netdev - Re: [PATCH nf-next] netfilter: xt_osf: Use explicit member assignment to avoid implicit no padding rule

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 27 Sep 2016 14:00:31 +0800
From:   Gao Feng <fgao@...ai8.com>
To:     Liping Zhang <zlpnobody@...il.com>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Patrick McHardy <kaber@...sh.net>,
        Netfilter Developer Mailing List 
        <netfilter-devel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH nf-next] netfilter: xt_osf: Use explicit member assignment
 to avoid implicit no padding rule

Hi Liping,


On Tue, Sep 27, 2016 at 1:49 PM, Liping Zhang <zlpnobody@...il.com> wrote:
> Hi Feng,
>
> 2016-09-27 12:39 GMT+08:00  <fgao@...ai8.com>:
>> From: Gao Feng <fgao@...ai8.com>
>>
>> Current xt_osf codes use memcmp to check if two user fingers are same,
>> so it depends on that the struct xt_osf_user_finger is no padding.
>> It is one implicit rule, and is not good to maintain.
>>
>> Now use zero memory and assign the members explicitly.
>>
>> Signed-off-by: Gao Feng <fgao@...ai8.com>
>> ---
>>  net/netfilter/xt_osf.c | 32 ++++++++++++++++++++++++++++++--
>>  1 file changed, 30 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/netfilter/xt_osf.c b/net/netfilter/xt_osf.c
>> index 2455b69..9793670 100644
>> --- a/net/netfilter/xt_osf.c
>> +++ b/net/netfilter/xt_osf.c
>> @@ -61,6 +61,34 @@ static const struct nla_policy xt_osf_policy[OSF_ATTR_MAX + 1] = {
>>         [OSF_ATTR_FINGER]       = { .len = sizeof(struct xt_osf_user_finger) },
>>  };
>>
>> +static void copy_user_finger(struct xt_osf_user_finger *dst,
>> +                            const struct xt_osf_user_finger *src)
>> +{
>> +#define OSF_COPY_MEMBER(mem)   dst->mem = src->mem
>> +
>> +       int i;
>> +
>> +       OSF_COPY_MEMBER(wss.wc);
>> +       OSF_COPY_MEMBER(wss.val);
>> +
>> +       OSF_COPY_MEMBER(ttl);
>> +       OSF_COPY_MEMBER(df);
>> +       OSF_COPY_MEMBER(ss);
>> +       OSF_COPY_MEMBER(mss);
>> +       OSF_COPY_MEMBER(opt_num);
>> +
>> +       memcpy(dst->genre, src->genre, sizeof(dst->genre));
>> +       memcpy(dst->version, src->version, sizeof(dst->version));
>> +       memcpy(dst->subtype, src->subtype, sizeof(dst->subtype));
>> +
>> +       for (i = 0; i < MAX_IPOPTLEN; ++i) {
>> +               OSF_COPY_MEMBER(opt[i].kind);
>> +               OSF_COPY_MEMBER(opt[i].length);
>> +               OSF_COPY_MEMBER(opt[i].wc.wc);
>> +               OSF_COPY_MEMBER(opt[i].wc.val);
>> +       }
>> +}
>> +
>
> This xt_osf_user_finger{} is carefully designed, no padding now, and
> will not be changed in the future, otherwise backward compatibility will
> be broken.

Yes, there is no padding now. So it is ok to use memcmp now.
I am afraid the struct would be modified for other requirements.

If it is never changed forever, it is ok certainly.

>
> I don't think this convert is necessary, actually it is a little ugly, and will
> increase the maintenance burden.

I just want the codes don't depend any implicit rule.

It is a tradeoff. If never change, needn't do any convert.
If may change, the memcmp is a little dangerous.

Regards
Feng