netdev - RE: [PATCH v2] net: netfilter: LLVMLinux: vlais-netfilter

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Date:	Tue, 18 Mar 2014 15:24:13 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Behan Webster' <behanw@...verseincode.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

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);

	David