linux-kernel - Re: [PATCH] KVM: x86: Replace 0-length arrays with flexible arrays

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Message-ID: <202301121444.104E492D@keescook>
Date:   Thu, 12 Jan 2023 14:44:47 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Replace 0-length arrays with flexible arrays

On Mon, Jan 09, 2023 at 05:30:48PM +0000, Sean Christopherson wrote:
> On Thu, Jan 05, 2023, Kees Cook wrote:
> > Zero-length arrays are deprecated[1]. Replace struct kvm_nested_state's
> > "data" union 0-length arrays with flexible arrays. (How are the
> > sizes of these arrays verified?)
> 
> It's not really interpreted as an array, it's a mandatory single-entry "array".
[...]
> >  
> >  	/*
> > -	 * Define data region as 0 bytes to preserve backwards-compatability
> > +	 * Define union of flexible arrays to preserve backwards-compatability
> 
> I think I'd actually prefer the "as 0 bytes" comment.  The important part is that
> the size of "data" be zero, how that happens is immaterial.

Oh, dur, I can read the comment. :)

It has to stay the old size -- this was a way to add an optional extra
struct to the end. Got it!

-- 
Kees Cook