Re: [PATCH net v2] net/rds: include the scope id in the sendmsg connection cache check

[sashiko-bot@kernel.org]

> 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.
> 
> A request without a scope keeps matching the cached connection.  A
> send with a destination always carries one for a link-local peer, but
> a send without a destination takes rs_bound_scope_id, and a bind() to
> a non-link-local address after connect() to a link-local peer resets
> that to 0.  Such a send never named an interface, so the connection
> the socket last used is the one it means; making it miss the cache
> would send it to look up a connection with no interface, which cannot
> be established.
> 
> Fixes: 1e2b44e78eea ("rds: Enable RDS IPv6 support")
> Assisted-by: Claude-Code:claude-fable-5
> Signed-off-by: Allison Henderson <achender@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

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