linux-kernel - Re: [PATCH v2] net: netfilter: LLVMLinux: vlais-netfilter

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Date:	Tue, 18 Mar 2014 10:37:58 -0700
From:	Behan Webster <behanw@...verseincode.com>
To:	David Laight <David.Laight@...LAB.COM>,
	"davem@...emloft.net" <davem@...emloft.net>
CC:	"pablo@...filter.org" <pablo@...filter.org>,
	"kaber@...sh.net" <kaber@...sh.net>,
	"kadlec@...ckhole.kfki.hu" <kadlec@...ckhole.kfki.hu>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
	"netfilter@...r.kernel.org" <netfilter@...r.kernel.org>,
	"coreteam@...filter.org" <coreteam@...filter.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"pageexec@...email.hu" <pageexec@...email.hu>,
	Mark Charlebois <charlebm@...il.com>,
	Vinícius Tinti <viniciustinti@...il.com>
Subject: Re: [PATCH v2] net: netfilter: LLVMLinux: vlais-netfilter

On 03/18/14 08:24, David Laight wrote:
> From: Behan Webster
>> On 03/18/14 02:41, David Laight wrote:
>>> From:  behanw@...verseincode.com
>>>> From: Mark Charlebois <charlebm@...il.com>
>>>>
>>>> Replaced non-standard C use of Variable Length Arrays In Structs (VLAIS) in
>>>> xt_repldata.h with a C99 compliant flexible array member and then calculated
>>>> offsets to the other struct members. These other members aren't referenced by
>>>> name in this code, however this patch maintains the same memory layout and
>>>> padding as was previously accomplished using VLAIS.
>>>>
>>>> Had the original structure been ordered differently, with the entries VLA at
>>>> the end, then it could have been a flexible member, and this patch would have
>>>> been a lot simpler. However since the data stored in this structure is
>>>> ultimately exported to userspace, the order of this structure can't be changed.
>>> Why not just remove the last element and allocate space for it after the
>>> structure?
>> Because that would still be employing VLAIS to solve the problem. The
>> last element may be a zero-length array (a flexible member), not a VLA.
>> Sadly both the last 2 elements in the struct need to be manually
>> calculated, which is what we've done.
> So make the last element a 'flexible member' and then work out where
> the final field goes.
> Something like:
> 	struct p {
> 		struct a a;
> 		struct b b[];
> 	} p = malloc(sizeof *p + n * sizeof (struct b) + alignof (struct c)
> 		+ sizeof (struct c);
> 	struct c *c = (void *)&p->b[n] + (-offsetof(struct p, b[n]) & (alignof(struct c) - 1);
Oh, I see. Will fix.

Thanks!

Behan

-- 
Behan Webster
behanw@...verseincode.com

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