From: Nicolas Dichtel <nicolas.dichtel@6wind•com>
To: David Laight <David.Laight@ACULAB•COM>
Cc: tgraf@suug•ch, netdev@vger•kernel.org, davem@davemloft•net
Subject: Re: [PATCH] netlink: align attributes on 64-bits
Date: Mon, 17 Dec 2012 17:53:46 +0100 [thread overview]
Message-ID: <50CF4E1A.1060207@6wind.com> (raw)
In-Reply-To: <AE90C24D6B3A694183C094C60CF0A2F6026B70F0@saturn3.aculab.com>
Le 17/12/2012 10:59, David Laight a écrit :
>> - if (unlikely(skb_tailroom(skb) < nla_total_size(attrlen)))
>> + int align = IS_ALIGNED((unsigned long)skb_tail_pointer(skb), sizeof(void *)) ? 0 : 4;
>> +
>> + if (unlikely(skb_tailroom(skb) < nla_total_size(attrlen) + align))
>> return -EMSGSIZE;
>>
>> + if (align) {
>> + /* Goal is to add an attribute with size 4. We know that
>> + * NLA_HDRLEN is 4, hence payload is 0.
>> + */
>> + __nla_reserve(skb, 0, 0);
>> + }
>> +
>
> Shouldn't the size of the dummy parameter be based on the value
> of 'align' - and that be based on the amount of padding needed?
>
Align is 4 or 0. Instead of the comment and 0, I can put 'NLA_HDRLEN - align',
which will always be 0, because we made this patch because we don't want to
change values like NLA_HDRLEN, because many user apps have these values
/structures hardcoded.
> That aligns the write pointer, what guarantees the alignment of
> the start of the buffer - so that the reader will find aligned data?
As Thomas said, skb->head will be aligned, am I wrong?
>
> What guarantees that the reader will read the data into an
> 8-byte aligned buffer.
>
> There is also the lurking issue of items that require more
> than 8-byte alignment.
> (x86/amd64 requires 16-byte alignment for 16-byte SSE2 regs and
> 32-byte alignment for the AVX regs.)
>
> Will anyone ever want to put such items into a netlink message?
>
> David
next prev parent reply other threads:[~2012-12-17 16:53 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-12-04 11:13 [PATCH net-next 0/7] Allow to monitor multicast cache event via rtnetlink Nicolas Dichtel
2012-12-04 11:13 ` [PATCH net-next 1/7] netconf: advertise mc_forwarding status Nicolas Dichtel
2012-12-04 11:13 ` [PATCH net-next 2/7] ip6mr: use nla_nest_* helpers Nicolas Dichtel
2012-12-04 11:13 ` [PATCH net-next 3/7] ipmr/ip6mr: advertise mfc stats via rtnetlink Nicolas Dichtel
2012-12-04 11:13 ` [PATCH net-next 4/7] ipmr/ip6mr: report origin of mfc entry into rtnl msg Nicolas Dichtel
2012-12-04 11:13 ` [PATCH net-next 5/7] ipmr/ip6mr: allow to get unresolved cache via netlink Nicolas Dichtel
2012-12-04 11:13 ` [PATCH net-next 6/7] ipmr: advertise new mfc entries via rtnl Nicolas Dichtel
2012-12-04 11:13 ` [PATCH net-next 7/7] ip6mr: " Nicolas Dichtel
2012-12-04 18:09 ` [PATCH net-next 0/7] Allow to monitor multicast cache event via rtnetlink David Miller
2012-12-04 20:02 ` Nicolas Dichtel
2012-12-05 11:02 ` Nicolas Dichtel
2012-12-05 11:41 ` David Laight
2012-12-05 17:54 ` David Miller
2012-12-06 8:43 ` Nicolas Dichtel
2012-12-06 17:49 ` Thomas Graf
2012-12-06 21:49 ` Nicolas Dichtel
2012-12-07 10:38 ` David Laight
2012-12-07 10:58 ` Thomas Graf
2012-12-11 15:03 ` Nicolas Dichtel
2012-12-11 18:40 ` Thomas Graf
2012-12-12 17:30 ` Nicolas Dichtel
2012-12-14 13:16 ` [PATCH] netlink: align attributes on 64-bits Nicolas Dichtel
2012-12-14 15:49 ` Ben Hutchings
2012-12-14 16:04 ` Nicolas Dichtel
2012-12-17 16:49 ` [PATCH v2] " Nicolas Dichtel
2012-12-17 17:06 ` David Laight
2012-12-17 17:35 ` Nicolas Dichtel
2012-12-18 9:19 ` David Laight
2012-12-18 10:18 ` Nicolas Dichtel
2012-12-18 12:57 ` Thomas Graf
2012-12-18 16:23 ` Nicolas Dichtel
2012-12-18 16:50 ` David Laight
2012-12-18 17:11 ` Thomas Graf
2012-12-19 9:17 ` David Laight
2012-12-19 17:20 ` Thomas Graf
2012-12-20 9:37 ` David Laight
2012-12-20 9:40 ` David Laight
2012-12-18 17:08 ` Thomas Graf
2012-12-18 22:07 ` Nicolas Dichtel
2012-12-19 11:22 ` Nicolas Dichtel
2012-12-19 17:09 ` Thomas Graf
2012-12-19 18:07 ` Nicolas Dichtel
2012-12-17 9:59 ` [PATCH] " David Laight
2012-12-17 16:53 ` Nicolas Dichtel [this message]
2012-12-05 17:53 ` [PATCH net-next 0/7] Allow to monitor multicast cache event via rtnetlink David Miller
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