netdev - Re: [PATCH] netfilter: xt_hashlimit: avoid 64-bit division

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Date:   Wed, 6 Sep 2017 22:48:22 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Vishwanath Pai <vpai@...mai.com>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        Josh Hunt <johunt@...mai.com>, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] netfilter: xt_hashlimit: avoid 64-bit division

On Wed, Sep 6, 2017 at 10:22 PM, Vishwanath Pai <vpai@...mai.com> wrote:
> On 09/06/2017 03:57 PM, Arnd Bergmann wrote:
>> 64-bit division is expensive on 32-bit architectures, and
>> requires a special function call to avoid a link error like:
>>
>> net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common':
>> xt_hashlimit.c:(.text+0x1328): undefined reference to `__aeabi_uldivmod'
>>
>> In the case of hashlimit_mt_common, we don't actually need a
>> 64-bit operation, we can simply rewrite the function slightly
>> to make that clear to the compiler.
>>
>> Fixes: bea74641e378 ("netfilter: xt_hashlimit: add rate match mode")
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>> ---
>>  net/netfilter/xt_hashlimit.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c
>> index 10d48234f5f4..50b53d86eef5 100644
>> --- a/net/netfilter/xt_hashlimit.c
>> +++ b/net/netfilter/xt_hashlimit.c
>> @@ -531,7 +531,10 @@ static u64 user2rate_bytes(u64 user)
>>  {
>>       u64 r;
>>
>> -     r = user ? 0xFFFFFFFFULL / user : 0xFFFFFFFFULL;
>> +     if (user > 0xFFFFFFFFULL)
>> +             return 0;
>> +
>> +     r = user ? 0xFFFFFFFFULL / (u32)user : 0xFFFFFFFFULL;
>>       r = (r - 1) << 4;
>>       return r;
>>  }
>>
>
> I have submitted another patch to fix this:
> https://patchwork.ozlabs.org/patch/809881/
>
> We have seen this problem before, I was careful not to introduce this
> again in the new patch but clearly I overlooked this particular line :(
>
> In the other cases we fixed it by replacing division with div64_u64().

div64_u64() seems needlessly expensive here since the dividend
is known to be a 32-bit number. I guess the function is not called
frequently though, so it doesn't matter much.

      Arnd