Re: MPSAFE and KERNEL_LOCK

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

On Fri, May 11, 2018 at 3:53 PM Michael van Elst <mlelstv%serpens.de@localhost> wrote:

> A traditional ethernet driver receives input via an interrupt routine
> and passes packets to ether_input(). The data is protected by disabling
> the receive interrupt.

> In multiprocessor land, the same is achieved by also acquiring the
> big kernel lock.

> Current code however queues incoming packets in a percpuq and passes
> things through a softint tagged as MPSAFE. The softint handler is calling
> ether_input without holding the kernel lock.

> There is another unprotected path in bpf_write calling into the
> _if_input vector directly without locking (bpf device is tagged MPSAFE)
> and without disabling interrupts.

> Did I miss something?

You're right.

Not holding KERNEL_LOCK in ether_input can be problematic if we use
CARP and/or AGR (and NETATALK) that are not MP-safe. Also calling
_if_ipnput without splsoftnet from bpf_write is problematic as you said.

The following patch should fix the issue:
   http://www.netbsd.org/~ozaki-r/fix-if_input.diff

The patch works for the !NET_MPSAFE case. NET_MPSAFE needs some more
cares, but that's another story.

Thanks,
   ozaki-r