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

[Ido Schimmel <idosch@idosch.org>]

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:
> > 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.

> 
> > 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 <cmi@nvidia.com>
> > ---
> >  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);

> 
> >  	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;
> >  
>