port-amd64/55483: SMEP exception skip topmost stack frame

[martin%NetBSD.org@localhost]

>Number:         55483
>Category:       port-amd64
>Synopsis:       SMEP exception skip topmost stack frame
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jul 12 12:50:00 +0000 2020
>Originator:     Martin Husemann
>Release:        NetBSD 9.99.69
>Organization:
The NetBSD Foundation, Inc.
>Environment:
System: NetBSD seven-days-to-the-wolves.aprisoft.de 9.99.69 NetBSD 9.99.69 (GENERIC) #406: Tue Jun 30 16:36:57 CEST 2020 martin%seven-days-to-the-wolves.aprisoft.de@localhost:/work/src/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:

The SMEP exception when the kernel jumps through a NULL pointer is nice,
but the ddb trace it ends up with misses the topmost stack frame, so it is
mostly useless for driver developement/kernel debugging.

>How-To-Repeat:

Make the kernel jump via a NULL function pointer, watch the db backtrace
afterwards and wonder where the jump really happens.

>Fix:
n/a