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

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Message-ID: <4ADF59F8.7010205@redhat.com>
Date:	Wed, 21 Oct 2009 13:59:04 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Arjan van de Ven <arjan@...radead.org>
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.

Arjan van de Ven wrote:
> 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....
> 
> 

Here's a sample of some of the largest xfs stack users,
and the effect stack-protector had on them.  This was just
done with objdump -d xfs.ko | scripts/checkstack.pl; I don't
know if there's extra runtime stack overhead w/ stackprotector?

-Eric

function                  nostack stackprot delta delta %
xfs_bmapi                      376      408    32  9%
xfs_bulkstat                   328      344    16  5%
_xfs_trans_commit              296      312    16  5%
xfs_iomap_write_delay          264      280    16  6%
xfs_file_ioctl                 248      312    64 26%
xfs_symlink                    248      264    16  6%
xfs_bunmapi                    232      280    48 21%
xlog_do_recovery_pass          232      248    16  7%
xfs_trans_unreserve_and_mod_sb 224      240    16  7%
xfs_bmap_del_extent            216      248    32 15%
xfs_cluster_write              216      232    16  7%
xfs_file_compat_ioctl          216      296    80 37%
xfs_attr_set_int               200      216    16  8%
xfs_bmap_add_extent_delay_real 200      248    48 24%

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