Re: [PATCH] Linux Kernel Markers

Mathieu Desnoyers
2006-09-19T17:54:09Z

* Martin Bligh (mbligh@google.com) wrote:
> 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.
>

Very good idea.. However, overwriting the second instruction with a jump could
be dangerous on preemptible and SMP kernels, because we never know if a thread
has an IP in any of its contexts that would return exactly at the middle of the
jump. I think it would be doable to overwrite a 5+ bytes instruction with a NOP
non-atomically in all cases, but as the instructions nin the prologue seems to
be smaller :

prologue on x86
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
epilogue on x86
3: 5d pop %ebp
4: c3 ret

Then is can be a problem. Ideas are welcome.

Mathieu

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