Re: [PATCH v3 1/2] kretprobe: produce sane stack traces

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Thu, Nov 08, 2018 at 07:04:48PM +1100, Aleksa Sarai wrote:
> On 2018-11-08, Aleksa Sarai <cyphar@cyphar.com> wrote:
> > I will attach what I have at the moment to hopefully explain what the
> > issue I've found is (re-using the kretprobe architecture but with the
> > shadow-stack idea).
> 
> Here is the patch I have at the moment (it works, except for the
> question I have about how to handle the top-level pt_regs -- I've marked
> that code with XXX).
> 
> -- 
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> <https://www.cyphar.com/>
> 
> --8<---------------------------------------------------------------------
> 
> Since the return address is modified by kretprobe, the various unwinders
> can produce invalid and confusing stack traces. ftrace mostly solved
> this problem by teaching each unwinder how to find the original return
> address for stack trace purposes. This same technique can be applied to
> kretprobes by simply adding a pointer to where the return address was
> replaced in the stack, and then looking up the relevant
> kretprobe_instance when a stack trace is requested.
> 
> [WIP: This is currently broken because the *first entry* will not be
>       overwritten since it looks like the stack pointer is different
>       when we are provided pt_regs. All other addresses are correctly
>       handled.]

When you see this problem, what does regs->ip point to?  If it's
pointing to generated code, then we don't _currently_ have a way of
dealing with that.  If it's pointing to a real function, we can fix that
with unwind hints.

-- 
Josh