Re: XFS stack overhead

Ingo Molnar
2009-10-21T19:16:58Z

* Eric Sandeen <sandeen@redhat.com> wrote:

> 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%

Note that those are very large stack frames to begin with.

3496 bytes - that's a _lot_ - can anyone even run XFS with 4K stacks on?

With stackprotector it's 3928 - a 12% increase - which certainly does
not help - but the basic problem is the large stack footprint to begin
with.

Also, the posting apparently mixes 'stack overhead' with 'runtime
overhead'.

Ingo