Subversion Repositories or1k_soc_on_altera_embedded_dev_kit

Table of contents

=================

Last updated: 20 December 2005

Contents

========

- Introduction

- Devices not appearing

- Finding patch that caused a bug

-- Finding using git-bisect

-- Finding it the old way

- Fixing the bug

Introduction

============

Always try the latest kernel from kernel.org and build from source. If you are

not confident in doing that please report the bug to your distribution vendor

instead of to a kernel developer.

Finding bugs is not always easy. Have a go though. If you can't find it don't

give up. Report as much as you have found to the relevant maintainer. See

MAINTAINERS for who that is for the subsystem you have worked on.

Before you submit a bug report read REPORTING-BUGS.

Devices not appearing

=====================

Often this is caused by udev. Check that first before blaming it on the

kernel.

Finding patch that caused a bug

===============================

Finding using git-bisect

------------------------

Using the provided tools with git makes finding bugs easy provided the bug is

reproducible.

Steps to do it:

- start using git for the kernel source

- read the man page for git-bisect

- have fun

Finding it the old way

----------------------

[Sat Mar  2 10:32:33 PST 1996 KERNEL_BUG-HOWTO lm@sgi.com (Larry McVoy)]

This is how to track down a bug if you know nothing about kernel hacking.

It's a brute force approach but it works pretty well.

You need:

        . A reproducible bug - it has to happen predictably (sorry)

        . All the kernel tar files from a revision that worked to the

          revision that doesn't

You will then do:

        . Rebuild a revision that you believe works, install, and verify that.

        . Do a binary search over the kernels to figure out which one

          introduced the bug.  I.e., suppose 1.3.28 didn't have the bug, but

          you know that 1.3.69 does.  Pick a kernel in the middle and build

          that, like 1.3.50.  Build & test; if it works, pick the mid point

          between .50 and .69, else the mid point between .28 and .50.

        . You'll narrow it down to the kernel that introduced the bug.  You

          can probably do better than this but it gets tricky.

        . Narrow it down to a subdirectory

          - Copy kernel that works into "test".  Let's say that 3.62 works,

            but 3.63 doesn't.  So you diff -r those two kernels and come

            up with a list of directories that changed.  For each of those

            directories:

                Copy the non-working directory next to the working directory

                as "dir.63".

                One directory at time, try moving the working directory to

                "dir.62" and mv dir.63 dir"time, try

                        mv dir dir.62

                        mv dir.63 dir

                        find dir -name '*.[oa]' -print | xargs rm -f

                And then rebuild and retest.  Assuming that all related

                changes were contained in the sub directory, this should

                isolate the change to a directory.

                Problems: changes in header files may have occurred; I've

                found in my case that they were self explanatory - you may

                or may not want to give up when that happens.

        . Narrow it down to a file

          - You can apply the same technique to each file in the directory,

            hoping that the changes in that file are self contained.

        . Narrow it down to a routine

          - You can take the old file and the new file and manually create

            a merged file that has

                #ifdef VER62

                routine()

                {

                        ...

                }

                #else

                routine()

                {

                        ...

                }

                #endif

            And then walk through that file, one routine at a time and

            prefix it with

                #define VER62

                /* both routines here */

                #undef VER62

            Then recompile, retest, move the ifdefs until you find the one

            that makes the difference.

Finally, you take all the info that you have, kernel revisions, bug

description, the extent to which you have narrowed it down, and pass

that off to whomever you believe is the maintainer of that section.

A post to linux.dev.kernel isn't such a bad idea if you've done some

work to narrow it down.

If you get it down to a routine, you'll probably get a fix in 24 hours.

My apologies to Linus and the other kernel hackers for describing this

brute force approach, it's hardly what a kernel hacker would do.  However,

it does work and it lets non-hackers help fix bugs.  And it is cool

because Linux snapshots will let you do this - something that you can't

do with vendor supplied releases.

Fixing the bug

==============

Nobody is going to tell you how to fix bugs. Seriously. You need to work it

out. But below are some hints on how to use the tools.

To debug a kernel, use objdump and look for the hex offset from the crash

output to find the valid line of code/assembler. Without debug symbols, you

will see the assembler code for the routine shown, but if your kernel has

debug symbols the C code will also be available. (Debug symbols can be enabled

in the kernel hacking menu of the menu configuration.) For example:

    objdump -r -S -l --disassemble net/dccp/ipv4.o

NB.: you need to be at the top level of the kernel tree for this to pick up

your C files.

If you don't have access to the code you can also debug on some crash dumps

e.g. crash dump output as shown by Dave Miller.

>    EIP is at ip_queue_xmit+0x14/0x4c0

>     ...

>    Code: 44 24 04 e8 6f 05 00 00 e9 e8 fe ff ff 8d 76 00 8d bc 27 00 00

>    00 00 55 57  56 53 81 ec bc 00 00 00 8b ac 24 d0 00 00 00 8b 5d 08

>    <8b> 83 3c 01 00 00 89 44  24 14 8b 45 28 85 c0 89 44 24 18 0f 85

>

>    Put the bytes into a "foo.s" file like this:

>

>           .text

>           .globl foo

>    foo:

>           .byte  .... /* bytes from Code: part of OOPS dump */

>

>    Compile it with "gcc -c -o foo.o foo.s" then look at the output of

>    "objdump --disassemble foo.o".

>

>    Output:

>

>    ip_queue_xmit:

>        push       %ebp

>        push       %edi

>        push       %esi

>        push       %ebx

>        sub        $0xbc, %esp

>        mov        0xd0(%esp), %ebp        ! %ebp = arg0 (skb)

>        mov        0x8(%ebp), %ebx         ! %ebx = skb->sk

>        mov        0x13c(%ebx), %eax       ! %eax = inet_sk(sk)->opt

In addition, you can use GDB to figure out the exact file and line

number of the OOPS from the vmlinux file. If you have

CONFIG_DEBUG_INFO enabled, you can simply copy the EIP value from the

OOPS:

 EIP:    0060:[]    Not tainted VLI

And use GDB to translate that to human-readable form:

  gdb vmlinux

  (gdb) l *0xc021e50e

If you don't have CONFIG_DEBUG_INFO enabled, you use the function

offset from the OOPS:

 EIP is at vt_ioctl+0xda8/0x1482

And recompile the kernel with CONFIG_DEBUG_INFO enabled:

  make vmlinux

  gdb vmlinux

  (gdb) p vt_ioctl

  (gdb) l *(0x
 + 0xda8)

Another very useful option of the Kernel Hacking section in menuconfig is

Debug memory allocations. This will help you see whether data has been

initialised and not set before use etc. To see the values that get assigned

with this look at mm/slab.c and search for POISON_INUSE. When using this an

Oops will often show the poisoned data instead of zero which is the default.

Once you have worked out a fix please submit it upstream. After all open

source is about sharing what you do and don't you want to be recognised for

your genius?

Please do read Documentation/SubmittingPatches though to help your code get

accepted.