[uml-devel] How to properly load debug symbols for module and set breakpoint

[Mark K. <mar...@gm...>]

I have build custom 64bit UML Linux kernel with TUN/TAP kernel module
(tun.ko) and then I have installed it on small root filesystem with busybox
. These module and kernel seems to work and I would like to make simple
changes in TUN module and measure it's performance .

So I have tried to follow guide from
http://user-mode-linux.sourceforge.net/hacking.html which explains how to
attach gdb to UML and set breakpoint for module init function .

(gdb) p modules
$1 = {next = 0x6385cf38, prev = 0x6385cf38}

(gdb) p *((struct module *)0x6385cf30)
$2 = {state = MODULE_STATE_LIVE, list = {next = 0x6036aee0, prev =
0x6036aee0}, name = "tun", '\000' <repeats 52 times>, mkobj = {kobj =
{name = 0x62e95b00 "tun", entry = {next = 0x62c30440, prev =
0x62c30148}, parent = 0x62c30450,
kset = 0x62c30440, ktype = 0x603694d0, sd = 0x62dd6fa0, kref =
{refcount = {counter = 3}}, state_initialized = 1, state_in_sysfs = 1,
state_add_uevent_sent = 1, state_remove_uevent_sent = 0,
uevent_suppress = 0},
mod = 0x6385cf30, drivers_dir = 0x0, mp = 0x0}, modinfo_attrs =
0x62ea0400, version = 0x0, srcversion = 0x0, holders_dir = 0x62da7400,
syms = 0x0, crcs = 0x0, num_syms = 0, kp = 0x0, num_kp = 0,
num_gpl_syms = 1,
gpl_syms = 0x6385c260, gpl_crcs = 0x0, gpl_future_syms = 0x0,
gpl_future_crcs = 0x0, num_gpl_future_syms = 0, num_exentries = 0,
extable = 0x0, init = 0x63860000, module_init = 0x0, module_core =
0x6385a000, init_size = 0,
core_size = 13990, init_text_size = 0, core_text_size = 8244,
init_ro_size = 127, core_ro_size = 11432, arch = {<No data fields>},
taints = 0, num_bugs = 0, bug_list = {next = 0x60376a40, prev =
0x60376a40}, bug_table = 0x0,
symtab = 0x6385d120, core_symtab = 0x6385d120, num_symtab = 37,
core_num_syms = 37, strtab = 0x6385d498 "", core_strtab = 0x6385d498
"", sect_attrs = 0x62c52000, notes_attrs = 0x62e98240, args =
0x62e95b20 " ", source_list = {
next = 0x6385d0e0, prev = 0x6385d0e0}, target_list = {next =
0x6385d0f0, prev = 0x6385d0f0}, waiter = 0x62ea10c0, exit =
0x6385a101, refptr = 0x62176ca0}

(gdb) add-symbol-file
/home/mark/playground/linux-2.6.39.3/drivers/net/tun.ko 0x6385a000
add symbol table from file
"/home/mark/playground/linux-2.6.39.3/drivers/net/tun.ko" at
.text_addr = 0x6385a000
(y or n)
Please answer y or n.
(y or n) y
Reading symbols from
/home/mark/playground/linux-2.6.39.3/drivers/net/tun.ko...done.

(gdb) p tun_init
$3 = {int (void)} 0x24 <tun_init>


(gdb) break tun_init
Cannot access memory at address 0x24



I don't understand why tun_init is located at 0x24 and not at 0x6385a000 ?
Can somebody point what I am doing wrong and how to load debug symbols and
set breakpoint ?