linux-kernel - Re: [PATCH v12 net-next 03/11] bpf: add lookup/update/delete/iterate methods to BPF maps

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Date:	Tue, 16 Sep 2014 16:54:52 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ast@...mgrid.com
Cc:	mingo@...nel.org, torvalds@...ux-foundation.org,
	luto@...capital.net, dborkman@...hat.com,
	hannes@...essinduktion.org, chema@...gle.com, edumazet@...gle.com,
	a.p.zijlstra@...llo.nl, pablo@...filter.org, hpa@...or.com,
	akpm@...ux-foundation.org, keescook@...omium.org,
	linux-api@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 net-next 03/11] bpf: add
 lookup/update/delete/iterate methods to BPF maps

From: Alexei Starovoitov <ast@...mgrid.com>
Date: Tue, 16 Sep 2014 13:44:12 -0700

> the union indeed changes alignment from patch #1 to #3,
> but, imo, it is not a problem, since kernel does:

It changes the alignment of the datastructures in userspace.

> I haven't thought of 'aligned_u64' for this case.
> For counters and masks it would be perfect, but here user is
> passing real pointers to key and value, so they have to
> be 'void __user *', otherwise user would need to type cast
> them, which I want to avoid.

The cost of the compat layer must be considered and weighted
against this cast, which I think is really no big deal.

> I think compat layer is a better option.

It's overhead you'll have to support forever, I think you should
reconsider.

All of the "ugly casting" will be hidden, or can be hidden, in the
syscall wrappers and/or interfaces in userspace.
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