linux-kernel - Re: [PATCH 1/2] vfs: make sure struct filename->iname is word-aligned

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Message-ID: <878u2heq4y.fsf@rasmusvillemoes.dk>
Date:	Thu, 18 Feb 2016 21:10:21 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] vfs: make sure struct filename->iname is word-aligned

On Tue, Feb 16 2016, Al Viro <viro@...IV.linux.org.uk> wrote:

> On Tue, Feb 16, 2016 at 11:49:24PM +0100, Rasmus Villemoes wrote:
>> I noticed that offsetof(struct filename, iname) is actually 28 on 64
>> bit platforms, so we always pass an unaligned pointer to
>> strncpy_from_user. This is mostly a problem for those 64 bit platforms
>> without HAVE_EFFICIENT_UNALIGNED_ACCESS, but even on x86_64, unaligned
>> accesses carry a penalty, especially when done in a loop.
>> 
>> Let's try to ensure we always pass an aligned destination pointer to
>> strncpy_from_user. I considered making refcnt a long instead of doing
>> the union thing, and mostly ended up tossing a coin.
>
> Why not swap it with the previous field, then?

Sure, that would work as well. I don't really care how ->iname is pushed
out to offset 32, but I'd like to know if it's worth it.

Rasmus