I compiled my Linux kernel according to the Linux Device Driver Chapter 4: Debugging Techniques. After I loaded my first hello world module and then checked the output by dmesg, however, all I can see is evbug: ........
I know I can turn off evbug's output by execute sudo rmmod evbug. But, obviously, it is inconvenient to execute this command after each reboot.
How could I disable this module's autoloading? I just want to load it manually when I need it.
You need to blacklist the module. For debian systems see https://wiki.debian.org/KernelModuleBlacklisting. For redhat systems see https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Installation_Guide/rescuemode_drivers-blacklisting.html
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tl;dr I want to use Linux "Dynamic Debug" but the path /sys/kernel/debug/dynamic_debug/ is not found.
I want to use the "Dynamic Debug" feature of Linux to enable debug messages for a particular kernel module (wireguard). To enable a kernel module kernel debug messages requires writing instructions to file /sys/kernel/debug/dynamic_debug/control,
echo module wireguard +p > /sys/kernel/debug/dynamic_debug/control
But there is no directory /sys/kernel/debug/dynamic_debug/ and thus no file /sys/kernel/debug/dynamic_debug/control.
The debugfs is mounted to /sys/kernel/debug/ (as is typical) and it has other files (so it's not like debugfs is mounted unexpectedly). Nor does alternative path /proc/dynamic_debug/ exist.
Running Linux kernel 5.15.
Enabling "Dynamic Debug" requires setting Linux compilation option CONFIG_DYNAMIC_DEBUG. In my case, I'm using Raspbian, and that project decided not to enable CONFIG_DYNAMIC_DEBUG during Linux builds.
I could "roll my own" (compile Raspbian with CONFIG_DYNAMIC_DEBUG) but that's a lot of work and likely will upset the apt package management.
🙁
Debugging the Linux Kernel with kgdb over rs-232 needs several preparation steps. I found awesome documentation, but no single-source that is fully self-contained and summarizes all steps needed, does not explain for ages, and has been tested. And also covers Yocto.
Is there any source that covers all that is needed in one single and short description ?
I.e.:
What files are needed in the directory GDB is started from (e.g. kernel awareness, source, vmlinux) and how to get theese, where to put it ?
When and where to get a cross-gdb from ?
ALL kernel config options needed, also the not-obvious ones (like CONFIG_RANDOMIZE_BASE)
How to configure the serial ports
Explaining a working back and forth of breaking into debugee and debugger to get started.
Explaining one rock-solid option of stopping the kernel that runs everywhere.
Explaining how to get this done not only for PC-PC debugging, but also for Yocto targets.
Debugging the Linux Kernel via a Nullmodem-Cable:
It took me a while to get a kgdb connection with Linux kernel awareness fully running. I share my way of doing this with Ubuntu Eoan (optional: Yocto Warrior) in 2020 here:
Tested with:
Debugging a linux based Intel PC from an Intel MacBook running MacOS Catalina. Using the gdb from the Homebrew package "i386-elf-gdb“. (wituout „-tui“ option in GDB)
Debugging a linux based ARM target (i.mx6, Yocto) from a linux based Intel PC.
Prerequisites:
You need two computers and a serial nullmodem cable. Check the cable by firiing up a serial termianl (e.g. screen or putty) on both hosts, connecting to your serial port (e.g. /dev/ttyS0 or /dev/ttyUSB0) and print characters from each station to the other. Remember the /dev/tty ports you confirmed.
Preparation:
You need on the first debuggee computer, we call it „target":
Special kernel installed that contains symbols, kgdb support etc.
Learn how to compile and install a kernel and use in make menuconfig belows configuration. You can search for Sybmbols with F8 or the / key in menuconfig.
(E.g. wiki.ubuntu.com. There take care in the first paragraph to execute deb-src before apt-get :)
# CONFIG_SERIAL_KGDB_NMI is not set
CONFIG_CONSOLE_POLL=y
# CONFIG_DEBUG_INFO is not set
CONFIG_KGDB=y
CONFIG_KGDB_SERIAL_CONSOLE=y
# CONFIG_KGDB_TESTS is not set
# CONFIG_KGDB_KDB is not set
CONFIG_FRAME_POINTER=y
CONFIG_DEBUG_INFO=y
# CONFIG_DEBUG_INFO_REDUCED is not set
# CONFIG_DEBUG_INFO_SPLIT is not set
CONFIG_DEBUG_INFO_DWARF4=y
CONFIG_GDB_SCRIPTS=y
CONFIG_STRIP_ASM_SYMS=y
# CONFIG_RANDOMIZE_BASE is not set
(Note for advanced Yocto use, skip if you're debugging a PC:
In yocto I created in my layer a file: recipes-kernel/linux/linux-mainline_%.bbappend with the content:
FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
SRC_URI += "file://kgdb.cfg“
And in files/kgdb.cfg I added the config fragment shown above (without the on ARM unavailable options CONFIG_RANDOMIZE_BASE and CONFIG_FRAME_POINTER)
)
You need on the second debugger computer, we call it „debugger pc":
Full kernel source code, same code you used to compile the kernel above. (If you compiled the .o and .ko objects in place and not in a build-folder you better not copy the directory from the other pc, where you called make etc. in, but then better grab fresh sources again.)
vmlinux file containing the symbols (lies in the kernel source root, or build folder on the highest level after kernel make).
vmlinux-gdb.py file that was made during the kernel build (also lies at the same position on the highest level.).
All scripts in the folder scripts/gdb (Folder scripts in the same toplevel-position. If you use a dedicated build folder use the script folder from there, not from the source folder.)
(Advanced: If both computers don’t match in CPU, like Intel and Arm, a cross-gdb build. Ignore if you're on Intel/AMD.)
Note for advanced Yocto use, I did something like (ignore if you debug a PC):
bitbake -c patch virtual/kernel #(apply the changed kernel config from above)
bitbake -f -c compile virtual/kernel #(unpack is not sufficient because of vmlinux-gdb.py)
mkdir ~/gdbenv
cp -a tmp/work-shared/phyboard-mira-imx6-14/kernel-source/. ~/gdbenv
cp tmp/work/phyboard_mira_imx6_14-phytec-linux-gnueabi/linux-mainline/4.19.100-phy1-r0.0/build/vmlinux ~/gdbenv
cp tmp/work/phyboard_mira_imx6_14-phytec-linux-gnueabi/linux-mainline/4.19.100-phy1-r0.0/build/vmlinux-gdb.py ~/gdbenv
mkdir ~/gdbenv/scripts
cp -r tmp/work/phyboard_mira_imx6_14-phytec-linux-gnueabi/linux-mainline/4.19.100-phy1-r0.0/build/scripts/gdb ~/gdbenv/scripts
Then (ignore if you're on a PC)
yocto bitbake -c populate_sdk [my-image]
Then (still ignore on PC) install the sdk .sh-installation file from your deploy directory on the debugger pc and start the environment as guided by the output of the install script (remember that command), then use "$GDB" for starting the cross-gdb instead of „gdb".
Debug execution
Launch on the debugger two console screens:
Console 1, ssh: +++++++++++++++++++++++++++++++++++++++
ssh user#192.168.x.y
sudo -s
echo ttyS0,9600n8 > /sys/module/kgdboc/parameters/kgdboc
echo 1 > /proc/sys/kernel/sysrq
echo g > /proc/sysrq-trigger
Console 2, local: ++++++++++++++++++++++++++++++++++++++++
cd ~/gdbenv
gdb -tui ./vmlinux
add-auto-load-safe-path ~/gdbenv
source ~/gdbenv/vmlinux-gdb.py
set serial baud 9600
target remote /dev/ttyS0 (use the tty port you confirmed in the beginning)
b [name of the c funtion you want to debug]
cont
Back to console 1, ssh: +++++++++++++++++++++++++++++++++++++++
[Now trigger the function, e.g. sudo modprobe yourFancyKernelModule]
Back to console 2, local: ++++++++++++++++++++++++++++++++++++++++
Now use gdb functions, like bt, step, next, finish ...
You can also use linux-aware commands. Call "apropos lx“ in gdb for a list of commands.
I'm working with a raspberry pi connected with xrusb to a controller using python.I use make file to Compile and install the common usb serial driver module and it works fine. After reboot i have problem. The driver is lost. I have to install the module again using this
modprobe usbserial
insmod ./xr_usb_serial_common.ko
Any idea?
Now my answer might be off because of the way you say "install the driver". I bet the make script most likely just loaded the driver just like you did via modprobe.
In order to get the module to be loaded at boot time, you need to tell udev what to load/do during bootup. And tell the kernel to load your driver.. Otherwise it assumes you don't want it to be loaded at boot time.
Either you can do a automatic module handling via:
#nano /etc/modules-load.d/usbserial.conf
usbserial
or, you can specify options:
#nano /etc/modprobe.d/usbserial.conf
options usbserial parameter_name=parameter_value
Here's some documentation on how this works:
http://man7.org/linux/man-pages/man5/modules-load.d.5.html
https://wiki.archlinux.org/index.php/kernel_modules
(Even if you're not running Arch on your RPi, they still have one of the best documentation websites for Linux out there. User friendly, in depth etc. So apply the information there to your Distribution, they should be very much the same this day and age)
I found maybe a temporary solution so i can finish my project and look for the best way later.
I make a script to run after reboot to load the driver.
use:
sudo crontab -e
then go to the bottom and write
#reboot bash /your/path/script/script.sh
`
Im running linux on my board and have to read info in /proc/[pid]/io. But it is not found.
For ex:
$ dd if=/dev/zero of=/tmp/aa &
[1] 926
$ cat /proc/926/io
cat: /proc/926/io: No such file or directory
Which I need enable to have kernel export that?
Many thanks for your help!
I just discovered that another thing is necessary.
I just recompiled a 4.4 kernel (for an embedded system) and enabling the CONFIG_TASKSTATS was not enough. I have to enable
CONFIG_TASKSTATS=y
CONFIG_TASK_XACCT=y
CONFIG_TASK_IO_ACCOUNTING=y
in order for the /proc/<pid>/io to appear.
According to this, you need CONFIG_TASKSTATS enabled in your kernel.
You can check your current kernel's config in various ways depending on distribution, but looking at /boot/config-$(uname -r) works in Redhat flavors.
If you don't have that option configured, you'll need to recompile your kernel, or investigate why your distro doesn't enable it.
I am trying to understand an existing Linux Wi-Fi driver for a USB Wi-Fi adapter. While I can read the C code, I would also like to be able to insert debug/print statements at certain critical spots in the driver to see how it behaves when executed. On a Linux system, after modifying the driver code, how does one go about loading it into the kernel in such a way that it replaces the old driver? Is there a way to "hotplug substitute" it straight over the old driver, or is it more complicated than that?
I intend on doing this inside of an expendible virtual machine, so I am not concerned about messing up the original driver, for what it counts.
If the driver is compiled as a module, all you need to do is to add your debug print outs, compile the module, rmmod the original module, insmod your new module and initiate the WLAN connection as usual.
If you want to test and edit on the fly :
lsmod to find the module name
rmmod it
edit source
Make sur you got a Makefile : obj-m := module_name.o
recompile (on Ubuntu) : make -C /usr/src/linux-headers-'uname -r' M='pwd' modules
insmod module_name.ko
If it's a device module, you might want to rm any devices in /dev, then do a mknod to remake them (see mknod man) and finally chmod to correct rights.