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    SubjectRe: [PATCH 02/16] x86/dumpstack: Add get_stack_info() support for the SYSENTER stack
    On Mon, Nov 20, 2017 at 12:46:13PM -0800, Andy Lutomirski wrote:
    > On Mon, Nov 20, 2017 at 12:42 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
    > > On Mon, Nov 20, 2017 at 09:07:33AM -0800, Andy Lutomirski wrote:
    > >> +bool in_SYSENTER_stack(unsigned long *stack, struct stack_info *info)
    > >
    > > Can you make it lowercase for consistency with the other in_*_stack()
    > > functions? For example, in_irq_stack() is all lowercase even though
    > > "IRQ" is normally written in uppercase.
    > >
    > > But also, I'm wondering whether this get_stack_info() support is even
    > > really needed.
    > >
    > > As currently written, the trampoline code doesn't have any ORC data
    > > associated with it. So the unwinder would never have the need to
    > > actually read the SYSENTER stack.
    > >
    > > You _could_ add an UNWIND_HINT_IRET_REGS annotation after the simulated
    > > iret frame is written, which would allow the unwinder to dump those regs
    > > when unwinding from an NMI.
    >
    > There's some ORC data in the non-trampoline SYSENTER path

    But that's *after* the stack switch to the real kernel stack, right?

    > but, more importantly, the OOPS unwinder will just bail without this
    > patch. With the patch, we get a valid unwind, except that everything
    > has a ? in front.

    Hm. I can't even fathom how that's possible. Are you talking about the
    "unwind from NMI to SYSENTER stack" path? Or any unwind to a syscall?
    Either way I'm baffled... If the unwinder only encounters the SYSENTER
    stack at the end, how could that cause everything beforehand to have a
    question mark?

    > > But there's only a tiny window where that would be possible: only a few
    > > instructions. I'm not sure that would be worth the effort, unless we
    > > got to the point where we expect to have 100% unwinder coverage. But
    > > that's currently unrealistic anyway because of generated code and
    > > runtime patching.
    >
    > I tripped it myself several times when debugging this code.

    Again I don't see how this patch would help if there's no ORC data for
    the code which uses the SYSENTER stack. I must be missing something.

    --
    Josh

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