netdev - Re: alignment faults in 3.6

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Date:	Fri, 12 Oct 2012 12:07:50 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Måns Rullgård <mans@...sr.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org,
	David Laight <David.Laight@...lab.com>,
	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	Jon Masters <jonathan@...masters.org>
Subject: Re: alignment faults in 3.6

On Fri, Oct 12, 2012 at 12:00:03PM +0100, Måns Rullgård wrote:
> Russell King - ARM Linux <linux@....linux.org.uk> writes:
> 
> > On Fri, Oct 12, 2012 at 08:11:42AM +0000, Arnd Bergmann wrote:
> >> On Thursday 11 October 2012, Måns Rullgård wrote:
> >> > > But, the IP header is expected to be aligned.
> >> > 
> >> > Everything tells the compiler the struct is perfectly aligned.  When the
> >> > buggy driver passes a misaligned pointer, bad things happen.
> >> 
> >> Would it be appropriate to add a WARN_ON_ONCE() in the alignment fault path
> >> then?
> 
> I think that's an excellent idea.

Well, I get the last word here and it's no.

> >> If all alignment faults in the kernel are caused by broken drivers,
> >> that would at least give us some hope of finding those drivers while
> >> at the same time not causing much overhead in the case where we need
> >> to do the fixup in the meantime.
> >
> > No.  It is my understanding that various IP option processing can also
> > cause the alignment fault handler to be invoked, even when the packet is
> > properly aligned, and then there's jffs2/mtd which also relies upon
> > alignment faults being fixed up.
> 
> As far as I'm concerned, this is all hearsay, and I've only ever heard
> it from you.  Why can't you let those who care fix these bugs instead?

You know, I'm giving you the benefit of my _knowledge_ which has been
built over the course of the last 20 years.  I've been in these
discussions with networking people before.  I ended up having to develop
the alignment fault handler because of those discussions.  And oh look,
Eric confirmed that the networking code isn't going to get "fixed" as
you were demanding, which is exactly what I said.

I've been in discussions with MTD people over these issues before, I've
discussed this with David Woodhouse when it came up in JFFS2.  I *KNOW*
these things.

You can call it hearsay if you wish, but it seems to be more accurate
than your wild outlandish and pathetic statements.
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