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<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>Kernel Build</h2>
<pre>
$ tar xf linux-4.9.48.tar.xz
$ cd linux-4.9.48
</pre>
<p>Default configuration disable some security
configurations that allow us to debug (random memory
layout).</p>
<pre>
$ make x86_64_defconfig
</pre>
<p>Enable CONFIG_DEBUG_INFO, CONFIG_DEBUG_INFO_DWARF4
and CONFIG_GDB_SCRIPTS in the kernel;</p>
<pre>
make x86_64_defconfig
cat <<EOF >.config-fragment
CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_KERNEL=y
CONFIG_GDB_SCRIPTS=y
EOF
./scripts/kconfig/merge_config.sh .config .config-fragment
</pre>
<p>Check or change to your needs the configuration;</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 'break 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>
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<p>
This is part of the Hive System Documentation.
Copyright (C) 2019
Hive Team.
See the file <a href="../../fdl-1.3-standalone.html">Gnu Free Documentation License</a>
for copying conditions.</p>
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