Re: Unnecessary overhead with stack protector.

Eric Sandeen
2009-10-21T18:59:46Z

Arjan van de Ven wrote:
> On Wed, 21 Oct 2009 10:50:02 -0500
> Eric Sandeen <sandeen@redhat.com> wrote:
>
>> Ingo Molnar wrote:
>>> (Cc:-ed Arjan too.)
>>>
>>> * Dave Jones <davej@redhat.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%