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<!DOCTYPE html>
<html dir="ltr" lang="en">
<head>
<meta charset='utf-8'>
<title>System Development & GDB</title>
</head>
<body>
<a href="index.html">C Index</a>
<h1>System Development</h1>
<p>System development requires knowing how to debug
kernel know how, in this example will be used Qemu
and GDB. Qemu creates the virtual machine that kernel
will run on and GDB will connect to it to help us
understand how things tick.</p>
<h2>Build Kernel</h2>
<pre>
$ tar xf linux-4.9.258.tar.xz
$ cd linux-4.9.258
</pre>
<p>Default configuration disable some security
configurations that allow us to debug (random memory
layout KALSR), CONFIG_COMPAT_BRK don't randomize
position of the programs (randomize_va_space) useful when
debugging a program (<a href="tracing.html">tracing</a>).
Configuration flags to enable;</p>
<ul>
<li>CONFIG_64BIT</li>
<li>CONFIG_DEBUG_KERNEL</li>
<li>CONFIG_HAVE_ARCH_KGDB</li>
<li>CONFIG_FTRACE</li>
<li>CONFIG_PRINTK</li>
<li>CONFIG_BLK_DEV_INITRD</li>
<li>CONFIG_BINFMT_ELF</li>
<li>CONFIG_TTY</li>
<li>CONFIG_DEBUG_INFO</li>
<li>CONFIG_DEBUG_INFO_DWARF4</li>
<li>CONFIG_GDB_SCRIPTS</li>
<li>CONFIG_READABLE_ASM</li>
<li>CONFIG_FRAME_POINTER</li>
<li>CONFIG_KGDB</li>
<li>CONFIG_KGDB_LOW_LEVEL_TRAP</li>
<li>CONFIG_EARLY_PRINTK</li>
<li>CONFIG_COMPAT_BRK</li>
</ul>
<p>And to disable;</p>
<ul>
<li>CONFIG_CC_OPTIMIZE_FOR_SIZE</li>
</ul>
<p>This changes can be achieved by creating a config-fragment and then
merge it with the configuration.</p>
<pre>
$ cat <<EOF >.config-fragment
CONFIG_64BIT=y
CONFIG_DEBUG_KERNEL=y
CONFIG_HAVE_ARCH_KGDB=y
CONFIG_COMPAT_BRK=y
CONFIG_FTRACE=y
CONFIG_PRINTK=y
CONFIG_BLK_DEV_INITRD=y
CONFIG_BINFMT_ELF=y
CONFIG_TTY=y
CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_INFO_DWARF4=y
CONFIG_GDB_SCRIPTS=y
CONFIG_READABLE_ASM=y
CONFIG_FRAME_POINTER=y
CONFIG_KGDB=y
CONFIG_KGDB_LOW_LEVEL_TRAP=y
CONFIG_EARLY_PRINTK=y
CONFIG_CC_OPTIMIZE_FOR_SIZE=n
EOF
</pre>
<p>Create a tiny config;</p>
<pre>
$ make ARCH=x86_64 tinyconfig
</pre>
<p>Merge config with the following script;</p>
<pre>
$ ./scripts/kconfig/merge_config.sh .config .config-fragment
</pre>
<p>Check or change the configuration according to your needs;</p>
<pre>
$ make nconfig
</pre>
<p>Build Kernel and modules;</p>
<pre>
$ make -j $(nproc)
</pre>
<h2>Simple Init</h2>
<p>Now that you have the kernel compiled you can
create a simple init program, this program is called
when kernel finish to load and setup its internals
and is ready to launch first process.
Init program should not exit or kernel will panic.
Create init.S;</p>
<pre>
.global _start
_start:
mov $1, %rax
mov $1, %rdi
mov $message, %rsi
mov $message_len, %rdx
syscall
jmp .
message: .ascii "FOOBAR FOOBAR FOOBAR FOOBAR FOOBAR FOOBAR FOOBAR\n"
.equ message_len, . - message
</pre>
<p>Assemble, link and create simple initial ram disk;</p>
<pre>
mkdir d
as --64 -o init.o init.S # assemble
ld -o d/init init.o # link
cd d
find . | cpio -o -H newc | gzip > ../rootfs.cpio.gz
</pre>
<p>Clean temporary directory;</p>
<pre>
cd ..
rm -r d/
</pre>
<p>Can be used C to create init program;</p>
<pre>
#include <stdio.h>
#include <unistd.h>
int main() {
printf("FOOBAR FOOBAR FOOBAR FOOBAR FOOBAR FOOBAR FOOBAR\n");
sleep(0xFFFFFFFF);
return 0;
}
</pre>
<pre>
$ gcc -static init.c -o init
</pre>
<h2>Start Debugging</h2>
<p>Test qemu, kernel and simple init program, you
should see
"FOOBAR FOOBAR FOOBAR FOOBAR FOOBAR FOOBAR";</p>
<pre>
$ qemu-system-x86_64 -enable-kvm --kernel arch/x86_64/boot/bzImage \
--initrd rootfs.cpio.gz
</pre>
<p>If everything goes well you can start qemu without
starting the CPU (-S) and with gdb server on TCP port 1234 (-s).</p>
<pre>
$ qemu-system-x86_64 -enable-kvm --kernel arch/x86/boot/bzImage \
--initrd rootfs.cpio.gz \
-S -s
</pre>
<p>On another terminal start gdb;</p>
<pre>
gdb \
-ex "add-auto-load-safe-path $(pwd)" \
-ex "file vmlinux" \
-ex 'set arch i386:x86-64:intel' \
-ex 'target remote localhost:1234' \
-ex 'hbreak start_kernel' \
-ex 'continue' \
-ex 'disconnect' \
-ex 'set arch i386:x86-64' \
-ex 'target remote localhost:1234'
</pre>
<pre>
(gdb) info thread
Id Target Id Frame
* 1 Thread 1 (CPU#0 [running]) start_kernel () at init/main.c:480
</pre>
<pre>
(gdb) info frame
Stack level 0, frame at 0xffffffff81e03f90:
rip = 0xffffffff81f4db2d in start_kernel (init/main.c:480); saved rip = 0xffffffff81f4d28e
called by frame at 0xffffffff81e03fa0
source language c.
Arglist at 0xffffffff81e03f80, args:
Locals at 0xffffffff81e03f80, Previous frame's sp is 0xffffffff81e03f90
Saved registers:
rip at 0xffffffff81e03f88
</pre>
<pre>
(gdb) print $rip
$2 = (void (*)()) 0xffffffff81f4db2d <start_kernel>
(gdb)
</pre>
<p>lx-symbols allows to debug kernel modules, after starting the vm and loading
the module use lx-symbols to load the symbols from all the modules loaded in
the kernel.</p>
<pre>
(gdb) apropos lx
(gdb) lx-symbols
</pre>
<p>It's useful to set conditional breakpoints or a break point can be trigger
by unrelated tasks, example of a break point on do_exit function but only by
the process with pid 1;</p>
<pre>
(gdb) br do_exit if $lx_current()->pid == 1
</pre>
<a href="index.html">C Index</a>
<p>
This is part of the LeetIO System Documentation.
Copyright (C) 2021
LeetIO Team.
See the file <a href="../../fdl-1.3-standalone.html">Gnu Free Documentation License</a>
for copying conditions.</p>
</body>
</html>
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