linux-kernel - Re: [PATCH] x86/paravirt: Detect oversized patching bugs as they happen and BUG

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Message-ID: <20190425092131.GL4038@hirez.programming.kicks-ass.net>
Date:   Thu, 25 Apr 2019 11:21:31 +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: [PATCH] x86/paravirt: Detect oversized patching bugs as they
 happen and BUG_ON() to avoid later crashes

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.

Not patching should at the very least cause a WARN with RETPOLINE
kernels though, we hard rely on the patching actually working and
writing at least a direct call.