Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault

From: Linus Torvalds
Date: Tue Feb 19 2019 - 13:44:02 EST

On Tue, Feb 19, 2019 at 8:18 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > So it would be good to not just say "user or kernel", but actually say
> > what *kind* of kernel access it expects.
> Note, kprobes are a different kind of beast. I've used kprobes to probe
> userspace information as well as kernel. Heck, I could see someone
> even using kprobes to probe IO memory to check if a device is doing
> what they expect it's doing.

Note that even if that is the case, you _need_ to special "user vs
kernel" information.

Because the exact same address might exist in both.

Right now I think that only happens on sparc32, but vendors used to
have that issue on x86-32 too (if they had the 4G:4G patches).

> Basically, a kprobe is mostly used for debugging what's happening in a
> live kernel, to read any address.

My point is that "any address" is not sufficient to begin with. You
need "kernel or user".

Having a flag for what _kind_ of kernel address is ok might then be
required for other cases if they might not be ok with following page
tables to IO space..