linux-kernel - Re: [lvc-project] [PATCH] netfilter: ebtables: remove unnecessary NULL check

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Message-ID: <20230621104603.GA24035@breakpoint.cc>
Date:   Wed, 21 Jun 2023 12:46:03 +0200
From:   Florian Westphal <fw@...len.de>
To:     "Igor A. Artemiev" <Igor.A.Artemiev@...t.ru>
Cc:     Florian Westphal <fw@...len.de>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        "David S . Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        lvc-project@...uxtesting.org
Subject: Re: [lvc-project] [PATCH] netfilter: ebtables: remove unnecessary
 NULL check

Igor A. Artemiev <Igor.A.Artemiev@...t.ru> wrote:
> On 6/20/23 19:38, Florian Westphal wrote:
> > Igor Artemiev <Igor.A.Artemiev@...t.ru> wrote:
> > > In ebt_do_table() 'private->chainstack' cannot be NULL
> > > and the 'cs' pointer is dereferenced below, so it does not make
> > > sense to compare 'private->chainstack' with NULL.
> > ?  Why do you think that?
> > 
> The 'cs' pointer is dereferenced below without checking, as it is assumed to
> always be initialized with 'private->chainstack[smp_processor_id()]'.

No, its not.  The dereferencing is conditional, as is the allocation
of the chainstack.

No user defined chains, no chain stack.

With this change, "ebtables-legacy -A INPUT" causes kernel panic.