SysRq debugger

SysRq debugger can help you to obtain some information from oops-ed kernel.

Features

SysRq debugger can do one of the following:

1. Dump memory at specified address
2. Write arbitrary value to memory at specified address
3. Resolve symbol address by its name

How to use it

To enter debug mode press SysRq-g. After this the following commands will be available:

1. SysRq-d - dump memory
2. SysRq-w - write to memory
3. SysRq-r - resolve symbol
4. SysRq-x - go on dumping memory
   • ... from the latest address dumped with -d
   • ... from the address you wrote to with -w
   • ... from the begginning of the symbol resolved with -r
5. SysRq-q - quit debugger

After entering -d, -w or -r state you must enter address to work with (and the value you want to write if you do). Simply do this by pressing according keys while holding Alt-StsRq.

If you need to enter the upper case letter or the underbar (_) while typing a symbol name then you need to press and release shift and then the letter or minus (-).

At the end press enter.

Working with proc

There is a /proc/sysrq-trigger entry which triggers sysrq events. You may send a string to it to trigger commands one by one. There is one note about it - string must end with '\r' character.

Examples

Here are some examples of how to use debugger

Dump memory

# echo -n -e 'gd0xc0000000\rq' > /proc/sysrq-trigger

will dump memory region of 512 bytes

# echo -n -e 'gxq' > /proc/sysrq-trigger

will dump the next 512 bytes of memory

Write to memory

# echo -n -e 'gw0xc0000000-0x12345678\rq' > /proc/sysrq-trigger

will write 0x12345678 at 0xc0000000

Resolve symbol

# echo -n -e 'grschedule\rq' > /proc/sysrq-trigger

will print you an address schedule() function starts from

# echo -n -e 'gxq' > /proc/sysrq-trigger

will dump first 512 bytes of it

Note -n and -e keys to echo command.