I work in a system that does not allow me to enable tracepoints at runtime. To work around this, I manually add printks near a trace point to during debugging. This seems to be very inefficient and I am looking for methods to enable a tracepoint at compile time and have it pushed to the kernel log ring buffer.
You can use the kernel kprobes to dynamically register probe functions into the kernel. probe functions are just kernel modules that are dynamically inserted into your kernel and from there you can print to ring buffer. kprobes are automatically enabled on nearly all major linux distro's by default. see the documentation to know how to register and unregister kprobes.
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I know that in kernel I can call get_cpu() to disable kernel preemption and local_irq_save(flags) to disable interrupt. I'm curious how can I still achieve the same goal without calling these functions. I have three questions,
I found that cli instruction is the one responsible for disabling interrupts. Therefore, I was wondering if the following in-line assembly,
asm volatile("cli": : :"memory");
is enough to disable interrupts. If It works, how can I check if it actually disables interrupts?
Is there any inline assembly instruction that I can use to disable kernel preemption?
I know it is not possible to disable interrupts from the userspace. I just wondering if somehow someone manages to run that cli instruction from the userspace. Will it still be able to disable the interrupts? If the answer is no then why?
As a part of an experiment i need to make a read-only page writable in kernel address space from a non-IOKit kext. In user mode i can do vm_protect, but there is surprisingly little info on how this can be done in kernel mode (or i am completely blind). Is there a call to do this?
In the kernel, it's declared in <mach/vm_map.h>, and is part of the "unsupported" KPI. (Add com.apple.kpi.unsupported to OSBundleRequired of your kext's info.plist) I unfortunately haven't used this function from the kernel before, so I can't really comment beyond that. vm_map_t appears to be a Mach Port reference, but I'm not sure where you'd get it from.
I have ARM board at remote location. Some time I had a kernel panic error in it. At this same time there is no option to hardware restart. bus no one is available at this place to restart it.
I want to restart my board automatically after kernel panic error. so what to do in kernel.
If your hardware contains watchdog timer, then compile the kernel with watchdog support and configure it. I suggest to follow this blog http://www.jann.cc/2013/02/02/linux_watchdog.html
Caution :: I never tried this. If the problem is solved, request you to update here.
You can modify the panic() function kernel/panic.c to call the kernel_restart(*cmd) at the point you want it to restart (like probably after printing the required debug information).
I am assuming you are bringing up a board, so Please note that you need to supply the ops for the associated functions in machine_restart() - (called by kernel_restart) in accordance to the MACH . If you are just using the board as is , then i guess rebuilding the kernel with kernel_restart(*cmd) should do.
The panic() is usually due to events that the kernel can not recover from. If you do not have a watchdog, you need to look at your hardware to see if a GPIO, etc is connected to the RESET line. If so, you can toggle this pin to reboot the CPU. Trying to alter panic() may just make things worse, depending on the root cause and the type of features you use.
You may hook arm_pm_restart with your custom restart functionality. You can test it with the shell command reboot, if present. panic() should call the same routine. With current ARM Linux versions
You may wish to turn off the MMU and block interrupts in this routine. It will make it more resilient when called from panic(). As you are going to reset, you can copy the routine to any physical address you like.
The watchdog maybe better; it may catch cases where even panic() may not be called. You may have a watchdog and not realize it. Many Cortex-A CPUs, have one built in. It is fairly rare for hardware not to have a watchdog.
However, if you don't have the watchdog, you can use the GPIO mechanism above; hardware should usually provide someway for software to restart the device (and peripherals). The panic() maybe due to some mis-behaving device tromping memory, latched up DRAM/Flash, etc. Toggling a RESET line maybe better than a watchdog in this case; if the RESET is also connected to other hardware, besides the CPU.
Related: How to debug kernel freeze, How to change watchdog timer
AFAIK, a simple way to restart the board after kernel panic is to pass a kernel parameter (from the bootloader usually)
panic=1
The board will then auto-reboot '1' second(s) after a panic.
Search the Documentation for more.
Some examples from the documentation:
...
panic= [KNL] Kernel behaviour on panic: delay <timeout>
timeout > 0: seconds before rebooting
timeout = 0: wait forever
timeout < 0: reboot immediately
Format: <timeout>
...
oops=panic Always panic on oopses. Default is to just kill the
process, but there is a small probability of
deadlocking the machine.
This will also cause panics on machine check exceptions.
Useful together with panic=30 to trigger a reboot.
...
As suggested in previous comments watchdog timer is your friend here. If your hardware contains watchdog timer, Enable it in kernel option and configure it.
Other alternative is use Phidget. If you usb connection available at remote location. Phidget controller/software is used to control your board using USB. Check for board support.
I am programming a pci device with verilog and also writing its driver,
I have probably inserted some bug in the hardware design and when i load the driver with insmod the kernel just gets stuck and doesnt respond. Now Im trying to figure out what's the last driver code line that makes my computer stuck. I have inserted printk in all relevant functions like probe and init but non of them get printed.
What other code is running when i use insmod before it gets to my init function? (I guess the kernel gets stuck over there)
printks are often not useful debugging such a problem. They are buffered sufficiently that you won't see them in time if the system hangs shortly after printk is called.
It is far more productive to selectively comment out sections of your driver and by process of elimination determine which line is the (first) problem.
Begin by commenting out the entire module's init section leaving only return 0;. Build it and load it. Does it hang? Reboot system, reenable the next few lines (class_create()?) and repeat.
From what you are telling, it is looks like that Linux scheduler is deadlocking by your driver. That's mean that interrupts from the system timer doesn't arrive or have a chance to be handled by kernel. There are two possible reasons:
You hang somewhere in your driver interrupt handler (handler starts its work but never finish it).
Your device creates interrupts storm (Device generates interrupts too frequently as a result your system do the only job -- handling of your device interrupts).
You explicitly disable all interrupts in your driver but doesn't reenable them.
In all other cases system will either crash, either oops or panic with all appropriate outputs or tolerate potential misbehavior of your device.
I guess that printk won't work for such extreme scenario as hang in kernel mode. It is quite heavy weight and due to this unreliable diagnostic tool for scenarios like your.
This trick works only in simpler environments like bootloaders or more simple kernels where system runs in default low-end video mode and there is no need to sync access to the video memory. In such systems tracing via debugging output to the display via direct writing to the video memory can be great and in many times the only tool that can be used for debugging purposes. Linux is not the case.
What techniques can be recommended from the software debugging point of view:
Try to review you driver code devoting special attention to interrupt handler and places where you disable/enable interrupts for synchronization.
Commenting out of all driver logic with gradual uncommenting can help a lot with localization of the issue.
You can try to use remote kernel debugging of your driver. I advice to try to use virtual machine for that purposes, but I'm not aware about do they allow to pass the PCI device in the virtual machine.
You can try the trick with in-memory tracing. The idea is to preallocate the memory chunk with well known virtual and physical addresses and zeroes it. Then modify your driver to write the trace data in this chunk using its virtual address. (For example, assign an unique integer value to each event that you want to trace and write '1' into the appropriate index of bytes array in the preallocated memory cell). Then when your system will hang you can simply force full memory dump generation and then analyze the memory layout packed in the dump using physical address of the memory chunk with traces. I had used this technique with VmWare Workstation VM on Windows. When the system had hanged I just pause a VM instance and looked to the appropriate .vmem file that contains raw memory latout of the physical memory of the VM instance. Not sure that this trick will work easy or even will work at all on Linux, but I would try it.
Finally, you can try to trace the messages on the PCI bus, but I'm not an expert in this field and not sure do it can help in your case or not.
In general kernel debugging is a quite tricky task, where a lot of tricks in use and all they works only for a specific set of cases. :(
I would put a logic analyzer on the bus lines (on FPGA you could use chipscope or similar). You'll then be able to tell which access is in cause (and fix the hardware). It will be useful anyway in order to debug or analyze future issues.
Another way would be to use the kernel crash dump utility which saved me some headaches in the past. But depending your Linux distribution requires installing (available by default in RH). See http://people.redhat.com/anderson/crash_whitepaper/
There isn't really anything that is run before your init. Bus enumeration is done at boot, if that goes by without a hitch the earliest cause for freezing should be something in your driver init AFAIK.
You should be able to see printks as they are printed, they aren't buffered and should not get lost. That's applicable only in situations where you can directly see kernel output, such as on the text console or over a serial line. If there is some other application in the way, like displaying the kernel logs in a terminal in X11 or over ssh, it may not have a chance to read and display the logs before the computer freezes.
If for some other reasons the printks still do not work for you, you can instead have your init function return early. Just test and move the return to later in the init until you find the point where it crashes.
It's hard to say what is causing your freezes, but interrupts is one of those things I would look at first. Make sure the device really doesn't signal interrupts until the driver enables them (that includes clearing interrupt enables on system reset) and enable them in the driver only after all handlers are registered (also, clear interrupt status before enabling interrupts).
Second thing to look at would be bus master transfers, same thing applies: Make sure the device doesn't do anything until it's asked to and let the driver make sure that no busmaster transfers are active before enabling busmastering at the device level.
The fact that the kernel gets stuck as soon as you install your driver module makes me wonder if any other driver (built in to kernel?) is already driving the device. I made this mistake once which is why i am asking. I'd look for the string "kernel driver in use" in the output of 'lspci' before installing the module. In any case, your printk's should be visible in dmesg output.
in addition to Claudio's suggestion, couple more debug ideas:
1. try kgdb (https://www.kernel.org/doc/htmldocs/kgdb/EnableKGDB.html)
2. use JTAG interfaces to connect to debug tools (these i think vary between devices, vendors so you'll have to figure out which debug tools you need to the particular hardware)
I want to be able to monitor kernel panics - know if and when they have happened.
Is there a way to know, after the machine has booted, that it went down due to a kernel panic (and not, for example, an ordered reboot or a power failure)?
The machine may be configured with KDUMP and/or KDB, but I prefer not to assume that either is or is not installed.
Patching the kernel is an option, though I prefer to avoid it. But even if I do it, I'm not sure what can the patch do.
I'm using kernel 2.6.18 (ancient, I know). Solutions for newer kernels may be interesting too.
Thanks.
The kernel module 'netconsole' may help you to log kernel printk messages over UDP.
You can view the log message in remote syslog server, event if the machine is rebooted.
Introduction:
=============
This module logs kernel printk messages over UDP allowing debugging of
problem where disk logging fails and serial consoles are impractical.
It can be used either built-in or as a module. As a built-in,
netconsole initializes immediately after NIC cards and will bring up
the specified interface as soon as possible. While this doesn't allow
capture of early kernel panics, it does capture most of the boot
process.
Check kernel document for more information: https://www.kernel.org/doc/Documentation/networking/netconsole.txt