linux-hardening - Re: [PATCH v2][next] dlm: Replace one-element array with flexible-array member

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Message-ID: <20221011152031.GA11089@redhat.com>
Date:   Tue, 11 Oct 2022 10:20:31 -0500
From:   David Teigland <teigland@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>,
        Christine Caulfield <ccaulfie@...hat.com>,
        cluster-devel@...hat.com, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] dlm: Replace one-element array with
 flexible-array member

On Mon, Oct 10, 2022 at 03:35:24PM -0700, Kees Cook wrote:
> On Mon, Oct 10, 2022 at 04:00:39PM -0500, David Teigland wrote:
> > On Sat, Oct 08, 2022 at 09:03:28PM -0700, Kees Cook wrote:
> > > On Sun, Oct 09, 2022 at 03:05:17PM +1300, Paulo Miguel Almeida wrote:
> > > > On Sat, Oct 08, 2022 at 05:18:35PM -0700, Kees Cook wrote:
> > > > > This is allocating 1 more byte than before, since the struct size didn't change. But this has always allocated too much space, due to the struct padding. For a "no binary changes" patch, the above "+ 1" needs to be left off.
> > > > 
> > > > That's true. I agree that leaving "+ 1" would work and produce a
> > > > no-binary-changes patch due to the existing padding that the structure
> > > > has. OTOH, I thought that relying on that space could bite us in the
> > > > future if anyone tweaks the struct again...so my reaction was to ensure 
> > > > that the NUL-terminator space was always guaranteed to be there.
> > > > Hence, the change on c693 (objdump above).
> > > > 
> > > > What do you think? Should we keep or leave the above
> > > > "+ 1" after the rationale above?
> > > 
> > > I think it depends on what's expected from this allocation. Christine or
> > > David, can you speak to this?
> > 
> > Hi, thanks for picking through that.  Most likely the intention was to
> > allow up to 64 (DLM_LOCKSPACE_LEN) character names, and then use the
> > ls_name[1] for the terminating byte.  I'd be happy to take the patch
> 
> Should this just use:
> 
> 	char			ls_name[DLM_LOCKSPACE_LEN + 1];
> 
> instead, or is the byte savings worth keeping it dynamically sized?

Yes, I think that's the best option.
Dave