Re: [PATCH 2/2] x86/entry: Fix unwinding from kprobe on PUSH/POP instruction

[Peter Zijlstra]

On Thu, Feb 16, 2023 at 11:46:30AM +0100, Peter Zijlstra wrote:
> On Wed, Feb 15, 2023 at 03:16:37PM -0800, Josh Poimboeuf wrote:
> > On Wed, Feb 15, 2023 at 11:25:54AM +0100, Peter Zijlstra wrote:
> 
> > > Well, I was specifically thinking about #DB, why don't we need to
> > > decrement when we put a hardware breakpoint on a stack modifying op?
> > 
> > I assume you mean the INT1 instruction.  Yeah, maybe we should care
> > about that.
> 
> Nah, I was thinking #DB from DR7, but ...
> 
> > I'm struggling to come up with any decent ideas about how to implement
> > that.  Presumably the #DB handler would have to communicate to the
> > unwinder somehow whether the given frame is a trap.
> 
> ... I had forgotten that #DB is not unconditionally trap :/ The worst
> part seems to be that code breakpoints are faults while data breakpoints
> are traps.
> 
> And you so don't want to go decode the DR registers in the unwinder,
> quality mess this :/
> 
> Put a breakpoint on the stack and you've got PUSH doing a trap, put a
> breakpoint on the PUSH instruction and you get a fault, and lo and
> behold, you get a different unwind :-(

It could be I'm just confusing things... when #DB traps it is actually
because the instruction is complete, so looking up the ORC based on the
next instruction is correct, while when #DB faults, it is because the
instruction has not yet completed and again ORC lookup on IP just works.

So while determining if #DB is trap or fault is a giant pain in the
arse, it does not actually matter for the unwinder in this case.

And with the INT3 thing the problem is that we've replaced an
instruction that was supposed to do a stack op.