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[164.138.29.33]) by smtp.gmail.com with ESMTPSA id bw9-20020a170906c1c900b00a4650ec48d0sm1645765ejb.140.2024.03.15.04.48.49 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Fri, 15 Mar 2024 04:48:49 -0700 (PDT) Date: Fri, 15 Mar 2024 12:48:08 +0100 From: Stefano Brivio To: Jakub Kicinski Cc: davem@davemloft.net, netdev@vger.kernel.org, edumazet@google.com, pabeni@redhat.com, jiri@resnulli.us, idosch@idosch.org, johannes@sipsolutions.net, fw@strlen.de, pablo@netfilter.org, Martin Pitt , Paul Holzinger , David Gibson Subject: Re: [PATCH net-next v2 3/3] genetlink: fit NLMSG_DONE into same read() as families Message-ID: <20240315124808.033ff58d@elisabeth> In-Reply-To: <20240303052408.310064-4-kuba@kernel.org> References: <20240303052408.310064-1-kuba@kernel.org> <20240303052408.310064-4-kuba@kernel.org> Organization: Red Hat X-Mailer: Claws Mail 4.2.0 (GTK 3.24.36; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable Hi, On Sat, 2 Mar 2024 21:24:08 -0800 Jakub Kicinski wrote: > Make sure ctrl_fill_info() returns sensible error codes and > propagate them out to netlink core. Let netlink core decide > when to return skb->len and when to treat the exit as an > error. Netlink core does better job at it, if we always > return skb->len the core doesn't know when we're done > dumping and NLMSG_DONE ends up in a separate read(). While this change is obviously correct, it breaks... well, broken applications that _wrongly_ rely on the fact that NLMSG_DONE is delivered in a separate datagram. This was the (embarrassing) case for passt(1), which I just fixed: https://archives.passt.top/passt-dev/20240315112432.382212-1-sbrivio@redh= at.com/ but the "separate" NLMSG_DONE is such an established behaviour, I think, that this might raise a more general concern. =46rom my perspective, I'm just happy that this change revealed the issue, but I wanted to report this anyway in case somebody has similar possible breakages in mind. > Reviewed-by: Eric Dumazet > Signed-off-by: Jakub Kicinski > --- > CC: jiri@resnulli.us > --- > net/netlink/genetlink.c | 12 +++++++----- > 1 file changed, 7 insertions(+), 5 deletions(-) >=20 > diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c > index 50ec599a5cff..3b7666944b11 100644 > --- a/net/netlink/genetlink.c > +++ b/net/netlink/genetlink.c > @@ -1232,7 +1232,7 @@ static int ctrl_fill_info(const struct genl_family = *family, u32 portid, u32 seq, > =20 > hdr =3D genlmsg_put(skb, portid, seq, &genl_ctrl, flags, cmd); > if (hdr =3D=3D NULL) > - return -1; > + return -EMSGSIZE; > =20 > if (nla_put_string(skb, CTRL_ATTR_FAMILY_NAME, family->name) || > nla_put_u16(skb, CTRL_ATTR_FAMILY_ID, family->id) || > @@ -1355,6 +1355,7 @@ static int ctrl_dumpfamily(struct sk_buff *skb, str= uct netlink_callback *cb) > struct net *net =3D sock_net(skb->sk); > int fams_to_skip =3D cb->args[0]; > unsigned int id; > + int err =3D 0; > =20 > idr_for_each_entry(&genl_fam_idr, rt, id) { > if (!rt->netnsok && !net_eq(net, &init_net)) > @@ -1363,16 +1364,17 @@ static int ctrl_dumpfamily(struct sk_buff *skb, s= truct netlink_callback *cb) > if (n++ < fams_to_skip) > continue; > =20 > - if (ctrl_fill_info(rt, NETLINK_CB(cb->skb).portid, > - cb->nlh->nlmsg_seq, NLM_F_MULTI, > - skb, CTRL_CMD_NEWFAMILY) < 0) { > + err =3D ctrl_fill_info(rt, NETLINK_CB(cb->skb).portid, > + cb->nlh->nlmsg_seq, NLM_F_MULTI, > + skb, CTRL_CMD_NEWFAMILY); > + if (err) { > n--; > break; > } > } > =20 > cb->args[0] =3D n; > - return skb->len; > + return err; > } > =20 > static struct sk_buff *ctrl_build_family_msg(const struct genl_family *f= amily, --=20 Stefano