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    SubjectRe: Unnecessary overhead with stack protector.
    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%



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