 00 00 00 e0 43 00 00 00 00 00 00 00 00 |.. ....C........|
00010180 00 00 00 00 00 00 40 00 10 42 2e 74 65 78 74 00 |......@..B.text.|
00010190 00 00 20 93 80 00 00 44 00 00 20 93 80 00 00 44 |.. ....D.. ....D|
000101a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 20 00 |.............. .|
000101b0 50 60 2e 62 73 73 00 00 00 00 e0 b8 79 01 20 d7 |P`.bss......y. .|
000101c0 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000101d0 00 00 00 00 00 00 80 00 00 c8 00 00 00 00 00 00 |................|
000101e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ff |................|
000101f0 ff 21 01 00 32 09 08 00 00 00 ff ff 00 00 55 aa |.!..2.........U.|
```
The bootloader has now loaded the Linux kernel into memory, filled the header fields, and then jumped to the corresponding memory address. We now move directly to the kernel setup code.
The Beginning of the Kernel Setup Stage