linux-kernel - Re: [PATCH 1/7] x86/entry: Fix #UD vs WARN more

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Date:   Thu, 18 Jun 2020 11:36:53 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Mark Rutland <Mark.Rutland@....com>,
        Matthew Helsley <mhelsley@...are.com>,
        Steven Rostedt <rostedt@...dmis.org>, jthierry@...hat.com,
        Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH 1/7] x86/entry: Fix #UD vs WARN more

On Thu, Jun 18, 2020 at 8:50 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Jun 18, 2020 at 07:57:35AM -0700, Andy Lutomirski wrote:
> >
> >
> > > On Jun 18, 2020, at 7:50 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > vmlinux.o: warning: objtool: exc_invalid_op()+0x47: call to probe_kernel_read() leaves .noinstr.text section
> > >
> > > Since we use UD2 as a short-cut for 'CALL __WARN', treat it as such.
> > > Have the bare exception handler do the report_bug() thing.
> >
> > I think you should consider inlining or noinstr-ifying report_bug()
> > too if you want to make this more bulletproof. I admit the scenario
> > where someone instruments it and it goes wrong is farfetched.
>
> How far down that rabbit hole do we go? Because then we need to noinstr
> printk, the console drivers, those will very quickly pull in lovely bits
> like PCI, USB, DRM :/
>
> At some point we have to just give up.

I wasn't imagining going far down the rabbit hole at all -- I think
that, at most, we should cover the path for when the fault wasn't a
BUG/WARN in the first place.  I admit that, for #UD in particular,
this isn't a big deal, but if it were a different vector, this could
matter.

I suppose report_bug() could be split into lookup_bug() and
handle_bug(), and just lookup_bug() could be noinstr.

--Andy