linux-kernel - Re: [PATCH] efi-bgrt: Add error handling; inform the user when ignoring the BGRT

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Message-ID: <20140805163932.GA3146@thin>
Date:	Tue, 5 Aug 2014 09:39:39 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	Matt Fleming <matt.fleming@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Srihari Vijayaraghavan <linux.bug.reporting@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [PATCH] efi-bgrt: Add error handling; inform the user when
 ignoring the BGRT

On Mon, Aug 04, 2014 at 01:19:59PM +0100, Matt Fleming wrote:
> On Fri, 01 Aug, at 09:11:54AM, Josh Triplett wrote:
> > 
> > The original bug report was about an allocation failure for a fairly
> > reasonable BGRT size.  We can certainly prohibit absurdly huge ones (for
> > instance, bigger than the maximum likely screen resolution times 4 bytes
> > per pixel), but allocation failures may well occur for smaller sizes,
> > and I don't think we want to spew a massive warning for that either.
> 
> Oh, dammit, that's my bad. I misread the allocation size and thought it
> was huge, but now realise it was only 6MB or so. Sorry Josh.
> 
> I was worried that this was the first reported instance of a BGRT
> claiming to be valid but with a bogusly large image size. I've never
> been so happy to be wrong.

:)

> However, the fact that the allocation failed is worth investigating -
> this machine appears to have GBs of ram. Perhaps we should switch to
> requesting pages directly instead of relying on kmalloc()? 
> 
> I appreciate that the BGRT code isn't mission critical or anything like
> that, and that failing the alloc isn't the end of the world, but if we
> have code in the kernel it should really be as robust as possible. I
> don't think trying to kmalloc() ~6MB can claim to be robust.

vmalloc or flex_array could potentially help here.  However, I'd suggest
we go ahead and merge this patch to improve the existing error handling
before doing a more extensive rewrite to use one of those.

Would anything go horrifically wrong if this allocation used vmalloc?
We really don't care deeply about the performance of this memory; it
just needs a single copy in and a small number of copies out in the
lifetime of a system.

- Josh Triplett
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