#### How Linux Kernel Prints Text on Screen

https://www.youtube.com/watch?v=aAuw2EVCBBg
https://www.bilibili.com/video/BV1ex4y1t7WF
https://www.kernel.org/
https://docs.kernel.org/dev-tools/gdb-kernel-debugging.html
ubuntu22
https://www.qemu.org/docs/master/system/introduction.html


















# download latest release
www.kernel.org




# open windows subsystem for linux, ubuntu22, this time

explorer.exe .

# copy to linux folder

# unzip
tar -xf linux-6.11.1.tar.xz


# this will download all linux-kernel, do not run:
git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git





# or, you use codespace: 
wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.11.1.tar.xz

tar -xf linux-6.11.1.tar.xz




















cd linux-6.11.1


make defconfig


sudo apt update
sudo apt upgrade
sudo apt install flex
# sudo apt install bison



make defconfig

#### output:

  YACC    scripts/kconfig/parser.tab.[ch]
  HOSTCC  scripts/kconfig/lexer.lex.o
  HOSTCC  scripts/kconfig/menu.o
  HOSTCC  scripts/kconfig/parser.tab.o
  HOSTCC  scripts/kconfig/preprocess.o
  HOSTCC  scripts/kconfig/symbol.o
  HOSTCC  scripts/kconfig/util.o
  HOSTLD  scripts/kconfig/conf

*** Default configuration is based on 'x86_64_defconfig'
#
# configuration written to .config
#

####



done

tired... next time update















make menuconfig






# error:
# * Unable to find the ncurses package
####
*
* Unable to find the ncurses package.
* Install ncurses (ncurses-devel or libncurses-dev
* depending on your distribution).
*
* You may also need to install pkg-config to find the
* ncurses installed in a non-default location.
*
####






sudo apt install pkg-config
sudo apt install libncurses-dev







make menuconfig






# down to the lowest

Kernel hacking   
--->  Compile-time checks and compiler options  
        --->   Debug information (Disable debug information)   
		--->   (X) Rely on the toolchain's implicit default DWARF versiLon
		--->   [*] Provide GDB scripts for kernel debugging






make -j 8





# <stdin>:1:10: fatal error: libelf.h: No such file or directory




# search for bing.com: libelf.h: No such file or directory
# fatal error: gelf.h: No such file or directory





sudo apt install libelf-dev







make






# certs/extract-cert.c:21:10: fatal error: openssl/bio.h: No such file or directory





sudo apt-get install aptitude
sudo aptitude install libssl-dev





make -j 8






# my cpu fans go crazy, i go away from my computer, what i have done







# after a long-not-happy time, output these:

####
  ...
  OBJCOPY arch/x86/boot/vmlinux.bin
  AS      arch/x86/boot/header.o
  LD      arch/x86/boot/setup.elf
  OBJCOPY arch/x86/boot/setup.bin
  BUILD   arch/x86/boot/bzImage
Kernel: arch/x86/boot/bzImage is ready  (#1)
####








man make

/-j

-j [jobs], --jobs[=jobs]
            Specifies  the number of jobs (commands) to run simultaneously.  If there is more than one -j
            option, the last one is effective.  If the -j option is given without an argument, make  will
            not  limit  the number of jobs that can run simultaneously. When make invokes a sub-make, all
            instances of make will coordinate to run the specified number of jobs at a time; see the sec‐
            tion PARALLEL MAKE AND THE JOBSERVER for details.
			
			
		


	
	

make






####
  CALL    scripts/checksyscalls.sh
  DESCEND objtool
  INSTALL libsubcmd_headers
Kernel: arch/x86/boot/bzImage is ready  (#1)
####






done





tired... next time
















make scripts_gdb



cd ~/distro



mkdir ~/distro



cd ~/distro



vim init.c




###########   init.c   
#include <stdio.h>

int main()
{
        printf("Hello, new world. \n");
}
###########








gcc -static init.c -o init

ls

./init




strace ./init




man 2 write









echo init | cpio -H newc -o > init.cpio

sudo apt install cpio


echo init | cpio -H newc -o > init.cpio







vim

# open a file
:e fs/read_write.c 

# search a file
/sys_write






# create a new file: .gdbinit
# in this directory: /linux-6.11.1
:new .gdbinit



########## edit this: 
add-auto-load-safe-path /path/to/linux-build


########## into this file: .gdbinit
add-auto-load-safe-path /linux-6.11.1





gdb vmlinux










# open a new terminal
qemu-system-x86_64






# need to install qemu





tired... next time...















# download qemu

# QEMU is packaged by most Linux distributions:
# Debian/Ubuntu:
# For full system emulation: apt-get install qemu-system
# For emulating Linux binaries: apt-get install qemu-user-static





sudo apt install qemu-system




qemu-system-x86_64



# Unable to init server: Could not connect: Connection refused
# gtk initialization failed

# maybe, can not run form a codespace server
# need to run from local, for example, windows subsystem for linux 







# this could run:
qemu-system-x86_64 -nographic



# open a new terminal in vscode of codespace: ctrl+shift+~
cd linux-6.11.1






# change this:
# qemu-system-x86_64 -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append nokaslr

# add: -nographic
qemu-system-x86_64 -nographic -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append nokaslr





# open a new terminal:
gdb vmlinux


# press enter, enter. until see: (gdb)



target remote :1234
 


break ksys_write


c







# maybe, can not run form a codespace server
# try again in local










# MarsCode.com

# Exit QEMU: 
To exit QEMU, you can press Ctrl+A and then X.





qemu-system-x86_64 -kernel <kernel-image> -m 2G -smp 4 -device virtio-net-pci,netdev=net0 -netdev user,id=net0
# In this example, we're allocating 2GB of memory, using 4 CPU cores, and creating a virtual network device.





# ask artificial intelligence，large language model, marscode:
# qemu-system-x86_64 -nographic -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append nokaslr
# 
# qemu-system-x86_64: This is the QEMU emulator for the x86_64 architecture.
# -nographic: This option tells QEMU not to create a graphical display. Instead, it will use a serial console for output.
# -kernel arch/x86_64/boot/bzImage: This option specifies the path to the Linux kernel image (bzImage) that QEMU will load and boot.
# -initrd ~/distro/init.cpio: This option tells QEMU to use the specified initial ramdisk (initrd) file. The initrd is a temporary file system that is loaded into memory during the boot process and is used to provide early userspace support.
# -s: This option enables the QEMU gdbserver on TCP port 1234. This allows you to debug the kernel using a remote GDB debugger.
# -S: This option tells QEMU to start the emulation in a paused state. This is useful if you want to attach a debugger before the emulation starts.
# -append nokaslr: This option appends the "nokaslr" parameter to the kernel command line. "kaslr" stands for "kernel address space layout randomization", which is a security feature that randomizes the memory addresses of the kernel. The "nokaslr" parameter disables this feature.






# echo init | cpio -H newc -o > init.cpio
# 
# echo init: This part of the command simply prints the word "init" to the standard output.
# |: This is a pipe symbol that is used to redirect the output of the echo command to the input of the cpio command.
# cpio -H newc -o: This is the cpio command that is used to create a new archive. The -H newc option specifies the format of the archive, and the -o option tells cpio to create an archive from the standard input.
# > init.cpio: This redirects the output of the cpio command to a file named init.cpio.















########################### https://kviccn.github.io/posts/%E5%B0%86Qemu%E8%BE%93%E5%87%BA%E8%AE%BE%E7%BD%AE%E5%88%B0%E7%BB%88%E7%AB%AF.html

########################## https://fadeevab.com/how-to-setup-qemu-output-to-console-and-automate-using-shell-script/

wget https://people.debian.org/~aurel32/qemu/i386/debian_wheezy_i386_standard.qcow2 -O wheezy.qcow2

qemu-system-x86_64 -nographic wheezy.qcow2

username:root
password:root



















# try in wsl:
qemu-system-x86_64 -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append nokaslr





# output:
# Guest has not initialized the display (yet).


what?


tired...














#run this:
qemu-system-x86_64 -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio

#output:
end Kernel panic - not syncing: Attempted tto kill init!







cd linux-6.11
make defconfig
make menuconfig

		--->   (X) Rely on the toolchain's implicit default DWARF versiLon
		--->   [*] Provide GDB scripts for kernel debugging




gdb vmlinux

qemu-system-x86_64 -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append nokaslr



i got nothing...



















try again...


make defconfig
make menuconfig
make


mkdir ~/distro
vim init.c

echo init | cpio -H newc -o > init.cpio




gdb





#####################
warning: File "/home/codespace/linux-6.11.7/scripts/gdb/vmlinux-gdb.py" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
To enable execution of this file add
        add-auto-load-safe-path /home/codespace/linux-6.11.7/scripts/gdb/vmlinux-gdb.py
line to your configuration file "/home/codespace/.gdbinit".
To completely disable this security protection add
        set auto-load safe-path /
line to your configuration file "/home/codespace/.gdbinit".






vim /home/codespace/.gdbinit

########### add this line to ignore auto-load
set auto-load safe-path /






it is turns out to run gdb...
# Guest has not initialized the display (yet).


# get this error is ok...






gdb vmlinux

qemu-system-x86_64 -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append nokaslr






(gdb) target remote :1234

(gdb) break ksys_write

(gdb) continue
Continuing.

Breakpoint 1, ksys_write (fd=1, buf=0x36846b50 "Hello, new world. \n", count=19) at fs/read_write.c:633
633     {




(gdb) list

(gdb) info break

(gdb) enable delete breakpoint-number

(gdb) delete breakponit 2








still do not get output on terminal...at least get the breakpoint:
# Breakpoint 1, ksys_write (fd=1, buf=0x36846b50 "Hello, new world. \n", count=19) at fs/read_write.c:633








# qemu-system-x86_64 -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append nokaslr

# run this, could see a print:
qemu-system-x86_64 -nographic -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append "root=/dev/sda console=ttyS0" 



# run this, could see a  breakpoint:
qemu-system-x86_64 -nographic -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append nokaslr





gdb vmlinux





############################### get a breakpoint 

Reading symbols from vmlinux...
(gdb) target remote :1234
Remote debugging using :1234
0x000000000000fff0 in exception_stacks ()
(gdb) info b
No breakpoints or watchpoints.
(gdb) break ksys_write
Breakpoint 1 at 0xffffffff8127f4c0: file fs/read_write.c, line 633.
(gdb) c
Continuing.

Breakpoint 1, ksys_write (fd=1, buf=0x1826ab50 "Hello, new world", '.' <repeats 35 times>, " \n", count=53) at fs/read_write.c:633
633     {
(gdb) list
628     {
629             return ksys_read(fd, buf, count);
630     }
631
632     ssize_t ksys_write(unsigned int fd, const char __user *buf, size_t count)
633     {
634             struct fd f = fdget_pos(fd);
635             ssize_t ret = -EBADF;
636
637             if (f.file) {
(gdb) 

############################### get a breakpoint 












(gdb) advance vfs_write
Couldn't get information on specified line.








what happend...









# repeatly run:
qemu-system-x86_64 -nographic -kernel arch/x86_64/boot/bzImage -initrd ~/distro/init.cpio -s -S -append nokaslr
gdb vmlinux
target remote :1234
break vfs_write
continue
list


ctrl+A, X: stop the qemu
quit the gdb








# again set breakpoint

break new_sync_write






tired...