Division by zero on UP (was: Re: netfilter: nat: use keyed locks)

[Geert Uytterhoeven]

Hi Florian, Pablo,

On Sat, Sep 9, 2017 at 9:21 PM, Linux Kernel Mailing List
<linux-kernel@xxxxxxxxxxxxxxx> wrote:
> Web:        https://git.kernel.org/torvalds/c/8073e960a03bf7b5d5ebfc5ff18ac475e1688f46
> Commit:     8073e960a03bf7b5d5ebfc5ff18ac475e1688f46
> Parent:     e1bf1687740ce1a3598a1c5e452b852ff2190682
> Refname:    refs/heads/master
> Author:     Florian Westphal <fw@xxxxxxxxx>
> AuthorDate: Wed Sep 6 14:39:52 2017 +0200
> Committer:  Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
> CommitDate: Fri Sep 8 18:55:52 2017 +0200
>
>     netfilter: nat: use keyed locks
>
>     no need to serialize on a single lock, we can partition the table and
>     add/delete in parallel to different slots.
>     This restores one of the advantages that got lost with the rhlist
>     revert.
>
>     Cc: Ivan Babrou <ibobrik@xxxxxxxxx>
>     Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
>     Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
> ---
>  net/netfilter/nf_nat_core.c | 36 ++++++++++++++++++++++++------------
>  1 file changed, 24 insertions(+), 12 deletions(-)
>
> diff --git a/net/netfilter/nf_nat_core.c b/net/netfilter/nf_nat_core.c
> index f090419f5f97..f393a7086025 100644
> --- a/net/netfilter/nf_nat_core.c
> +++ b/net/netfilter/nf_nat_core.c
> @@ -30,7 +30,7 @@
>  #include <net/netfilter/nf_conntrack_zones.h>
>  #include <linux/netfilter/nf_nat.h>
>
> -static DEFINE_SPINLOCK(nf_nat_lock);
> +static spinlock_t nf_nat_locks[CONNTRACK_LOCKS];
>
>  static DEFINE_MUTEX(nf_nat_proto_mutex);
>  static const struct nf_nat_l3proto __rcu *nf_nat_l3protos[NFPROTO_NUMPROTO]
> @@ -425,13 +425,15 @@ nf_nat_setup_info(struct nf_conn *ct,
>
>         if (maniptype == NF_NAT_MANIP_SRC) {
>                 unsigned int srchash;
> +               spinlock_t *lock;
>
>                 srchash = hash_by_src(net,
>                                       &ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple);
> -               spin_lock_bh(&nf_nat_lock);
> +               lock = &nf_nat_locks[srchash % ARRAY_SIZE(nf_nat_locks)];
> +               spin_lock_bh(lock);

If no spinlock debugging options (CONFIG_GENERIC_LOCKBREAK,
CONFIG_DEBUG_SPINLOCK, CONFIG_DEBUG_LOCK_ALLOC) are enabled on a UP platform
(e.g. m68k defconfig), arch_spinlock_t is an empty struct, and thus
ARRAY_SIZE(nf_nat_locks)] is zero, leading to:

    net/netfilter/nf_nat_core.c: In function ânf_nat_setup_infoâ:
    net/netfilter/nf_nat_core.c:432: warning: division by zero
    net/netfilter/nf_nat_core.c: In function â__nf_nat_cleanup_conntrackâ:
    net/netfilter/nf_nat_core.c:535: warning: division by zero
    net/netfilter/nf_nat_core.c:537: warning: division by zero
    net/netfilter/nf_nat_core.c: In function ânf_nat_initâ:
    net/netfilter/nf_nat_core.c:810: warning: division by zero
    net/netfilter/nf_nat_core.c:811: warning: division by zero
    net/netfilter/nf_nat_core.c:824: warning: division by zero

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds