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== Session 4: Kernel Debugging ==

* [[http://workshop.rosedu.org/res/kernel/sesiune-04.pdf|Slides]]

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== Tasks ==

Download and unzip the tasks archive [[http://workshop.rosedu.org/res/kernel/sesiune-04-code.zip|sesiune-04]] on the virtual machine.

# gdb
#* ''cd /usr/src/linux''
#* ''file vmlinux''
#* Kernel symbol table <code bash>
# nm vmlinux | grep sys_call_table
# cat System.map | grep sys_call_table </code>
#* Run ''gdb --quiet vmlinux''
#** Dissasemble ''sys_fork'' function
#** Print ''x/x 0x123'' where 0x123 is an address obtained above from the symbol table.
#** Use gdb to display the address and value of ''jiffies'' variable.
#* Debug dynamic modules:
#** Compile ''myproc'' module from 0-gdb directory with debug options ''make COPTS=-g''.
#** Load the module ''insmod myproc.ko''.
#** Obtain the module's load address ''cat /sys/module/myproc/sections/.text''.
#** In gdb run: ''add-symbol-file myproc.o load_address''.
#** Display the code/implementation of myproc_read implemented in the module (hint: list).
# sysrq-trigger
#* Use this file to generate the following events:
#** ''echo d > /proc/sysrq-trigger'', display all currently held locks
#** ''echo m > /proc/sysrq-trigger'', output current memory information to the console	
#** ''echo p > /proc/sysrq-trigger'', output the current registers and flags to the console
#** ''echo q > /proc/sysrq-trigger'', display all active high-resolution timers and clock sources
#** ''echo c > /proc/sysrq-trigger'', generate a kernel panic
#* Use ''dmesg'' to check the results.
# Penguin allocation
#* Follow the code in 1-penguin directory.
#* ''penguin.c'' implements a simple kernel modules that brings alive a penguin. 
#* Do you notice any problems with the penguin module?
#* Use ''kmemleak'' to solve the problem:
#** Clear the list of all possible memory leaks:
#*** ''echo clear > /sys/kernel/debug/kmemleak''
#** insert penguin module: ''insmod penguin.ko''
#** remove penguin module: ''rmmod penguin.ko''
#** Trigger a memory scan:
#*** ''echo scan > /sys/kernel/debug/kmemleak''
#** Check possible memory  leaks:
#*** cat /sys/kernel/debug/kmemleak
# Linux kernel real memory leak 
#* Reboot the system to cleanup the leaks information from the previous session
#* Use kmemleak to check all possible leaks existing after booting
#* cat /sys/kernel/debug/kmemleak
#* Explain why there is a memory leak in function ''udp_table_init''?
# Slab debug
#* Follow the code in 2-crusher/ directory.
#** ''crusher.c'' is a simple module which allocates memory and accesses it in different forms.
#** Use ''CRUSH_MODE'' to generate the following types of errros:
#*** use before init
#*** use after free
# Stack size
#* Use the code in 3-eat-stack/ directory to determine the size of the stack.
#** Hints: 
#*** Make use of a buffer or make use of recursive calls.  
# Netconsole and netcat
#* Netconsole module is already configured on the virtual machine in order to send kernel panic messages to the host to port 6000.
#* On the host, start listening for these messages using netcat ''nc -lu 6000''.
#* On the virtual machine, generate a panic ''echo c > /proc/sysrq-trigger''.
# HW breakpoint
#* In this example 4-hwbreak/ we use 3 modules in order to monitor the write accesses on a variable.
#* ''simple.ko'' module defines and exports ''myvar'' variable, ''simplet.ko'' modifies it and ''data_breakpoint.ko'' adds a hardware breakpoint in order to monitor the variable. Follow the steps below:
#** build ''simple.ko'' and load it: ''./build.sh simple; insmod simple/simple.ko''
#** build ''data_breakpoint.ko'' and load it: ''./build.sh data_breakpoint; insmod data_breakpoint/data_breakpoint.ko ksym=myvar''
#** modify simple/Kbuild in order to build ''simplet.ko'' and load it: ''./build.sh simplet; insmod simple/simplet.ko''
#** follow the messages in dmesg.
#* We can also monitor other kernel variables by adding the associated name while loading ''data_breakpoint.ko'': ''ksym=variable_to_monitor''.
# Kprobes
#* Build and run the 3 samples in ''5-kprobe/'', one for each type of kprobe.
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