Re: [PATCH net v4 2/2] net/rds: include the scope id in the sendmsg connection cache check

[Simon Horman <horms@kernel.org>]

On Wed, Oct 07, 2026 at 08:14:26PM -0700, Allison Henderson wrote:
> rds_sendmsg() reuses the connection cached in rs->rs_conn when its
> peer address and ToS match the request.  The interface index is part
> of a connection's identity as well: rds_conn_create_outgoing() passes
> the request's scope_id down as dev_if, and rds_conn_lookup() compares
> c_dev_if, so sends to the same link-local address through two
> interfaces are two different connections.  The cache-hit test never
> looked at it.
> 
> A socket bound to a non-link-local address has rs_bound_scope_id 0,
> and the scope check at the top of rds_sendmsg() accepts any non-zero
> destination scope for such a socket.  So after a send to fe80::x%ifA,
> a send to fe80::x%ifB hits the cached ifA connection and the datagram
> leaves through ifA, to whichever peer answers to that address there.
> 
> Compare c_dev_if with the request's scope_id in the cache test, so
> that such a send takes the lookup path and finds, or creates, the ifB
> connection - the same key the lookup uses.  With the previous patch a
> send without a destination carries the connected peer's scope, or the
> bound scope for a non-link-local peer, which is what an explicit send
> to that peer computes, so the comparison is exact for it too.
> 
> Found by inspection while reworking this cache for the connection
> lifetime series on net-next; not observed in the field.
> 
> Fixes: 1e2b44e78eea ("rds: Enable RDS IPv6 support")
> Assisted-by: Claude-Code:claude-fable-5
> Signed-off-by: Allison Henderson <achender@kernel.org>

Reviewed-by: Simon Horman <horms@kernel.org>