Re: [PATCH 02/16] x86/dumpstack: Add get_stack_info() support for the SYSENTER stack

From: Andy Lutomirski
Date: Mon Nov 20 2017 - 16:07:42 EST

On Mon, Nov 20, 2017 at 1:00 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxx> 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?

Hmm, maybe you're 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?

I mean that, if I put a ud2 or other bug in the code that runs on the
SYSENTER stack, without this patch, I get a totally blank call trace.

>> > 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.