To debug init_module, place a breakpoint in the kernel just before the point where it calls init_module function.

For kgdb versions 1.9 and later
Load the module with modprobe or insmod and wait for the breakpoint to occur. When the breakpoint occurs gdb has
already detected that the module was loaded. You can now place a breakpoint anywhere in the module.

For kgdb versions 1.8 and earlier
Once the kernel breaks into the debugger, wait for loadmodule.sh script to exit. At this point of time init_module isn't called and all the debugging information is available. Now load the module object file loading script generated by loadmodule.sh script to load the module object file into gdb.
Once the module object file is loaded, a breakpoint can be placed anywhere in the module such as a statement in init_module function.

A snippet of the kernel/module.c file from 2.4.10 kernel is shown below with the place where a breakpoint can be placed.

 /* Free our temporary memory.  */
 put_mod_name(n_name);
 put_mod_name(name);

 /* Initialize the module.  */
 atomic_set(&mod->uc.useWorking of loadmodule.sh script

This information applies only to kgdb versions 1.8 and earlier.

Object file sections

Consider a module object file trfs.  The sections contained in the file can be listed using objdump command.

$ objdump -h trfs

trfs:     file format elf32-i386

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         0000162c  00000000  00000000  00000040  2**4
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  1 .text.lock    0000010a  00000000  00000000  0000166c  2**0
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
  2 .rodata       000002a4  00000000  00000000  00001780  2**5
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  3 __ksymtab     00000020  00000000  00000000  00001a24  2**2
                  CONTENTS, ALLOC, LOAD, RELOC, READONLY, DATA
  4 .kstrtab      0000006f  00000000  00000000  00001a60  2**5
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  5 .modinfo      00000111  00000000  00000000  00001acf  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  6 .data         00000178  00000000  00000000  00001be0  2**5
                  CONTENTS, ALLOC, LOAD, RELOC, DATA
  7 .bss          00000010  00000000  00000000  00001d58  2**2
                  ALLOC
  8 .stab         0003b8b0  00000000  00000000  00001d58  2**2
                  CONTENTS, RELOC, READONLY, DEBUGGING
  9 .stabstr      000e32bd  00000000  00000000  0003d608  2**0
                  CONTENTS, READONLY, DEBUGGING
 10 .comment      000001ab  00000000  00000000  001208c5  2**0
                  CONTENTS, READONLY
 11 .note         0000008c  00000010  00000010  00120a70  2**0
                  CONTENTS, READONLY

.text is the code section. Sections .data and .bss contain initialized and uninitialized data respectively. Debugging information is contained in sections .stab and .stabstr.

VMA is the virtual address of the section. Since the file is not relocated yet, VMAs of all sections are zero.

Kernel object file, vmlinux, is relocated. Hence VMAs of sections in vmlinux are the actual addresses where they appear in virtual address space of a process.

[amit@askii-pc linux-2.4.6-kgdb]$ objdump -h vmlinux

vmlinux:     file format elf32-i386

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .text         0014ed2b  c0100000  c0100000  00001000  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  1 .text.lock    00009579  c024ed30  c024ed30  0014fd30  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  2 .rodata       00048061  c02582c0  c02582c0  001592c0  2**5
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  3 .kstrtab      0000a200  c02a0340  c02a0340  001a1340  2**5
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  4 __ex_table    00001848  c02aa540  c02aa540  001ab540  2**2
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  5 __ksymtab     000023f0  c02abd88  c02abd88  001acd88  2**2
                  CONTENTS, ALLOC, LOAD

, READONLY, DATA
  6 .data         0001a200  c02ae180  c02ae180  001af180  2**5
                  CONTENTS, ALLOC, LOAD, DATA
  7 .data.init_task 00002000  c02ca000  c02ca000  001ca000  2**5
                  CONTENTS, ALLOC, LOAD, DATA
  8 .text.init    00018718  c02cc000  c02cc000  001cc000  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  9 .data.init    0001ed18  c02e4720  c02e4720  001e4720  2**5
                  CONTENTS, ALLOC, LOAD, DATA
 10 .setup.init   00000170  c0303440  c0303440  00203440  2**2
                  CONTENTS, ALLOC, LOAD, DATA
 11 .initcall.init 000000bc  c03035b0  c03035b0  002035b0  2**2
                  CONTENTS, ALLOC, LOAD, DATA
 12 .data.page_aligned 00000800  c0304000  c0304000  00204000  2**5
                  CONTENTS, ALLOC, LOAD, DATA
 13 .data.cacheline_aligned 00004520  c0304800  c0304800  00204800  2**5
                  CONTENTS, ALLOC, LOAD, DATA
 14 .bss          00041f54  c0308d20  c0308d20  00208d20  2**5
                  ALLOC
 15 .stab         00615708  00000000  00000000  00208d20  2**2
                  CONTENTS, READONLY, DEBUGGING
 16 .stabstr      001b4ec1  00000000  00000000  0081e428  2**0
                  CONTENTS, READONLY, DEBUGGING
 17 .comment      00005bfa  00000000  00000000  009d32e9  2**0
                  CONTENTS, READONLY
 18 .note         00001e28  c034ac74  c034ac74  009d8ee3  2**0
                  CONTENTS, READONLY

As you can see from above output, the code section, .text, begins at address 0xc0100000.

Loading of the module

When the module is loaded into kernel, it has to be first relocated. Relocation of a module means changing all relative references in it to absolute addresses. After a module is relocated, the insmod command prints module map file if requested on command line. A module map file contains addresses of sections and all symbols in the module. Part of a module map file for module trfs is given below.

[amit@askii-pc linux-2.4.6-kgdb]$ head -n 20 /tmp/trfs.map
Sections:       Size      Address   Align
.this           00000060  c1808000  2**2
.text           0000162c  c1808060  2**4
.text.lock      0000010a  c180968c  2**0
.rodata         000002a4  c18097a0  2**5
__ksymtab       00000048  c1809a44  2**2
.kstrtab        00000109  c1809aa0  2**5
.data           00000178  c1809bc0  2**5
.bss            00000010  c1809d38  2**2
__archdata      00000000  c1809d50  2**4

Symbols:
00000000 a translators.c
00000000 a linkinfo.c
00000000 a view.c
00000000 a super.c
00000000 a dirinfo.c
00000000 a link.c
00000000 a dir.c
c1808000 d __this_module

The code section, .text, is loaded at address 0xc1808060.

Generation of gdb script

loadmodule.sh script generates a gdb script for loading the object file into gdb. The script tells gdb to load the module object file and gives it addresses where different sections are loaded. The gdb script for above module map is show below.

[amit@askii-pc linux-2.4.6-kgdb]$ cat /mnt/work/gdbscripts/loadtrfs
add-symbol-file /mnt/work/build/old-pc/trfs/modules/trfs/trfs 0xc1808060 -s .text.lock 0xc180968c -s .rodata 0xc18097a0 -s __ksymtab 0xc1809a44 -s .data 0xc1809bc0
[amit@askii-pc linux-2.4.6-kgdb]$

The loadmodule.sh script finds addresses of sections from the module map file.

Loading object files into gdb

When gdb loads a module object file with given addresses, it can calculate address of a symbol in it by adding their relative addresses to the absolute address of the section that contains it. Consider a function tr_initviewtree in a module trfs. The section that contains it and relative address of the function

in that section can be found out by using objdump.

[amit@askii-pc trfs]$ objdump -t trfs | grep tr_initviewtree
000014f0 g     F .text  00000016 tr_initviewtree
[amit@askii-pc trfs]$

.text is the section that contains the  function. character F indicates that it's a function. 0x14f0 is the relative address of the function in section .text.
0x16 is the length of the code of the function. The address where the function is loaded is 0xc1808060 + 0x14f0 = 0xc1809550. The address is also given in the module map file.

[amit@askii-pc trfs]$ grep tr_initviewtree /tmp/trfs.map
c1809550 T tr_initviewtree
[amit@askii-pc trfs]$

gdb calculates addresses by adding relative addresses to absolute addresses of sections as shown above.