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    SubjectRe: [PATCH] Gerd Hoffman's move-vsyscall-into-user-address-range patch
    Chris Wright wrote:
    > * Zachary Amsden (zach@vmware.com) wrote:
    >
    >> Let's dive into it. How do you get the randomization without
    >> sacrificing syscall performance? Do you randomize on boot, dynamically,
    >> or on a per-process level?
    >>
    >
    > The latter, on exec.
    >
    >
    >> Because I can see some issues with
    >> per-process randomization that will certainly cost some amount of cycles
    >> on the system call path. Marginal perhaps, but that is exactly where
    >> you don't want to shed cycles unnecessarily, and the complexity of the
    >> whole thing will go up quite a bit I think.
    >>
    >
    > The crux is here:
    >
    > + OFFSET(TI_sysenter_return, thread_info, sysenter_return);
    > ...
    >
    > - pushl $SYSENTER_RETURN
    > -
    > + /*
    > + * Push current_thread_info()->sysenter_return to the stack.
    > + * A tiny bit of offset fixup is necessary - 4*4 means the 4 words
    > + * pushed above; +8 corresponds to copy_thread's esp0 setting.
    > + */
    > + pushl (TI_sysenter_return-THREAD_SIZE+8+4*4)(%esp)
    >
    > ...
    >
    > and in binfmt_elf during exec thread_info->sysenter_return is setup
    > based on the randomized mapping it does for vdso
    >
    > + ti->sysenter_return = &SYSENTER_RETURN_OFFSET + addr;
    >
    >
    > I think it's not so bad, but I can't say I've benchmarked the cost.
    >

    Now that I see it, it doesn't look bad at all. I had imagined a host of
    holy horrors unfolding from it, but clearly that is not the case. I
    think there is still the sysexit path that needs some change, but in
    total, there should be almost zero cycle impact. I envisioned trying to
    get the thread info for the return address would be awkward, but you've
    already switched the stack at this point, so it is really almost free.

    Zach

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