Re: kern/50602: removing ethernet cable panics the kernel

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

On Mon, Jan 4, 2016 at 4:15 PM, Michael van Elst <mlelstv%serpens.de@localhost> wrote:
> The following reply was made to PR kern/50602; it has been noted by GNATS.
>
> From: mlelstv%serpens.de@localhost (Michael van Elst)
> To: gnats-bugs%netbsd.org@localhost
> Cc:
> Subject: Re: kern/50602: removing ethernet cable panics the kernel
> Date: Mon, 4 Jan 2016 07:09:51 +0000 (UTC)
>
>  ozaki-r%netbsd.org@localhost (Ryota Ozaki) writes:
>
>  >I remember that riastradh suggested me that we don't need the
>  >ifdef for kvm(3). IIUC, the added variables aren't accessed
>  >by kvm(3) so that we don't need to hide them to userland.
>
>  >I'd remove it if nobody objects.
>
>  The point is that the structure to userland must not change
>  without adding appropriate compatibility code.
>
>  But since the added variables are only accessed by the kernel
>  the simple solution is to put them inside #ifdef _KERNEL
>  as it was already done for the if_slowtimo_ch variable.

Hm. My understanding is that we can keep compatibility by
appending new variables to a data structure while inserting
variables to middle of a data structure or removing variables
breaks compatibility. Basically struct ifnet is accessed from
userland via struct ifreq and one exception is kvm(3) that is
used by netstat(1) and vmstat(1). The former guarantees
compatibility on changing ifnet and the compatibility for
the latter is guaranteed by always appending new variables
to ifnet when we change ifnet.

Correct me if I'm wrong (probably I am!).

  ozaki-r

>
>  --
>  --
>                                  Michael van Elst
>  Internet: mlelstv%serpens.de@localhost
>                                  "A potential Snark may lurk in every tree."
>