Re: [PATCH RFC] kprobes: Remove MODULES dependency

Jarkko Sakkinen
2020-07-13T05:06:05Z

On Fri, Jul 10, 2020 at 12:49:10PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 10, 2020 at 01:36:38PM +0300, Jarkko Sakkinen wrote:
> > Just so that I know (and learn), what did exactly disable optprobes?
>
> So regular, old-skool style kprobe is:
>
> - copy original instruction out
> - replace instruction with breakpoint (int3 on x86)
> - have exception handler return to the copied instruction with
> single-step on
> - have single step exception handler return to the original
> instruction stream
>
> which is 2 exceptions.

Out of pure interest, how does it handle a jump (as the original
opcode), given that it single steps a copy?

> optprobes avoid the single-step by not only writing a single
> instruction, but additionally placing a JMP instruction behind it such
> that it will automagically continue in the original instruction stream.
>
> This brings the requirement that the copied instruction is placed
> within the JMP displacement of the regular kernel text (s32 on x86).
>
> module_alloc() ensures the memory provided is within that range.

Right, a relative jump is placed instead of 0xcc to the breakpoint?

/Jarkko