Disassembling the kernel

This page describes how to obtain sufficient disassembling of your kernel when reporting a new BUG caught on a self-compiled kernel.

The objdump utility

The main utility used to do it is the objdump one. Most of the time it's enough just to run

# objdump -dr <the-binary-file-to-dump> > <the-output-file>

and the disassembled binary will be obtained.

Most often you'd need to provide the dump of the vmlinux file like this:

# objdump -dr vmlinux-<kernel-version> > vmlinux.decoded

We'll need the vmlinux.decoded file in this case.

Some hints that can be get from the kernel BUG report

Let's look at how a BUG report can look (in the dmesg output)

BUG: unable to handle kernel NULL pointer dereference at virtual address 00000044
 printing eip:
c05d575b
*pde = 00000000
Oops: 0000 [#1]
SMP
Modules linked in: ...
...
 Call Trace:
 [<c0429ea3>] local_bh_enable+0x95/0xa4
 ...
 [<c05bc1ba>] net_rx_action+0x8f/0x185
 [<c05b1aa9>] kernel_sendmsg+0x2f/0x3d
 [<d0a7e24f>] xs_send_kvec+0x92/0x9a [sunrpc]
 [<c048e3bc>] destroy_inode+0x24/0x33
 [<d0a7e38e>] xs_sendpages+0x82/0x128 [sunrpc]
 ...
 [<d0a80732>] rpc_async_schedule+0x0/0x8 [sunrpc]
 [<c0433d6a>] run_workqueue+0x78/0xf5
 [<c0433de7>] worker_thread+0x0/0xdc
 ...
 [<c04371fb>] kthread+0x0/0x55
 [<c0405913>] kernel_thread_helper+0x7/0x10
 =======================
 ...
EIP: [<c05d575b>] ip_route_output_slow+0x40/0x6fa SS:ESP 0068:cf73bbbc

I've thrown away some unneeded info, so as you can see some function names in the stack trace are accompanied with the [sunrpc] string. This is a module name, in you traces you can see some other modules. So, when you disassembled the vmlinux file and attached this to the BUG report, very likely we'll ask you to disassemble the modules, that are seen in the stack trace, so you'd have to run the

# objdump -dr /lib/modules/<version>/<path>/<module-name>.ko > <module-name>.decoded

command and attach this module dump as well. Note, that the string

EIP: [<c05d575b>] ip_route_output_slow+0x40/0x6fa SS:ESP 0068:cf73bbbc

also contains the function name, that can also belong to some module.