net: diag: document swapped src/dst in udp_dump_one.

Since its inception, udp_dump_one has had a bug where userspace
needs to swap src and dst addresses and ports in order to find
the socket it wants. This is because it passes the socket source
address to __udp[46]_lib_lookup's saddr argument, but those
functions are intended to find local sockets matching received
packets, so saddr is the remote address, not the local address.

This can no longer be fixed for backwards compatibility reasons,
so add a brief comment explaining that this is the case. This
will avoid confusion and help ensure SOCK_DIAG implementations
of new protocols don't have the same problem.

Fixes: a925aa00a55 ("udp_diag: Implement the get_exact dumping functionality")
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/udp_diag.c b/net/ipv4/udp_diag.c
index d9ad986c7b2c9e073616c63d6d5ab376d2b72d5f..5cbb9be0529507f15371cc3b66fde16ad2d94156 100644 (file)
--- a/net/ipv4/udp_diag.c
+++ b/net/ipv4/udp_diag.c
@@ -42,6 +42,7 @@ static int udp_dump_one(struct udp_table *tbl, struct sk_buff *in_skb,
 
        rcu_read_lock();
        if (req->sdiag_family == AF_INET)
+               /* src and dst are swapped for historical reasons */
                sk = __udp4_lib_lookup(net,
                                req->id.idiag_src[0], req->id.idiag_sport,
                                req->id.idiag_dst[0], req->id.idiag_dport,