Why do kprobes and uprobes singlestep?

[Andy Lutomirski]

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.

But I was contemplating the code, and I'm no longer convinced.
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.)  Why does kprobe single-step?  I spend a while staring at
the code, and it was entirely unclear to me what the purpose of the
single-step is.

--Andy