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

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Message-ID: <20201001101206.6e4beea7@xps13>
Date:   Thu, 1 Oct 2020 10:12:06 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Stefan Agner <stefan@...er.ch>, Lucas Stach <dev@...xeye.de>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        linux-mtd@...ts.infradead.org, linux-tegra@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] mtd: rawnand: Replace one-element array with
 flexible-array member

Hi Jann,

Jann Horn <jannh@...gle.com> wrote on Thu, 1 Oct 2020 00:32:24 +0200:

> On Wed, Sep 30, 2020 at 11:30 PM Gustavo A. R. Silva
> <gustavoars@...nel.org> wrote:
> > On Wed, Sep 30, 2020 at 11:10:43PM +0200, Jann Horn wrote:  
> > > On Wed, Sep 30, 2020 at 11:02 PM Gustavo A. R. Silva
> > > <gustavoars@...nel.org> wrote:  
> > > > There is a regular need in the kernel to provide a way to declare having
> > > > a dynamically sized set of trailing elements in a structure. Kernel code
> > > > should always use “flexible array members”[1] for these cases. The older
> > > > style of one-element or zero-length arrays should no longer be used[2].  
> > >
> > > But this is not such a case, right? Isn't this a true fixed-size
> > > array? It sounds like you're just changing it because it
> > > pattern-matched on "array of length 1 at the end of a struct".  
> >
> > Yeah; I should have changed that 'dynamically' part of the text above
> > a bit. However, as I commented in the text below, in the case that more
> > CS IDs are needed (let's wait for the maintainers to comment on this...)
> > in the future, this change makes the code more maintainable, as for
> > the allocation part, the developer would only have to update the CS_N
> > macro to the number of CS IDs that are needed.  
> 
> But in that case, shouldn't you change it to "int cs[CS_N]" and get
> rid of the struct_size() stuff?

I do agree with Jann, I think it's best to consider this a fixed-size
array for now. If we ever want to extend the number of supported CS,
there is much more rework involved anyway.

Thanks,
Miquèl