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    SubjectRe: [patch V3 01/13] entry: Provide generic syscall entry functionality
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    Andy Lutomirski <luto@kernel.org> writes:
    > On Sat, Jul 18, 2020 at 7:16 AM Thomas Gleixner <tglx@linutronix.de> wrote:
    >> Andy Lutomirski <luto@kernel.org> writes:
    >> > FWIW, TIF_USER_RETURN_NOTIFY is a bit of an odd duck: it's an
    >> > entry/exit word *and* a context switch word. The latter is because
    >> > it's logically a per-cpu flag, not a per-task flag, and the context
    >> > switch code moves it around so it's always set on the running task.
    >>
    >> Gah, I missed the context switch thing of that. That stuff is hideous.
    >
    > It's also delightful because anything that screws up that dance (such
    > as failure to do the exit-to-usermode path exactly right) likely
    > results in an insta-root-hole. If we fail to run user return
    > notifiers, we can run user code with incorrect syscall MSRs, etc.

    Looking at it deeper, having that thing in the loop is a pointless
    exercise. This really wants to be done _after_ the loop.

    Thanks,

    tglx

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