some questions about kernel debugging

some questions about kernel debugging

[loody]

Dear all:
I enable kernel hacking options about kernel debugging on my mips platform.
I can successfully connect to the target board.
My question is:
1. why I cannot p/x the value of variable?
    I have enabled CONFIG_ARCH_WANT_FRAME_POINTERS and CONFIG_FRAME_POINTER
    but I still get the message which tell me "value optimized out".
    did I miss anything?
    my kernel version is 2.6.30.9?
appreciate your help,
miloody

Re: some questions about kernel debugging

[Jan Kratochvil]

On Fri, 09 Jul 2010 13:52:31 +0200, loody wrote:
> I enable kernel hacking options about kernel debugging on my mips platform.
> I can successfully connect to the target board.
> My question is:
> 1. why I cannot p/x the value of variable?
>     I have enabled CONFIG_ARCH_WANT_FRAME_POINTERS and CONFIG_FRAME_POINTER
>     but I still get the message which tell me "value optimized out".
>     did I miss anything?
>     my kernel version is 2.6.30.9?

This is not a GDB problem, you should report it to GCC (you need a minimal
reproducer for the bugreport).

The problem is Linux kernel cannot (at least could not) compile with -O0, it
requires -O2 to successfully build.  -O2 was always mostly unsupported
together with -g (=debuggingo information).  For -O2 -g you should use the
latest GCC - SVN HEAD best - and definitely at least 4.5 (for VTA
- var-tracking-assignments).

Besides resolving this -O2 -g problem at GCC the easy way is always to just
`disassemble' the code in GDB, guess from the code+source in which
register/memory is the value probably located and access it directly there.


Regards,
Jan

Re: some questions about kernel debugging

[loody]

hi:
thanks for your kind reply :)

2010/7/9 Jan Kratochvil <jan.kratochvil@redhat.com>:
> On Fri, 09 Jul 2010 13:52:31 +0200, loody wrote:
>> I enable kernel hacking options about kernel debugging on my mips platform.
>> I can successfully connect to the target board.
>> My question is:
>> 1. why I cannot p/x the value of variable?
>>     I have enabled CONFIG_ARCH_WANT_FRAME_POINTERS and CONFIG_FRAME_POINTER
>>     but I still get the message which tell me "value optimized out".
>>     did I miss anything?
>>     my kernel version is 2.6.30.9?
>
> This is not a GDB problem, you should report it to GCC (you need a minimal
> reproducer for the bugreport).
>
> The problem is Linux kernel cannot (at least could not) compile with -O0, it
> requires -O2 to successfully build.  -O2 was always mostly unsupported
> together with -g (=debuggingo information).  For -O2 -g you should use the
> latest GCC - SVN HEAD best - and definitely at least 4.5 (for VTA
> - var-tracking-assignments).
>
> Besides resolving this -O2 -g problem at GCC the easy way is always to just
> `disassemble' the code in GDB, guess from the code+source in which
> register/memory is the value probably located and access it directly there.

I tried compile the kernel with -O1 -g but it still fail.
does that mean only program compiled with -O0 -g will not get the
error message when trying to see the variable?
appreciate your help,
miloody

Re: some questions about kernel debugging

[Jan Kratochvil]

On Fri, 09 Jul 2010 17:56:35 +0200, loody wrote:
> 2010/7/9 Jan Kratochvil <jan.kratochvil@redhat.com>:
> > Besides resolving this -O2 -g problem at GCC the easy way is always to just
> > `disassemble' the code in GDB, guess from the code+source in which
> > register/memory is the value probably located and access it directly there.
> 
> I tried compile the kernel with -O1 -g but it still fail.

-O1 should be like -O2 for the debugging...

> does that mean only program compiled with -O0 -g will not get the
> error message when trying to see the variable?

Right.  If you succeed to build Linux kernel with -O0 -g (which I doubt) then
GCC should produce correct enough debug information that GDB will be happy.


Regards,
Jan