How to find out which part of code is changing a particular data structure.

How to find out which part of code is changing a particular data structure.

[Rajat Jain]

Hi folks,

I have a memory location (One of the fields in a kernel data structure) and I want to track down the code that changes the value of that particular location. Some thing like a "watchpoint".

And this is early while I am still booting up. Can some one tell me if it is possible to do so?

Thanks,

Rajat

 

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Re: How to find out which part of code is changing a particular data structure.

[Chetan Nanda]

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You may attach debugger (like Lauterbach) and use its data breakpoint
functionality.

Thanks,
Chetan Nanda


On Fri, Nov 8, 2013 at 6:16 AM, Rajat Jain <rajatjain@juniper.net> wrote:

> Hi folks,
>
> I have a memory location (One of the fields in a kernel data structure)
> and I want to track down the code that changes the value of that particular
> location. Some thing like a "watchpoint".
>
> And this is early while I am still booting up. Can some one tell me if it
> is possible to do so?
>
> Thanks,
>
> Rajat
>
>
>
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies@kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>

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Re: How to find out which part of code is changing a particular data structure.

[kiran kumar]

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Hi Rjat,
Can you define that location as read only(i.e constant)?. This is only for debug prupose, later you can remove. 
I am expecting kernel panic with this change. 
Regards,
Kiran



On Friday, 8 November 2013, 9:03, Chetan Nanda <chetannanda@gmail.com> wrote:
  
You may attach debugger (like Lauterbach) and use its data breakpoint functionality. 

Thanks,
Chetan Nanda




On Fri, Nov 8, 2013 at 6:16 AM, Rajat Jain <rajatjain@juniper.net> wrote:

Hi folks,
>
>I have a memory location (One of the fields in a kernel data structure) and I want to track down the code that changes the value of that particular location. Some thing like a "watchpoint".
>
>And this is early while I am still booting up. Can some one tell me if it is possible to do so?
>
>Thanks,
>
>Rajat
>
>
>
>
>_______________________________________________
>Kernelnewbies mailing list
>Kernelnewbies@kernelnewbies.org
>http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>


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Kernelnewbies@kernelnewbies.org
http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

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Re: How to find out which part of code is changing a particular data structure.

[Valdis.Kletnieks]

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On Thu, 07 Nov 2013 23:23:18 -0800, kiran kumar said:
> Can you define that location as read only(i.e constant)?. This is only for
> debug prupose, later you can remove.

Note that this requires 2 things:

1) A kernel built with CONFIG_DEBUG_RODATA=y

2) The structure needs to have a compile-time initializer:

struct foo barbaz = { 5, 3, "quuz"};

Otherwise, your kernel will throw an oops when it tries to initialize
the structure at runtime (which is almost certainly *not* the time that
is causing the actual problem).

Simply declaring it as 'const struct' may or may not help - this will catch
at compile time lots of mistakes, but isn't guaranteed to stop all cases of
buggy uses of miscast pointers, etc....

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Re: How to find out which part of code is changing a particular data structure.

[Frank Ch. Eigler]

Rajat Jain <rajatjain@juniper.net> writes:

> [...]  I have a memory location (One of the fields in a kernel data
> structure) and I want to track down the code that changes the value
> of that particular location. Some thing like a "watchpoint".

See the register_wide_hw_breakpoint API.  You could write a small
module that runs early, and gets callbacks when a given address is
modified.


> And this is early while I am still booting up. Can some one tell me
> if it is possible to do so?

(Were it not for the 'while still booting up' part, I'd suggest trying
systemtap's 

        probe kernel.data("SYMBOL").write { }
 or     probe kernel.data(0xDEADBEEF).write { }

probes, but we haven't worked through initramfs'ifying the widget.)


- FChE
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