kernel page 0

[Masanobu SAITOH <msaitoh%execsw.org@localhost>]

 Hi.

 While debugging an device driver, I found the following behavior:

> s1200# sync
> s1200# ~fatal breakpoint trap in supervisor mode
> trap type 1 code 0 rip ffffffff80114a75 cs 8 rflags 202 cr2 14f9000 ilevel 8 rsp fffffe810e8a1d00
> curlwp 0xfffffe823f72d840 pid 0.2 lowest kstack 0xfffffe810e89e2c0
> Stopped in pid 0.2 (system) at  netbsd:breakpoint+0x5:  leave
> db{0}> x/l 0x0
> 0:
> db{0}> w/l 0x0 0
> 0fatal page fault in supervisor mode
> trap type 6 code 0 rip ffffffff8012ecbc cs 8 rflags 10246 cr2 0 ilevel 8 rsp fffffe810e8a1860
> curlwp 0xfffffe823f72d840 pid 0.2 lowest kstack 0xfffffe810e89e2c0
> kernel: page fault trap, code=0
> Faulted in DDB; continuing...
> db{0}> c

 Read access of kernel page 0 doesn't panic. Write access does.

Is this intended behavior? I tested this behavior on amd64-current
and i386 netbsd-6.

-- 
-----------------------------------------------
                SAITOH Masanobu (msaitoh%execsw.org@localhost
                                 msaitoh%netbsd.org@localhost)