Re: [PATCH] Linux Kernel Markers

[Karim Yaghmour <karim at opersys dot com>]

Martin Bligh wrote:
> be that many? Still doesn't fix the problem Matieu just pointed
> out though. Humpf.

There's one possibility if we're willing to insert a placeholder
at function entry that allows to essentially do what Andrew
suggests without much impact. Specifically, if you need a 5-byte
operation to jump to the alternate instrumented function, you
can then do something like:
1- At build time insert 5-byte unconditional jump to instruction
right after placeholder.
2- At runtime for diverting flow:
   - Replace first byte with int3 (atomically)
   - Replace next 4 bytes with instrumented function destination
   - Replace first byte
3- At runtime for returning flow:
   - Do #2 but for the original placeholder jump.

There's not race condition here or fear of interrupt return in
the middle of anything, or any need to stop the kernel from
operating and the likes, or even dependency on kprobes or need
for dprobes, at least in as far as I can see -- so this should
be trivial on m68k ;). The price to pay is an additional
unconditional jump at all times, which should be optimized at
runtime by the CPU. Benchmarks could help show the real impact,
but as Ingo said, these things should be minimal.

In sum, this would work for function pointers and wouldn't
require having to walk the code in search of instances of
"call foo" to replace.

Just a thought.

Karim