KGDB!

README.md

@@ -28,7 +28,7 @@ which are `printk` messages from `init` and `cleanup` methods of those modules.
 
 Each module comes from a C file under `kernel_module/`. For module usage see:
 
-    head kernel_module/*.c
+    head kernel_module/modulename.c
 
 Good bets inside guest are:
 
@@ -211,6 +211,62 @@ ARM TODOs:
     - <https://github.com/cloudius-systems/osv/issues/49>
     - <https://unix.stackexchange.com/questions/167165/how-to-pass-ctrl-c-in-qemu>
 
+## KGDB
+
+KGDB is kernel dark magic that allows you to GDB the kernel on real hardware without any extra hardware support.
+
+It is useless with QEMU since we already have full system visibility with `-gdb`, but this is a good way to learn it.
+
+Cheaper than JTAG (free) and easier to setup (no wires), but with less visibility as it depends on the kernel working, so e.g.: dies on panic, does not see boot sequence.
+
+Usage:
+
+    ./runqemu -k
+    ./rungdb -k
+
+In GDB:
+
+    c
+
+In QEMU:
+
+    /count.sh &
+    /kgdb.sh
+
+In GDB:
+
+    b sys_write
+    c
+    c
+    c
+    c
+
+And now you can count from GDB!
+
+If you do: `b sys_write` immediately after `./rungdb -k`, it fails with `KGDB: BP remove failed: <address>`. I think this is because it would break too early on the boot sequence, and KGDB is not yet ready.
+
+See also:
+
+- <https://github.com/torvalds/linux/blob/v4.9/Documentation/DocBook/kgdb.tmpl>
+
+### KGDB kernel modules
+
+In QEMU:
+
+    /kgdb-mod.sh
+
+In GDB:
+
+    lx-symbols ../kernel_module-1.0/
+    b fop_write
+    c
+    c
+    c
+
+and you now control the count.
+
+TODO: if I `-ex lx-symbols` to the `gdb` command, just like done for QEMU `-gdb`, the kernel oops. How to automate this step?
+
 ## Table of contents
 
 1.  [Introduction](introduction.md)

kernel_config_fragment

@@ -5,3 +5,18 @@ CONFIG_DEBUG_KERNEL=y
 CONFIG_DEBUG_FS=y
 CONFIG_DEBUG_INFO=y
 CONFIG_GDB_SCRIPTS=y
+
+# KGDB.
+CONFIG_CONSOLE_POLL=y
+CONFIG_KDB_CONTINUE_CATASTROPHIC=0
+CONFIG_KDB_DEFAULT_ENABLE=0x1
+CONFIG_KDB_KEYBOARD=y
+CONFIG_KGDB=y
+CONFIG_KGDB_KDB=y
+CONFIG_KGDB_LOW_LEVEL_TRAP=y
+CONFIG_KGDB_SERIAL_CONSOLE=y
+CONFIG_KGDB_TESTS=y
+CONFIG_KGDB_TESTS_ON_BOOT=n
+CONFIG_MAGIC_SYSRQ=y
+CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x1
+CONFIG_SERIAL_KGDB_NMI=n

rootfs_overlay/kgdb-mod.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+insmod /fops.ko
+cd /sys/kernel/debug/kernel_module_cheat
+i=0
+while true; do
+  printf "$i" >fops
+  i=$(($i+1))
+done &
+/kgdb.sh

rootfs_overlay/kgdb.sh

@@ -0,0 +1,2 @@
+#!/bin/sh
+echo g > /proc/sysrq-trigger

rungdb

@@ -3,11 +3,15 @@
 set -e
 
 arch=x86_64
-while getopts a: OPT; do
+kgdb=false
+while getopts a:k OPT; do
   case "$OPT" in
     a)
       arch=$OPTARG
       ;;
+    k)
+      kgdb=true
+      ;;
   esac
 done
 shift "$(($OPTIND - 1))"
@@ -19,6 +23,14 @@ fi
 
 gdb="$(pwd)/buildroot/output/host/usr/bin/${arch}-linux-gdb"
 cd buildroot/output/build/linux-*.*.*/
+if $kgdb; then
+    cmd="$gdb \
+      -q \
+      -ex 'add-auto-load-safe-path $(pwd)' \
+      -ex 'file vmlinux' \
+      -ex 'target remote localhost:1234'
+    "
+else
   case "$arch" in
     x86_64)
       # http://stackoverflow.com/questions/11408041/how-to-debug-the-linux-kernel-with-gdb-and-qemu/33203642#33203642
@@ -48,4 +60,5 @@ case "$arch" in
       "
       ;;
   esac
+fi
 eval "$cmd"

runqemu

@@ -8,7 +8,7 @@ debug=false
 nographic=false
 extra_append=''
 extra_flags=''
-while getopts a:dn OPT; do
+while getopts a:dkn OPT; do
   case "$OPT" in
     a)
       arch=$OPTARG
@@ -17,6 +17,11 @@ while getopts a:dn OPT; do
       debug=true
       extra_flags="$extra_flags -S -s"
       ;;
+    k)
+      debug=true
+      extra_append="$extra_append kgdbwait kgdboc=ttyS0,115200"
+      extra_flags="$extra_flags -serial tcp::1234,server,nowait"
+      ;;
     n)
       extra_append="$extra_append console=ttyS0"
       extra_flags="$extra_flags -nographic"