kern/53590: DTrace fbt_getargdesc() fails with "Unknown function"

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         53590
>Category:       kern
>Synopsis:       DTrace fbt_getargdesc() fails with "Unknown function"
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Sep 11 08:30:00 +0000 2018
>Originator:     Andreas Gustafsson
>Release:        NetBSD 8.0; also -current as of Sep 10
>Organization:
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

Examining the arguments of any kernel function call using the DTrace FBT
provider and the args[] array fails.  For example:

  guido# modload dtrace_fbt
  guido# dtrace -n 'fbt::vmem_alloc:entry { trace(args[0]); }'
  dtrace: invalid probe specifier fbt::vmem_alloc:entry { trace(args[0]); }: in action list: index 0 is out of range for fbt::vmem_alloc:entry args[ ]

The following message appears on the console:

  guido /netbsd: fbt_getargdesc(2067): Unknown function vmem_alloc!

It works on FreeBSD (tested on EC2 with ami-e83e6c97):

  root@freebsd:/usr/home/ec2-user # dtrace -n 'fbt::vmem_alloc:entry { trace(args[0]); }'
  dtrace: description 'fbt::vmem_alloc:entry ' matched 1 probe
  CPU     ID                    FUNCTION:NAME
    0  57284                 vmem_alloc:entry       -2116181376
    0  57284                 vmem_alloc:entry       -2116181376
  ^C
  root@freebsd:/usr/home/ec2-user # dtrace -n 'fbt::vmem_alloc:entry { trace(args[0]->vm_name); }'
  dtrace: description 'fbt::vmem_alloc:entry ' matched 1 probe
  CPU     ID                    FUNCTION:NAME
    0  57284                 vmem_alloc:entry   buffer arena
    0  57284                 vmem_alloc:entry   kmem arena
    0  57284                 vmem_alloc:entry   buffer arena
    0  57284                 vmem_alloc:entry   kernel arena
  ^C

>How-To-Repeat:

>Fix: