linux-kernel - Re: Unnecessary overhead with stack protector.

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Message-ID: <20091021110053.26ab9982@infradead.org>
Date:	Wed, 21 Oct 2009 11:00:53 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, esandeen@...hat.com,
	cebbert@...hat.com, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: Unnecessary overhead with stack protector.

On Wed, 21 Oct 2009 10:50:02 -0500
Eric Sandeen <sandeen@...hat.com> wrote:

> Ingo Molnar wrote:
> > (Cc:-ed Arjan too.)
> > 
> > * Dave Jones <davej@...hat.com> wrote:
> > 
> >> 113c5413cf9051cc50b88befdc42e3402bb92115 introduced a change that
> >> made CC_STACKPROTECTOR_ALL not-selectable if someone enables 
> >> CC_STACKPROTECTOR.
> >>
> >> We've noticed in Fedora that this has introduced noticable
> >> overhead on some functions, including those which don't even have
> >> any on-stack variables.
> >>
> >> According to the gcc manpage, -fstack-protector will protect
> >> functions with as little as 8 bytes of stack usage. So we're
> >> introducing a huge amount of overhead, to close a small amount of
> >> vulnerability (the >0 && <8 case).
> >>
> >> The overhead as it stands right now means this whole option is 
> >> unusable for a distro kernel without reverting the above commit.
> > 
> > Exactly what workload showed overhead, and how much?
> > 
> > 	Ingo
> 
> I had xfs blowing up pretty nicely; granted, xfs is not svelte but it
> was never this bad before.
> 

do you have any indication that SP actually increases the stack
footprint by that much? it's only a few bytes....


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
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