I am new to linux kernel.
wandering how to browse the complete flow, right from the power up of CPU.
Basic idea on BIOS/ROM code.
can I have some tool to debug the complete kernel ?
or
raw code browsing is preferable ?
The following tools may help you to debug Linux kernel
Dynamic Probes is one of the popular debugging tool for Linux which developed by IBM. This tool allows the placement of a “probe” at almost any place in the system, in both user and kernel space. The probe consists of some code (written in a specialized, stack-oriented language) that is executed when control hits the given point. Resources regarding dprobes / kprobes listed below
http://www-01.ibm.com/support/knowledgecenter/linuxonibm/liaax/dprobesltt.pdf
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.107.6212&rep=rep1&type=pdf
https://www.redhat.com/magazine/005mar05/features/kprobes/
https://sourceware.org/systemtap/kprobes/
http://www.ibm.com/developerworks/library/l-kprobes/index.html
https://doc.opensuse.org/documentation/html/openSUSE_121/opensuse-tuning/cha.tuning.kprobes.html
Linux Trace Toolkit is a kernel patch and a set of related utilities that allow the tracing of events in the kernel. The trace includes timing information and can create a reasonably complete picture of what happened over a given period of time. Resources of LTT, LTT Viewer and LTT Next Generation
http://elinux.org/Linux_Trace_Toolkit
http://www.linuxjournal.com/article/3829
http://multivax.blogspot.com/2010/11/introduction-to-linux-tracing-toolkit.html
MEMWATCH is an open source memory error detection tool. It works by defining MEMWATCH in gcc statement and by adding a header file to our code. Through this we can track memory leaks and memory corruptions. Resources regarding MEMWATCH
http://www.linuxjournal.com/article/6059
ftrace is a good tracing framework for Linux kernel. ftrace traces internal operations of the kernel. This tool included in the Linux kernel in 2.6.27. With its various tracer plugins, ftrace can be targeted at different static tracepoints, such as scheduling events, interrupts, memory-mapped I/O, CPU power state transitions, and operations related to file systems and virtualization. Also, dynamic tracking of kernel function calls is available, optionally restrictable to a subset of functions by using globs, and with the possibility to generate call graphs and provide stack usage. You can find a good tutorial of ftrace at https://events.linuxfoundation.org/slides/2010/linuxcon_japan/linuxcon_jp2010_rostedt.pdf
ltrace is a debugging utility in Linux, used to display the calls a user space application makes to shared libraries. This tool can be used to trace any dynamic library function call. It intercepts and records the dynamic library calls which are called by the executed process and the signals which are received by that process. It can also intercept and print the system calls executed by the program.
http://www.ellexus.com/getting-started-with-ltrace-how-does-it-do-that/?doing_wp_cron=1425295977.1327838897705078125000
http://developerblog.redhat.com/2014/07/10/ltrace-for-rhel-6-and-7/
KDB is the in-kernel debugger of the Linux kernel. KDB follows simplistic shell-style interface. We can use it to inspect memory, registers, process lists, dmesg, and even set breakpoints to stop in a certain location. Through KDB we can set breakpoints and execute some basic kernel run control (Although KDB is not source level debugger). Several handy resources regarding KDB
http://www.drdobbs.com/open-source/linux-kernel-debugging/184406318
http://elinux.org/KDB
http://dev.man-online.org/man1/kdb/
https://www.kernel.org/pub/linux/kernel/people/jwessel/kdb/usingKDB.html
KGDB is intended to be used as a source level debugger for the Linux kernel. It is used along with gdb to debug a Linux kernel. Two machines are required for using kgdb. One of these machines is a development machine and the other is the target machine. The kernel to be debugged runs on the target machine. The expectation is that gdb can be used to "break in" to the kernel to inspect memory, variables and look through call stack information similar to the way an application developer would use gdb to debug an application. It is possible to place breakpoints in kernel code and perform some limited execution stepping. Several handy resources regarding KGDB
http://landley.net/kdocs/Documentation/DocBook/xhtml-nochunks/kgdb.html
First, see related question Linux kernel live debugging, how it's done and what tools are used?. Try to use KDB or Ftrace.
If your intention is understanding whole flow of Linux kernel, running Linux kernel on QEMU can be easy way to learn how Linux works. Esp. you can emulate many CPU types without real H/W. or how about user mode Linux?
This document can be helpful to debug kernel on QEMU.
Just adding, the Linux kernel is not very suitable for debugging. Linus Torvalds once stated that he's againts supportng kernel debugging in Linux because it leads to badly written code.
I used kdbg, however I didn't find it very useful, what I suggest is to debug the kernel the oldschool way, using printk.
Related
both strace and ftrace seem to be used for tracing function calls in Linux. What's the difference?
strace is a utility which allows you to trace the system calls that an application makes. When an application makes a system call, it is basically asking the kernel to do something, eg file access. Use the command man strace to get strace documentation and man syscalls to get information on system calls.
ftrace is a tool used during kernel development and allows the developer to see what functions are being called within the kernel. The documentation is here and states:
Ftrace is an internal tracer designed to help out developers and
designers of systems to find what is going on inside the kernel.
It can be used for debugging or analyzing latencies and
performance issues that take place outside of user-space.
I'm new to device drivers in Linux. And my first day task is to debug driver using GDB in Linux.
I need to debug some XYZ (PCIe device driver supports ethernet) device driver to know about the flow and what is going on device's registers and all.
I have installed the driver with patch file and insmod command.
The device is working properly. But am not getting any solution to debug the device driver.
All I know is that how to debug C program using GDB in Linux(fedora20). I got one link similar to my Problem but from that also I have not got any knowledge.
Can anybody please share your thoughts that how can I start from scratch.
I am very specific to learn about Debugging device drivers in Linux. Especially that init or probe function inside my driver I need to know the flow.
The gdb debugger is useful to debug user-space application level programs (since it uses ptrace(2)).
For kernel code, things are different. Consider using kgdb (I don't know the details). You might also add debug prints ....
I recommend at least reading more about operating systems, e.g. Operating Systems: Three Easy Pieces (freely downloadable), and reading something about Linux programming (perhaps the old ALP, and also intro(2), syscalls(2) and related stuff). Don't dare coding Linux loadable kernel modules without good familiarity with Linux programming (in user-land). See also kernelnewbies.
BTW, you should prefer writing user-land code than kernel modules. A very simple rule of thumb is to avoid writing kernel code when possible.
To begin with, you may need to understand basics of device driver and kernel in linux. Subsequently focus as per the type of driver in-hand. You also need to understand the functionality (specification / manual / datasheet) of the device you are working on.
The very basic approach for debugging can be using printk. Normally there will be debug logs that can be enabled through compilation flags. If it is present, enable it so that it can give important pointers else you might need to add it on your own.
Start with verification of driver registration and verification of loading of your driver (static or loadable module as per your requirement). Check whether it is getting listed as part of sysfs or proc as applicable. Check whether probe is successful and subsequently the appropriate read/write/open/close/other calls as per your driver / device functionality.
The dmesg shall be very helpfull for viewing the kernel messages. There are also tools like kdb, LTT, strace that can be useful as per the scenario.
Now a days debugging become so advanced that even 'core kernel source code' can be debugged using Virtual environment.
But after reading couple of blog related to Kernel Core development it was not clear whether they are debugging using Virtual environment.
They have mentioned that they rely on 'Printing message' rather than using debugging tool, at-least for core component.
So, I Request from 'Linux Kernel Experts' to let me know what is good practice followed while debugging Kernel?
I've tried multiple approaches when trying to debug the kernel.
Sometimes, the easiest way is to just add a few printk statements based on my own conditional values, monitor the serial log and see what's going on. Its especially useful when the function in question is invoked quite often, but you are interested only in a subset of those.
QEMU GDB debugging. I have a buildroot filesystem setup. This means the kernel is lean and it boots up real fast. I start qemu with the -s -S flags, and attach gdb as target remote :1234. Additionally, there aren't very many userspace processes in this setup so its easier to debug the kernel.
VMWare stub. Assuming you are running an Ubuntu VM, it is possible that you can attach gdb to a VMware stub and debug the kernel. Personally, I never have had to pursue this route, but I look forward to trying it out someday.
If you have a kernel for a device that gets stuck in a bootloop and it does not print out any debug information out onto serial, it still might be helpful to try and boot it up using QEMU. Sure, the booting up will probably fail as the kernel tries to load up drivers, but you should be able to attach gdb, get a stack trace and see what the root cause is(perhaps a recursive call).
Linux n00b here. How does one switch from User Mode to Kernel Mode? I'm running Linux Ubuntu 12.10. Is there an interrupt that I can call using inline assembly code that will do this? If not, how can it be done?
I'm asking this question because I am wishing to write a SCTP (network)protocol stack which has access to the kernel and runs in the background constantly though the UI cannot directly access the kernel. Never done anything like this before so tips from pros would definitely be appreciated.
All switches to kernel mode are made via system calls. In the case of network protocols these system calls are socket, listen, accept, ioctl, read, write, recvmsg, etc.
You write a Linux kernel module. There is already a SCTP protocol stack for Linux though. You would likely be better off modifying it to do what you want.
Once you have written and compiled your module you can load it into the kernel using insmod and rmmod. In my experience you rarely get a chance to use rmmod because if you made a mistake the system crashes or freezes. So use a virtual machine for your testing. It is faster to reboot, you lose less data, and it is easier to hook up a virtual serial console for debugging.
I am sure this question is a duplicate by the way. You can find a lot of questions on this topic.
I would like to monitor the the context switching behavior in a multi-threaded pthread application.
In other RTOSes(Micro C OS) I have been able to register a context switch callback for each thread in the application, and then log (or toggle a gpio) and watch the thread context switching in real time. This was a valuable tool for debugging the real time behavior and interaction of the multiple threads.
My current environment is embedded linux utilizing the pthread api. Is there a way to monitor each of the context switches?
Not in the way you describe, however there are various profiling tools for Linux, like oprofile, SystemTap and perf events, I'm not sure how well they'd fit into embedded development though.
EDIT: perf is probably best (if you are running a recent enough kernel to use it) since it's in the mainline so you just have to turn it on, and it's really basic.
EDIT: if none of those work for you you can always modify the kernel context switch code...
EDIT: I missed one of the tracing frameworks, there is also LTTng
If you're using busybox and can compile your own kernel perf is probably the most minimal way to go, it's consists of turning on perf events in the kernel and compiling the perf tool that comes with the kernel source (it's in tools/perf)