Re: [PATCH v15 13/25] x86/reboot: Add ljmp instructions to stacktool whitelist

From: Ingo Molnar
Date: Fri Jan 15 2016 - 06:00:13 EST

* Borislav Petkov <bp@xxxxxxxxx> wrote:

> On Fri, Jan 15, 2016 at 12:06:52AM -0600, Josh Poimboeuf wrote:
> > - xen_cpuid() uses some custom xen instructions which start with
> > XEN_EMULATE_PREFIX. It corresponds to the following x86 instructions:
> >
> > ffffffff8107e572: 0f 0b ud2
> > ffffffff8107e574: 78 65 js ffffffff8107e5db <xen_get_debugreg+0xa>
> > ffffffff8107e576: 6e outsb %ds:(%rsi),(%dx)
> >
> > Apparently(?) xen treats the ud2 special when it's followed by "78 65
> > 6e". This is confusing for stacktool because ud2 is normally a dead
> > end, and it thinks the instructions after it will never run.
> >
> > (In theory stacktool could be taught to understand this hack, but
> > that's a bad idea IMO)
> Why, because it is not generic enough?
> Well, you could add a cmdline option "--kernel" which is supplied when
> checking the kernel and such kernel "idiosyncrasies" are handled only
> then and there. And since the tool is part of the kernel, changes to
> XEN_EMULATE_PREFIX, will have to be updated in stacktool too...

So I think because we are talking about less than a dozen annotations, these are
technicalities - and it might in fact be better to have a single line of obvious
annotation in a function that does something weird (and arguably all of these
functions do something weird), than having dozens of lines of code on the tooling
side to avoid that single line on the kernel side.

That has a documentation value as well.

As long as the annotation itself is not stacktool specific, it should serve as
documentation as well - such as:





All of the cases Josh listed involve some sort of special case where we do
something non-standard. (Where 'standard' == 'regular kernel C function'.)