Re: Why do kprobes and uprobes singlestep?

[Andy Lutomirski]

On Mon, Mar 1, 2021 at 8:51 AM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> Hi Andy,
>
> sorry for delay.
>
> On 02/23, Andy Lutomirski wrote:
> >
> > A while back, I let myself be convinced that kprobes genuinely need to
> > single-step the kernel on occasion, and I decided that this sucked but
> > I could live with it.  it would, however, be Really Really Nice (tm)
> > if we could have a rule that anyone running x86 Linux who single-steps
> > the kernel (e.g. kgdb and nothing else) gets to keep all the pieces
> > when the system falls apart around them.  Specifically, if we don't
> > allow kernel single-stepping and if we suitably limit kernel
> > instruction breakpoints (the latter isn't actually a major problem),
> > then we don't really really need to use IRET to return to the kernel,
> > and that means we can avoid some massive NMI nastiness.
>
> Not sure I understand you correctly, I know almost nothing about low-level
> x86  magic.
>
> But I guess this has nothing to do with uprobes, they do not single-step
> in kernel mode, right?

They single-step user code, though, and the code that makes this work
is quite ugly.  Single-stepping on x86 is a mess.

>
> > Uprobes seem to single-step user code for no discernable reason.
> > (They want to trap after executing an out of line instruction, AFAICT.
> > Surely INT3 or even CALL after the out-of-line insn would work as well
> > or better.)
>
> Uprobes use single-step from the very beginning, probably because this
> is the most simple and "standard" way to implement xol.
>
> And please note that CALL/JMP/etc emulation was added much later to fix the
> problems with non-canonical addresses, and this emulation it still incomplete.

Is there something like a uprobe test suite?  How maintained /
actively used is uprobe?

--Andy