netdev - Re: [PATCH net-next v2 3/3] genetlink: fit NLMSG

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Message-ID: <20240411085206.1d127414@kernel.org>
Date: Thu, 11 Apr 2024 08:52:06 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Ilya Maximets <i.maximets@....org>
Cc: Eric Dumazet <edumazet@...gle.com>, Stefano Brivio <sbrivio@...hat.com>,
 davem@...emloft.net, netdev@...r.kernel.org, pabeni@...hat.com,
 jiri@...nulli.us, idosch@...sch.org, johannes@...solutions.net,
 fw@...len.de, pablo@...filter.org, Martin Pitt <mpitt@...hat.com>, Paul
 Holzinger <pholzing@...hat.com>, David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH net-next v2 3/3] genetlink: fit NLMSG_DONE into same
 read() as families

On Thu, 11 Apr 2024 17:39:20 +0200 Ilya Maximets wrote:
> nlmsg_type=0x1a /* NLMSG_??? */  --> RTM_GETROUTE
> nlmsg_flags=NLM_F_REQUEST|0x300  --> NLM_F_REQUEST|NLM_F_DUMP

Thanks!

Also:

"\x02\x20\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x08\x00\x01\x00\x0a\x01\x01\x02"
 ^^^^

So it's dumping AF_INET. I'm guessing its also going to dump v6,
separately? To fix v4 I think something like this would do (untested):

diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index 48741352a88a..749baa74eee7 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1050,6 +1050,11 @@ static int inet_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 			e++;
 		}
 	}
+
+	/* Don't let NLM_DONE coalesce into a message, even if it could.
+	 * Some user space expects NLM_DONE in a separate recv().
+	 */
+	err = skb->len;
 out:
 
 	cb->args[1] = e;