linux-kernel - Re: [RFC] netfilter: cttimeout: remove VLA in ctnl_timeout_parse

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Date:   Sun, 11 Mar 2018 23:21:10 +0100
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <garsilva@...eddedor.com>
Subject: Re: [RFC] netfilter: cttimeout: remove VLA in
 ctnl_timeout_parse_policy

On Sun, Mar 11, 2018 at 05:12:09PM -0500, Gustavo A. R. Silva wrote:
> Hi Pablo,
> 
> On 03/11/2018 05:04 PM, Pablo Neira Ayuso wrote:
> > On Tue, Mar 06, 2018 at 12:47:55PM -0600, Gustavo A. R. Silva wrote:
> > > In preparation to enabling -Wvla, remove VLA and replace it
> > > with dynamic memory allocation.
> > 
> > Looks good but...
> > 
> > > Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> > > ---
> > >   net/netfilter/nfnetlink_cttimeout.c | 12 ++++++++++--
> > >   1 file changed, 10 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/net/netfilter/nfnetlink_cttimeout.c b/net/netfilter/nfnetlink_cttimeout.c
> > > index 95b0470..a2f7d92 100644
> > > --- a/net/netfilter/nfnetlink_cttimeout.c
> > > +++ b/net/netfilter/nfnetlink_cttimeout.c
> > > @@ -52,18 +52,26 @@ ctnl_timeout_parse_policy(void *timeouts,
> > >   			  struct net *net, const struct nlattr *attr)
> > >   {
> > >   	int ret = 0;
> > > +	struct nlattr **tb = NULL;
> > 
> > I think we don't need to initialize this, right?
> > 
> 
> We actually do have to initialized it because in the unlikely case that the
> code block inside the 'if' below is not executed, then we will end up
> freeing an uninitialized pointer.

I see, you're right indeed.

We can probably simplify this code, but just doing:

        if (!l4proto->ctnl_timeout.nlattr_to_obj))
                return 0;

        netlink attribute parsing here.

You could even remove the likely() thing, which doesn't make much
sense for control plane code.

I understand this is a larger change, but I think this function will
look better while we're removing VLA.

Would you mind having a look? I'd appreciate if so.

Thanks.