Re: [PATCH net] net: mctp: Don't access ifa_index when missing

[Matt Johnston <matt@codeconstruct.com.au>]

On Tue, 2025-05-06 at 18:06 -0700, Jakub Kicinski wrote:
> On Mon, 05 May 2025 17:05:12 +0800 Matt Johnston wrote:
> > +		/* Userspace programs providing AF_MCTP must be expecting ifa_index filter
> > +		 * behaviour, as will those setting strict_check.
> > +		 */
> > +		if (hdr->ifa_family == AF_MCTP || cb->strict_check)
> > +			ifindex = hdr->ifa_index;
> 
> The use of cb->strict_check is a bit strange here. I could be wrong but
> I though cb->strict_check should only impact validation. Not be used
> for changing behavior.

It was following behaviour of inet_dump_addr()/inet6_dump_addr() where
filtering is applied if strict check is set.
I don't have strong opinion whether strict_check makes sense for MCTP though
- it depends on
whether userspace expects strict_check to apply to all families, or just
inet4/inet6.

> If you have a reason to believe all user space passes a valid header -
> how about we just return an error if message is too short?
> IPv4 and IPv6 seem to return an error if message is short and
> cb->strict_check, so they are more strict. MCTP doesn't have a ton of
> legacy user space, we don't have to be lenient at all. My intuition
> would be to always act like IP acts under cb->strict_check

The problem is that programs will pass ifa_family=AF_UNSPEC with a short
header, no strict_check 
(eg busybox "ip addr show").
An AF_UNSPEC request will reach mctp_dump_addrinfo(), so we don't want that
returning an error.
Maybe mctp_dump_addrinfo() should ignore AF_UNSPEC requests entirely, and
only populate
a response when ifa_family=AF_MCTP. That would be OK for the existing mctp
userspace programs 
I know about, though there may be other users that are calling with AF_UNSPEC
but filtering 
userspace-side for AF_MCTP addresses.

Matt