Re: [PATCH net] net: psample: Fix the netlink skb length

On Wed, Feb 03, 2021 at 06:21:03PM -0800, Jakub Kicinski wrote:

On Wed,  3 Feb 2021 11:10:28 +0800 Chris Mi wrote:

quoted

Currently, the netlink skb length only includes metadata and data
length. It doesn't include the psample generic netlink header length.

But what's the bug? Did you see oversized messages on the socket? Did
one of the nla_put() fail?

I didn't ask, but I assumed the problem was nla_put(). Agree it needs to
be noted in the commit message.

quoted

Fixes: 6ae0a6286171 ("net: Introduce psample, a new genetlink channel for packet sampling")
CC: Yotam Gigi <yotam.gi@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Chris Mi <redacted>
---
 net/psample/psample.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/psample/psample.c b/net/psample/psample.c
index 33e238c965bd..807d75f5a40f 100644
--- a/net/psample/psample.c
+++ b/net/psample/psample.c

@@ -363,6 +363,7 @@ void psample_sample_packet(struct psample_group *group, struct sk_buff *skb,
 	struct ip_tunnel_info *tun_info;
 #endif
 	struct sk_buff *nl_skb;
+	int header_len;
 	int data_len;
 	int meta_len;
 	void *data;

@@ -381,12 +382,13 @@ void psample_sample_packet(struct psample_group *group, struct sk_buff *skb,
 		meta_len += psample_tunnel_meta_len(tun_info);
 #endif
 
+	/* psample generic netlink header size */
+	header_len = nlmsg_total_size(GENL_HDRLEN + psample_nl_family.hdrsize);

GENL_HDRLEN is already included by genlmsg_new() and fam->hdrsize is 0
/ uninitialized for psample_nl_family. What am I missing? Ido?

Yea, I missed that genlmsg_new() eventually accounts for 'GENL_HDRLEN'.

Chris, assuming the problem is nla_put(), I think some other attribute
is not accounted for when calculating the size of the skb. Does it only
happen with packets that include tunnel metadata? Because I think I see
a few problems there:

diff --git a/net/psample/psample.c b/net/psample/psample.c
index 33e238c965bd..1a233cd128c7 100644
--- a/net/psample/psample.c
+++ b/net/psample/psample.c

@@ -311,8 +311,10 @@ static int psample_tunnel_meta_len(struct ip_tunnel_info *tun_info)
        int tun_opts_len = tun_info->options_len;
        int sum = 0;
 
+       sum += nla_total_size(0);       /* PSAMPLE_ATTR_TUNNEL */
+
        if (tun_key->tun_flags & TUNNEL_KEY)
-               sum += nla_total_size(sizeof(u64));
+               sum += nla_total_size_64bit(sizeof(u64));
 
        if (tun_info->mode & IP_TUNNEL_INFO_BRIDGE)
                sum += nla_total_size(0);

quoted

 	data_len = min(skb->len, trunc_size);
-	if (meta_len + nla_total_size(data_len) > PSAMPLE_MAX_PACKET_SIZE)
-		data_len = PSAMPLE_MAX_PACKET_SIZE - meta_len - NLA_HDRLEN
+	if (header_len + meta_len + nla_total_size(data_len) > PSAMPLE_MAX_PACKET_SIZE)
+		data_len = PSAMPLE_MAX_PACKET_SIZE - header_len - meta_len - NLA_HDRLEN
 			    - NLA_ALIGNTO;
-
-	nl_skb = genlmsg_new(meta_len + nla_total_size(data_len), GFP_ATOMIC);
+	nl_skb = genlmsg_new(header_len + meta_len + nla_total_size(data_len), GFP_ATOMIC);
 	if (unlikely(!nl_skb))
 		return;