Re: [PATCH net v2 4/4] selftests: net: Add exception cache tests

[Ido Schimmel <idosch@nvidia.com>]

On Fri, Aug 28, 2026 at 10:23:44PM +0300, Ido Schimmel wrote:
> Add a test for the IPv4 and IPv6 exception caches, covering the
> exceptions that are created in response to ICMP errors quoting a UDP
> packet.
> 
> The topology consists of a host (h1) that reaches a remote host (h2)
> via a router (r1), with a second router (r2) attached to the segment
> shared by h1 and r1. UDP packets are injected using a packet socket, so
> that an ICMP error quoting them is only matched to a socket when one was
> opened separately with the same source port. PMTU errors are provoked by
> lowering the MTU of the far end of the path and redirects by pointing
> r1's route towards h2 back over the segment it received the packet from.
> 
> The following is tested for both address families and for both PMTU and
> redirect exceptions:
> 
> * An error that is not matched to a socket creates an exception that
>   carries the new MTU or gateway.
> * An error that is matched to a socket creates the same exception.
> 
> The PMTU tests further verify that a lower PMTU replaces the one stored
> in the exception whereas a higher one does not, and that a socket which
> disabled PMTU discovery using IP{,V6}_PMTUDISC_OMIT gets the same
> exception as the other cases.
> 
> Without "ipv4: udp: Create exceptions before socket matching" and "ipv6:
> udp: Create exceptions before socket matching", the tests that do not
> open a socket fail:

Both of the comments from Sashiko are nits. The test passes when it
should and fails otherwise, so it's doing its job.

"
This isn't a bug, but a question about what the "with a socket" cases
assert. pmtu_socket(), pmtu_omit() and redirect_socket() only look at the
route exception, and with the two accompanying kernel changes that
exception is created before the socket lookup:
"

It asserts that an exception is created regardless of whether the ICMP
error matched a socket or not.

"
Is the ICMP input counter a strong enough barrier for this check?
"

Yes.