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SubjectRe: [PATCH] Linux Kernel Markers
On Tue, Sep 19, 2006 at 10:17:53AM -0700, Martin Bligh wrote:
> >>>>It seems like all we'd need to do
> >>>>is "list all references to function, freeze kernel, update all
> >>>>references, continue"
> >>>
> >>>
> >>>"overwrite first 5 bytes of old function with `jmp new_function'".
> >>
> >>Yes, that's simple. but slower, as you have a double jump. Probably
> >>a damned sight faster than int3 though.
> >
> >
> >The advantage of using int3 over jmp to launch the instrumented
> >module is that int3 (or breakpoint in most architectures) is an
> >atomic operation to insert.
>
> Ah, good point. Though ... how much do we care what the speed of
> insertion/removal actually is? If we can tolerate it being slow,
> then just sync everyone up in an IPI to freeze them out whilst
> doing the insert.
>
I guess using IPI occasionally would be acceptable. But I think
using IPI for each probes will lots of overhead.

>
> Surely this still carries the overhead of doing the breakpoint,
> which was part of what we were trying to get away from? I suppose
> we get more flexibility this way. Or does the slowness not actually
> come from the int3, but only the single-stepping?
Yes, it comes from int3 as well.
>
> How about we combine all three ideas together ...
>
> 1. Load modified copy of the function in question.
> 2. overwrite the first instruction of the routine with an int3 that
> does what you say (atomically)
> 3. Then overwrite the second instruction with a jump that's faster
> 4. Now atomically overwrite the int3 with a nop, and let the jump
> take over.
>

That's a good solution.

Thanks
Prasanna

> >Adv:
> >Can be enabled/disabled dynamically by inserting/removing
> >breakpoints. No overhead of single stepping.
> >No restriction of running the handler in interrupt context.
> >You can have pre-compiled instrumented routines.
> >This mechanism can be used for pre-defined set of routines and for
> >arbiratory probe points, you can use kprobes/jprobes/systemtap.
> >No need to be super-user for predefined breakpoints.
> >
> >Dis:
> >Maintainence of the code, since it can code base need to be
> >duplicated and instrumented.
>
> CONFIG_FOO_BAR .... turn it on or off to turn on the instrumentation.
> compiled out by default. Compiled in when making the tracing functions.
>
> >The above idea is similar to runtime or dynamic patching, but here we
> >use int3(breakpoint) rather than jump instruction.
>
> Depends what we're trying to fix. I was trying to fix two things:
>
> 1. Flexibility - kprobes seem unable to access all local variables etc
> easily, and go anywhere inside the function. Plus keeping low overhead
> for doing things like keeping counters in a function (see previous
> example I mentioned for counting pages in shrink_list).
>
> 2. Overhead of the int3, which was allegedly 1000 cycles or so, though
> faster after Ingo had played with it, it's still significant.
>
> M.

--
Prasanna S.P.
Linux Technology Center
India Software Labs, IBM Bangalore
Email: prasanna@in.ibm.com
Ph: 91-80-41776329
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