Re: Dynamically altering code segments

[Jeremy Jackson <jerj@coplanar.net>]

"Collins, Tom" wrote:

> Hi...
>
> This is my first post, so if this is off topic for this list, please
direct
> me
> to another one that is more appropriate.  Thanks
>
> That said, I am wanting to dynamically modify the kernel in specific
places
> to
> implement a custom kernel trace mechanism.  The general idea is that,
when
> the
> "trace" is off, there are NOP instruction sequences at various places
in the
> kernel.  When the "trace" is turned on, those same NOPs are replaced
by JMPs
> to code that implements the trace (such as logging events, using the
MSR and
> PMC's etc..).
>
> This was a trick that was done in my old days of OS/2 performance
tools
> developement to get trace information from the running kernel.  In
that
> case,
> we simply remapped the appropriate code segments to data segments (I
think
> back then it was called 'aliasing code segments') and used that
segment to
> make changes to the kernel code on the fly.
>
> Is it possible to do the same thing in Linux?
>

the CS and DS segment descriptors already both map 0-4G, the DS being
read-write.
what you want is to change page protections, the system call mprotect()
comes
to mind.