netdev - Re: [PATCH 3/3] netfilter: xt_hashlimit: uses div

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Date:   Fri, 30 Sep 2016 19:39:54 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Patrick McHardy <kaber@...sh.net>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        "David S. Miller" <davem@...emloft.net>,
        Joshua Hunt <johunt@...mai.com>,
        Vishwanath Pai <vpai@...mai.com>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] netfilter: xt_hashlimit: uses div_u64 for division

On Friday 30 September 2016, Eric Dumazet wrote:
> On Fri, 2016-09-30 at 18:05 +0200, Arnd Bergmann wrote:
> >  net/netfilter/xt_hashlimit.c | 17 ++++++++++-------
> >  1 file changed, 10 insertions(+), 7 deletions(-)
> > 
> > diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c
> > index 44a095ecc7b7..3d5525df6eb3 100644
> > --- a/net/netfilter/xt_hashlimit.c
> > +++ b/net/netfilter/xt_hashlimit.c
> > @@ -464,20 +464,23 @@ static u32 xt_hashlimit_len_to_chunks(u32 len)
> >  static u64 user2credits(u64 user, int revision)
> >  {
> >  	if (revision == 1) {
> > +		u32 user32 = user; /* use 32-bit division */
> > +
> 
> This looks dangerous to me. Have you really tried all possible cases ?

Yes, I'm pretty certain about that: The 11d5f15723c9 patch that introduced this
kept the existing implementation for the revision==1 case, except for changing
the types.

> Caller (even if using revision == 1) does
> user2credits(cfg->avg * cfg->burst, revision);
> 
> Since this is not a fast path, I would prefer to keep the 64bit divide.
>
> Vishwanath version looks safer.

Ok, fair enough. I couldn't tell how much of a fast path this
was, and it's more a general issue that I see with other developers
blindly using div_u64() whenever getting this link error.

Since I already had the patch by the time I saw the other one
(which is also at v3 and got comments), I just sent it out along
with the other two patches I had for netfilter.

I also ended up introducing a typo in a last-minute change, so I'll let
Vishwanath and you work out the best implementation and withdraw my
version.

	Arnd