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Re: moving instructions to another address

Hi Sebastian,

> I'm not sure if this is the right place to ask but I'll do it
> nevertheless.

It is certainly a good place to ask. You might also want to try asking on the gdb mailing list, since this is the sort of thing that debuggers do.

Currently I try to write a library which can hook into any function you
know the address of. That means that the lib overwrites to first few
instructions of the function with a jmp (or something similar) to
another function. If you want to call the original function you need to
execute the instructions which are overwritten by the jmp instruction.
The lib currently mmaps memory and writes them there (trampoline). This
somehow worked well but I'm sure that moving instruction to another
address may cause undefined behavior. Is there something in binutils I
can use to make it safe?

Sorry no.

And is it even possible?

Possible yes. But to do it in a clean portable way that will continue to work as operating systems evolve - probably not.

There are similar sorts of things that you might want to consider - overlays for example, or ifuncs, or even the breakpoint mechanism used by debuggers.


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