17.4.3. pr_debug
Debug messages are not printable by default without recompiling.
But the awesome CONFIG_DYNAMIC_DEBUG=y option which we enable by default allows us to do:
echo 8 > /proc/sys/kernel/printk echo 'file kernel/module.c +p' > /sys/kernel/debug/dynamic_debug/control ./linux/myinsmod.out hello.ko
and we have a shortcut at:
./pr_debug.sh
Source: rootfs_overlay/lkmc/pr_debug.sh.
Wildcards are also accepted, e.g. enable all messages from all files:
echo 'file * +p' > /sys/kernel/debug/dynamic_debug/control
TODO: why is this not working:
echo 'func sys_init_module +p' > /sys/kernel/debug/dynamic_debug/control
Enable messages in specific modules:
echo 8 > /proc/sys/kernel/printk echo 'module myprintk +p' > /sys/kernel/debug/dynamic_debug/control insmod myprintk.ko
Source: kernel_modules/myprintk.c
This outputs the pr_debug message:
printk debug
but TODO: it also shows debug messages even without enabling them explicitly:
echo 8 > /proc/sys/kernel/printk insmod myprintk.ko
and it shows as enabled:
# grep myprintk /sys/kernel/debug/dynamic_debug/control /root/linux-kernel-module-cheat/out/kernel_modules/x86_64/kernel_modules/panic.c:12 [myprintk]myinit =p "pr_debug\012"
Enable pr_debug for boot messages as well, before we can reach userland and write to /proc:
./run --kernel-cli 'dyndbg="file * +p" loglevel=8'
Get ready for the noisiest boot ever, I think it overflows the printk buffer and funny things happen.