Split QEMU run, automate GDB debugging

README.md

@@ -1,9 +1,10 @@
 # Linux Kernel Module Cheat
 
-Run one command, get into QEMU Buildroot BusyBox with several minimal Linux kernel 4.9 module example tutorials. Tested in Ubuntu 14.04.
+Run one command, get into QEMU Buildroot BusyBox with several minimal Linux kernel 4.9 module example tutorials. Tested in Ubuntu 14.04 host.
 
 Usage:
 
+    sudo apt-get install qemu
     ./run
 
 First build will take a while (GCC, Linux kernel).
@@ -22,19 +23,72 @@ Each module comes from a C file under `kernel_module/`. For module usage do:
 
     grep BR2_LINUX_KERNEL_VERSION buildroot/.config
 
+After the first build, you can also run just:
+
+    ./runqemu
+
+to save a few seconds. `./run` wouldn't rebuild everything, but checking timestamps takes a few moments.
+
 We use `printk` a lot, and it shows on the QEMU terminal by default. If that annoys you (e.g. you want to see stdout separately), do:
 
     dmesg -n 1
 
 See also: <https://superuser.com/questions/351387/how-to-stop-kernel-messages-from-flooding-my-console>
 
-When your kernel starts crashing, get the full trace with:
+## Debugging
+
+GDB the linux kernel:
+
+    ./runqemu -d
+
+If you want to break immediately at a symbol, e.g. `start_kernel` of the boot sequence, open another terminal and run for example:
+
+    ./rungdb start_kernel
+
+Now QEMU will stop there, and you can use the normal GDB commands:
+
+    l
+    n
+    c
+
+To skip the boot, run just:
+
+    ./rungdb
+
+and when you want to break, do `Ctrl + C` from GDB.
+
+To have some fun, you can first run inside QEMU:
+
+    /count.sh
+
+which counts to infinity to stdout, and then:
+
+    Ctrl + C
+    break sys_write
+
+And now you can control the counting from GDB:
+
+    continue
+    continue
+    continue
+
+## Text mode
+
+Show serial output of QEMU directly on the current terminal, without opening a QEMU window:
 
     ./run -n
 
+To exit, just do a regular:
+
+    poweroff
+
+This is particularly useful to get full panic traces when you start making the kernel crashing :-)
+
+In case of a panic, you want your terminal back with `Ctrl + C, A` and type `quit`. See also: <http://stackoverflow.com/questions/14165158/how-to-switch-to-qemu-monitor-console-when-running-with-curses>
+
 See also: <https://unix.stackexchange.com/questions/208260/how-to-scroll-up-after-a-kernel-panic>
 
-Then if you want your terminal back, hit `Ctrl + C, A` and type `quit`. See also: <http://stackoverflow.com/questions/14165158/how-to-switch-to-qemu-monitor-console-when-running-with-curses>
+## Table of contents
 
 1.  [Introduction](introduction.md)
 1.  [Build](build.md)

kernel_config_fragment

@@ -1,4 +1,7 @@
 # If you change this file, you need to run:
 #     rm -rf buildroot/output/build/linux-*.*.*/
 # before ./run
+CONFIG_DEBUG_KERNEL=y
 CONFIG_DEBUG_FS=y
+CONFIG_DEBUG_INFO=y
+CONFIG_GDB_SCRIPTS=y

rootfs_overlay/count.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+# Generate infinitely many system calls :-)
+i=0
+while true; do
+  echo "$i"
+  i=$(($i+1))
+  sleep 1
+done

run

@@ -1,36 +1,8 @@
 #!/usr/bin/env bash
 set -e
-
-# CLI handling.
-nographic=false
-while getopts n OPT; do
-  case "$OPT" in
-    n)
-      nographic=true
-      ;;
-  esac
-done
-if $nographic; then
-  extra_append='console=ttyS0'
-  extra_flags='-nographic'
-else
-  extra_append=''
-  extra_flags=''
-fi
-
-# Work.
 cd buildroot
 make BR2_EXTERNAL="$(pwd)/../kernel_module" qemu_x86_64_defconfig
 cat ../buildroot_config_fragment >> .config
 env -u LD_LIBRARY_PATH make BR2_JLEVEL="$(($(nproc) - 2))" kernel_module-rebuild all
-qemu-system-x86_64 \
+cd ..
-  -M pc \
+./runqemu "$@"
-  -append "root=/dev/vda $extra_append" \
-  -drive file=output/images/rootfs.ext2,if=virtio,format=raw \
-  -kernel output/images/bzImage \
-  -m 128M \
-  -net nic,model=virtio \
-  -net user \
-  -smp 1 \
-  $extra_flags \
-;

rungdb

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+if [ "$#" -gt 0 ]; then
+  brk="-ex 'break $1'"
+else
+  brk=""
+fi
+cd buildroot/output/build/linux-*.*.*/
+cmd="gdb \
+  -ex 'add-auto-load-safe-path $(pwd)' \
+  -ex 'file vmlinux' \
+  -ex 'set arch i386:x86-64:intel' \
+  -ex 'target remote localhost:1234' \
+  $brk \
+  -ex 'continue' \
+  -ex 'disconnect' \
+  -ex 'set arch i386:x86-64' \
+  -ex 'target remote localhost:1234'
+"
+echo "$cmd"
+eval "$cmd"

runqemu

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+
+set -e
+
+# CLI handling.
+extra_append=''
+extra_flags=''
+while getopts dn OPT; do
+  case "$OPT" in
+    d)
+      extra_flags="$extra_flags -S -s"
+      ;;
+    n)
+      extra_append="$extra_append console=ttyS0"
+      extra_flags="$extra_flags -nographic"
+      ;;
+  esac
+done
+
+qemu-system-x86_64 \
+  -M pc \
+  -append "root=/dev/vda $extra_append" \
+  -drive file=buildroot/output/images/rootfs.ext2,if=virtio,format=raw \
+  -kernel buildroot/output/images/bzImage \
+  -m 128M \
+  -net nic,model=virtio \
+  -net user \
+  -smp 1 \
+  $extra_flags \
+;