netdev - [PATCH v2] sched: report if filter is too large to dump

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Date:   Mon, 19 Feb 2018 21:32:51 +0100
From:   Roman Kapl <code@...pl.cz>
To:     netdev@...r.kernel.org
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jiri Pirko <jiri@...nulli.us>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Benc <jbenc@...hat.com>, Roman Kapl <code@...pl.cz>
Subject: [PATCH v2] sched: report if filter is too large to dump

So far, if the filter was too large to fit in the allocated skb, the
kernel did not return any error and stopped dumping. Modify the dumper
so that it returns -EMSGSIZE when a filter fails to dump and it is the
first filter in the skb. If we are not first, we will get a next chance
with more room.

I understand this is pretty near to being an API change, but the
original design (silent truncation) can be considered a bug.

Note: The error case can happen pretty easily if you create a filter
with 32 actions and have 4kb pages. Also recent versions of iproute try
to be clever with their buffer allocation size, which in turn leads to

Signed-off-by: Roman Kapl <code@...pl.cz>
---
v1 -> v2: add the "progress" comment, fixed error name in commit message.

I've looked at other dumpers in rtnetnlink, there are various ways to handle
that. For example rtnl_stats_dump has:
    WARN_ON((err == -EMSGSIZE) && (skb->len == 0));
rtnl_dump_ifinfo has the same logic I am proposing:
    if (err < 0) 
        if (skb->len) goto out;
        goto out_err;
Other functions handle the error in the "wrong" way (= what we currently do).
Although it might be OK if there is no array in what they return. I have not
tested the behavior, since the only way I found was to have a device with many
VFs.

 net/sched/cls_api.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index f21610c5da1a..92e9308bb920 100644
--- a/net/sched/cls_api.c
+++ b/net/sched/cls_api.c
@@ -1399,13 +1399,18 @@ static int tc_dump_tfilter(struct sk_buff *skb, struct netlink_callback *cb)
 		    nla_get_u32(tca[TCA_CHAIN]) != chain->index)
 			continue;
 		if (!tcf_chain_dump(chain, q, parent, skb, cb,
-				    index_start, &index))
+				    index_start, &index)) {
+			err = -EMSGSIZE;
 			break;
+		}
 	}
 
 	cb->args[0] = index;
 
 out:
+	/* If we did no progress, the error (EMSGSIZE) is real */
+	if (skb->len == 0 && err)
+		return err;
 	return skb->len;
 }
 
-- 
2.16.1