Re: [PATCH] Stackgap

From: Doug Rabson (dfr_at_nlsystems.com)
Date: 05/29/05

  • Next message: Doug Rabson: "Re: [PATCH] Stackgap" 
    To: freebsd-arch@freebsd.org
Date: Sun, 29 May 2005 15:07:49 +0100

    On Saturday 28 May 2005 15:10, Robert Watson wrote:
    > On Fri, 27 May 2005, Suleiman Souhlal wrote:
    > > You can find an implementation of stackgap from OpenBSD at http://
    > > people.freebsd.org/~ssouhlal/testing/stackgap-20050527.diff
    > >
    > > You can control the range of the random stack gap with the
    > > kern.stackgap_random sysctl. A value of 0 disables it. Otherwise,
    > > it has to be a power of 2 and not too large. The default value is
    > > 64K.
    > >
    > > I've only had the chance to test this on i386. Could anyone test it
    > > on other architectures as well?
    > >
    > > Any comments/objections?
    >
    > In the past, substantial performance hits have been measured due to
    > poor stack alignment. Specifically, in combination with less optimal
    > compiler behavior, the results have been pretty nasty. Have you
    > tried micro-benchmarking a series of runs with this stack offset
    > randomness using floating point on stack arguments to see if there's
    > a measurable cost to moving the stack around? Hopefull if all is
    > well, there will be little or no difference, but a small error here
    > could result in a substantial performance hit...

    I recently modified the crt code to force the stack alignment to 16
    bytes on startup (so that I could safely write code that uses movaps).
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