netdev - Re: array bounds warning in xfrm_output

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Message-ID: <20180619081147.rehw2o5x6r2cb7cn@breakpoint.cc>
Date:   Tue, 19 Jun 2018 10:11:47 +0200
From:   Florian Westphal <fw@...len.de>
To:     David Ahern <dsahern@...il.com>
Cc:     Florian Westphal <fw@...len.de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: array bounds warning in xfrm_output_resume

David Ahern <dsahern@...il.com> wrote:
> $ make O=kbuild/perf -j 24 -s
> In file included from /home/dsa/kernel-3.git/include/linux/kernel.h:10:0,
>                  from /home/dsa/kernel-3.git/include/linux/list.h:9,
>                  from /home/dsa/kernel-3.git/include/linux/module.h:9,
>                  from /home/dsa/kernel-3.git/net/xfrm/xfrm_output.c:13:
> /home/dsa/kernel-3.git/net/xfrm/xfrm_output.c: In function
> ‘xfrm_output_resume’:
> /home/dsa/kernel-3.git/include/linux/compiler.h:252:20: warning: array
> subscript is above array bounds [-Warray-bounds]
>    __read_once_size(&(x), __u.__c, sizeof(x));  \
>                     ^~~~
> /home/dsa/kernel-3.git/include/linux/compiler.h:258:22: note: in
> expansion of macro ‘__READ_ONCE’
>  #define READ_ONCE(x) __READ_ONCE(x, 1)
>                       ^~~~~~~~~~~
> /home/dsa/kernel-3.git/include/linux/rcupdate.h:350:48: note: in
> expansion of macro ‘READ_ONCE’
>   typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
>                                                 ^~~~~~~~~
> /home/dsa/kernel-3.git/include/linux/rcupdate.h:487:2: note: in
> expansion of macro ‘__rcu_dereference_check’
>   __rcu_dereference_check((p), (c) || rcu_read_lock_held(), __rcu)
>   ^~~~~~~~~~~~~~~~~~~~~~~
> /home/dsa/kernel-3.git/include/linux/rcupdate.h:545:28: note: in
> expansion of macro ‘rcu_dereference_check’
>  #define rcu_dereference(p) rcu_dereference_check(p, 0)
>                             ^~~~~~~~~~~~~~~~~~~~~
> /home/dsa/kernel-3.git/include/linux/netfilter.h:218:15: note: in
> expansion of macro ‘rcu_dereference’
>    hook_head = rcu_dereference(net->nf.hooks_arp[hook]);

Hmpf. compiler can't know that this is only called for ipv4 and
ipv6 families.

> Line in question is the nf_hook in xfrm_output_resume.
> NF_INET_POST_ROUTING = 4 which is greater than NF_ARP_NUMHOOKS = 3
> 
> I believe ef57170bbfdd6 is the commit that introduced the warning

Yes.  I will see how to best fix this, probably needs an explicit
check on skb_dst(skb)->ops->family.