Messages in this thread |  | | | Date | Wed, 21 Oct 2009 14:21:54 -0500 | | From | Eric Sandeen <> | | Subject | Re: XFS stack overhead |
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Ingo Molnar wrote: > * 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?
The above isn't a callchain; those are just the biggest users.
Yes, xfs works w/4k stacks but sometimes not over complex storage.
> 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.
I can find plenty of examples of > 300 bytes stack users in the core kernel write path too, I'm just using xfs as an example... > Also, the posting apparently mixes 'stack overhead' with 'runtime > overhead'.
right, that's why I asked, I'm not sure if stackprotector has runtime overhead as well.
-Eric
> Ingo
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