Oops, Linux crashes

You've just started kernel development and got something like that:

Unable to handle kernel NULL pointer dereference at virtual address 00000000
pgd = c0004000
[00000000] *pgd=00000000
Internal error: Oops: 817 [#1] PREEMPT
Modules linked in:
CPU: 0    Not tainted  (2.6.23.1 #14)
pc : [<c0025720>]    lr : [<c019dca0>]    psr: 20000013
sp : c037dcfc  ip : c037dd0c  fp : c037dd08
r10: 00000000  r9 : 003c0000  r8 : 003c0000
r7 : c029eccc  r6 : 00000002  r5 : 00020000  r4 : 000fa000
r3 : 00000000  r2 : 00000002  r1 : c33e0000  r0 : c33c0000
Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
Control: c000717f  Table: 08e2c000  DAC: 00000017
Process pdflush (pid: 64, stack limit = 0xc037c258)
Stack: (0xc037dcfc to 0xc037e000)
dce0:                                                                c037dd18
dd00: c037dd0c c019dca0 c00256f0 c037dd9c c037dd1c c0199950 c019dc90 c037dd48
...
dfe0: 00000000 00000000 00000000 c037dff8 c00347dc c004711c e58d7000 e51b303c
Backtrace:
Function entered at [<c00256e0>] from [<c019dca0>]
Function entered at [<c019dc80>] from [<c0199950>]
Function entered at [<c019983c>] from [<c019c0b0>]
Function entered at [<c019bdb4>] from [<c0197dd0>]
...

Well don't worry we will try to look at how to interpret this beast.

ARM registers

To understand a kernel Oops you must understand how the underlaying processor architecture is working (here ARM one). ARM processors has a set of 32 bits registers:

• general purposes one: r0 -> r10
• pc: program counter, which handles the currently executed instruction address in memory
• lr: link register, which handles the address to which the program counter should be set when it will reach a "return from subroutine" instruction. To summarize lr holds the address of the function which called the currently executing code.
• sp: stack pointer, holds the current position in stack

You can see the registers content when the error occured at the beginning of the Oops.

Backtrace & System.map

Backtrace:
Function entered at [<c00256e0>] from [<c019dca0>]
Function entered at [<c019dc80>] from [<c0199950>]