netdev - Re: [PATCH] cfg80211: Fix array-bounds warning in fragment copy

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Date:   Mon, 27 Mar 2017 13:38:36 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Felix Fietkau <nbd@....name>, Matthias Kaehlcke <mka@...omium.org>,
        "David S . Miller" <davem@...emloft.net>,
        Felix Fietkau <nbd@...nwrt.org>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Grant Grundler <grundler@...omium.org>
Subject: Re: [PATCH] cfg80211: Fix array-bounds warning in fragment copy


> > > -	const skb_frag_t *frag = &sh->frags[-1];
> > > +	const skb_frag_t *frag = &sh->frags[0];
[...]
> > > +	frag--;
> > 
> > Isn't it just a question of time until the compiler will see
> > through this trick and warn about it?
> 
> Frag is incremented again before being accessed, so there is nothing
> for the compiler to see through here.

But by that argument the existing code was already fine. The compiler
flagged it nonetheless, perhaps because it couldn't prove it was
incremented unconditionally/in all branches?

johannes