linux-kernel - Re: x86/paravirt: Detect over-sized patching bugs in paravirt_patch

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Message-ID: <20190425102210.GM4038@hirez.programming.kicks-ass.net>
Date:   Thu, 25 Apr 2019 12:22:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: x86/paravirt: Detect over-sized patching bugs in
 paravirt_patch_call()

On Thu, Apr 25, 2019 at 11:50:39AM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Thu, Apr 25, 2019 at 11:17:17AM +0200, Ingo Molnar wrote:
> > > It basically means that we silently won't do any patching and the kernel 
> > > will crash later on in mysterious ways, because paravirt patching is 
> > > usually relied on.
> > 
> > That's OK. The compiler emits an indirect CALL/JMP to the pv_ops
> > structure contents. That _should_ stay valid and function correctly at
> > all times.
> 
> It might result in a correctly executing kernel in terms of code 
> generation, but it doesn't result in a viable kernel: some of the places 
> rely on the patching going through and don't know what to do when it 
> doesn't and misbehave or crash in interesting ways.
> 
> Guess how I know this. ;-)

What sites would that be? It really should work AFAIK.