[PATCH net-next 0/2] rtnetlink: avoid a warning in rtnl_newlink()

[PATCH net-next 0/2] rtnetlink: avoid a warning in rtnl_newlink()

[Jakub Kicinski]

Hi!

I've been hoping for some time that someone more competent would fix
the stack frame size warning in rtnl_newlink(), but looks like I'll
have to take a stab at it myself :)  That's the only warning I see
in most of my builds.

First patch refactors away a somewhat surprising if (1) code block.
Reindentation will most likely cause cherry-pick problems but OTOH
rtnl_newlink() doesn't seem to be changed often, so perhaps we can
risk it in the name of cleaner code?

Second patch fixes the warning in simplest possible way.  I was
pondering if there is any more clever solution, but I can't see it..
rtnl_newlink() is quite long with a lot of possible execution paths
so doing memory allocations half way through leads to very ugly
results.

Jakub Kicinski (2):
  rtnetlink: remove a level of indentation in rtnl_newlink()
  rtnetlink: avoid frame size warning in rtnl_newlink()

 net/core/rtnetlink.c | 331 ++++++++++++++++++++++---------------------
 1 file changed, 170 insertions(+), 161 deletions(-)

-- 
2.17.1

[PATCH net-next 1/2] rtnetlink: remove a level of indentation in rtnl_newlink()

[Jakub Kicinski]

rtnl_newlink() used to create VLAs based on link kind.  Since
commit ccf8dbcd062a ("rtnetlink: Remove VLA usage") statically
sized array is created on the stack, so there is no more use
for a separate code block that used to be the VLA's live range.

While at it christmas tree the variables.  Note that there is
a goto-based retry so to be on the safe side the variables can
no longer be initialized in place.  It doesn't seem to matter,
logically, but why make the code harder to read..

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
---
 net/core/rtnetlink.c | 313 +++++++++++++++++++++----------------------
 1 file changed, 154 insertions(+), 159 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 86f2d9cbdae3..8cbdc8398b8c 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -2974,17 +2974,22 @@ static int rtnl_group_changelink(const struct sk_buff *skb,
 static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 			struct netlink_ext_ack *extack)
 {
+	struct nlattr *slave_attr[RTNL_SLAVE_MAX_TYPE + 1];
+	unsigned char name_assign_type = NET_NAME_USER;
+	struct nlattr *linkinfo[IFLA_INFO_MAX + 1];
+	const struct rtnl_link_ops *m_ops = NULL;
+	struct nlattr *attr[RTNL_MAX_TYPE + 1];
+	struct net_device *master_dev = NULL;
 	struct net *net = sock_net(skb->sk);
 	const struct rtnl_link_ops *ops;
-	const struct rtnl_link_ops *m_ops = NULL;
+	struct nlattr *tb[IFLA_MAX + 1];
+	struct net *dest_net, *link_net;
+	struct nlattr **slave_data;
+	char kind[MODULE_NAME_LEN];
 	struct net_device *dev;
-	struct net_device *master_dev = NULL;
 	struct ifinfomsg *ifm;
-	char kind[MODULE_NAME_LEN];
 	char ifname[IFNAMSIZ];
-	struct nlattr *tb[IFLA_MAX+1];
-	struct nlattr *linkinfo[IFLA_INFO_MAX+1];
-	unsigned char name_assign_type = NET_NAME_USER;
+	struct nlattr **data;
 	int err;
 
 #ifdef CONFIG_MODULES
@@ -3040,195 +3045,185 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 		ops = NULL;
 	}
 
-	if (1) {
-		struct nlattr *attr[RTNL_MAX_TYPE + 1];
-		struct nlattr *slave_attr[RTNL_SLAVE_MAX_TYPE + 1];
-		struct nlattr **data = NULL;
-		struct nlattr **slave_data = NULL;
-		struct net *dest_net, *link_net = NULL;
-
-		if (ops) {
-			if (ops->maxtype > RTNL_MAX_TYPE)
-				return -EINVAL;
+	data = NULL;
+	if (ops) {
+		if (ops->maxtype > RTNL_MAX_TYPE)
+			return -EINVAL;
 
-			if (ops->maxtype && linkinfo[IFLA_INFO_DATA]) {
-				err = nla_parse_nested(attr, ops->maxtype,
-						       linkinfo[IFLA_INFO_DATA],
-						       ops->policy, extack);
-				if (err < 0)
-					return err;
-				data = attr;
-			}
-			if (ops->validate) {
-				err = ops->validate(tb, data, extack);
-				if (err < 0)
-					return err;
-			}
+		if (ops->maxtype && linkinfo[IFLA_INFO_DATA]) {
+			err = nla_parse_nested(attr, ops->maxtype,
+					       linkinfo[IFLA_INFO_DATA],
+					       ops->policy, extack);
+			if (err < 0)
+				return err;
+			data = attr;
+		}
+		if (ops->validate) {
+			err = ops->validate(tb, data, extack);
+			if (err < 0)
+				return err;
 		}
+	}
 
-		if (m_ops) {
-			if (m_ops->slave_maxtype > RTNL_SLAVE_MAX_TYPE)
-				return -EINVAL;
+	slave_data = NULL;
+	if (m_ops) {
+		if (m_ops->slave_maxtype > RTNL_SLAVE_MAX_TYPE)
+			return -EINVAL;
 
-			if (m_ops->slave_maxtype &&
-			    linkinfo[IFLA_INFO_SLAVE_DATA]) {
-				err = nla_parse_nested(slave_attr,
-						       m_ops->slave_maxtype,
-						       linkinfo[IFLA_INFO_SLAVE_DATA],
-						       m_ops->slave_policy,
-						       extack);
-				if (err < 0)
-					return err;
-				slave_data = slave_attr;
-			}
+		if (m_ops->slave_maxtype &&
+		    linkinfo[IFLA_INFO_SLAVE_DATA]) {
+			err = nla_parse_nested(slave_attr, m_ops->slave_maxtype,
+					       linkinfo[IFLA_INFO_SLAVE_DATA],
+					       m_ops->slave_policy, extack);
+			if (err < 0)
+				return err;
+			slave_data = slave_attr;
 		}
+	}
 
-		if (dev) {
-			int status = 0;
-
-			if (nlh->nlmsg_flags & NLM_F_EXCL)
-				return -EEXIST;
-			if (nlh->nlmsg_flags & NLM_F_REPLACE)
-				return -EOPNOTSUPP;
+	if (dev) {
+		int status = 0;
 
-			if (linkinfo[IFLA_INFO_DATA]) {
-				if (!ops || ops != dev->rtnl_link_ops ||
-				    !ops->changelink)
-					return -EOPNOTSUPP;
+		if (nlh->nlmsg_flags & NLM_F_EXCL)
+			return -EEXIST;
+		if (nlh->nlmsg_flags & NLM_F_REPLACE)
+			return -EOPNOTSUPP;
 
-				err = ops->changelink(dev, tb, data, extack);
-				if (err < 0)
-					return err;
-				status |= DO_SETLINK_NOTIFY;
-			}
+		if (linkinfo[IFLA_INFO_DATA]) {
+			if (!ops || ops != dev->rtnl_link_ops ||
+			    !ops->changelink)
+				return -EOPNOTSUPP;
 
-			if (linkinfo[IFLA_INFO_SLAVE_DATA]) {
-				if (!m_ops || !m_ops->slave_changelink)
-					return -EOPNOTSUPP;
+			err = ops->changelink(dev, tb, data, extack);
+			if (err < 0)
+				return err;
+			status |= DO_SETLINK_NOTIFY;
+		}
 
-				err = m_ops->slave_changelink(master_dev, dev,
-							      tb, slave_data,
-							      extack);
-				if (err < 0)
-					return err;
-				status |= DO_SETLINK_NOTIFY;
-			}
+		if (linkinfo[IFLA_INFO_SLAVE_DATA]) {
+			if (!m_ops || !m_ops->slave_changelink)
+				return -EOPNOTSUPP;
 
-			return do_setlink(skb, dev, ifm, extack, tb, ifname,
-					  status);
+			err = m_ops->slave_changelink(master_dev, dev, tb,
+						      slave_data, extack);
+			if (err < 0)
+				return err;
+			status |= DO_SETLINK_NOTIFY;
 		}
 
-		if (!(nlh->nlmsg_flags & NLM_F_CREATE)) {
-			if (ifm->ifi_index == 0 && tb[IFLA_GROUP])
-				return rtnl_group_changelink(skb, net,
+		return do_setlink(skb, dev, ifm, extack, tb, ifname, status);
+	}
+
+	if (!(nlh->nlmsg_flags & NLM_F_CREATE)) {
+		if (ifm->ifi_index == 0 && tb[IFLA_GROUP])
+			return rtnl_group_changelink(skb, net,
 						nla_get_u32(tb[IFLA_GROUP]),
 						ifm, extack, tb);
-			return -ENODEV;
-		}
+		return -ENODEV;
+	}
 
-		if (tb[IFLA_MAP] || tb[IFLA_PROTINFO])
-			return -EOPNOTSUPP;
+	if (tb[IFLA_MAP] || tb[IFLA_PROTINFO])
+		return -EOPNOTSUPP;
 
-		if (!ops) {
+	if (!ops) {
 #ifdef CONFIG_MODULES
-			if (kind[0]) {
-				__rtnl_unlock();
-				request_module("rtnl-link-%s", kind);
-				rtnl_lock();
-				ops = rtnl_link_ops_get(kind);
-				if (ops)
-					goto replay;
-			}
-#endif
-			NL_SET_ERR_MSG(extack, "Unknown device type");
-			return -EOPNOTSUPP;
+		if (kind[0]) {
+			__rtnl_unlock();
+			request_module("rtnl-link-%s", kind);
+			rtnl_lock();
+			ops = rtnl_link_ops_get(kind);
+			if (ops)
+				goto replay;
 		}
+#endif
+		NL_SET_ERR_MSG(extack, "Unknown device type");
+		return -EOPNOTSUPP;
+	}
 
-		if (!ops->setup)
-			return -EOPNOTSUPP;
-
-		if (!ifname[0]) {
-			snprintf(ifname, IFNAMSIZ, "%s%%d", ops->kind);
-			name_assign_type = NET_NAME_ENUM;
-		}
+	if (!ops->setup)
+		return -EOPNOTSUPP;
 
-		dest_net = rtnl_link_get_net_capable(skb, net, tb, CAP_NET_ADMIN);
-		if (IS_ERR(dest_net))
-			return PTR_ERR(dest_net);
+	if (!ifname[0]) {
+		snprintf(ifname, IFNAMSIZ, "%s%%d", ops->kind);
+		name_assign_type = NET_NAME_ENUM;
+	}
 
-		if (tb[IFLA_LINK_NETNSID]) {
-			int id = nla_get_s32(tb[IFLA_LINK_NETNSID]);
+	dest_net = rtnl_link_get_net_capable(skb, net, tb, CAP_NET_ADMIN);
+	if (IS_ERR(dest_net))
+		return PTR_ERR(dest_net);
 
-			link_net = get_net_ns_by_id(dest_net, id);
-			if (!link_net) {
-				NL_SET_ERR_MSG(extack, "Unknown network namespace id");
-				err =  -EINVAL;
-				goto out;
-			}
-			err = -EPERM;
-			if (!netlink_ns_capable(skb, link_net->user_ns, CAP_NET_ADMIN))
-				goto out;
-		}
+	if (tb[IFLA_LINK_NETNSID]) {
+		int id = nla_get_s32(tb[IFLA_LINK_NETNSID]);
 
-		dev = rtnl_create_link(link_net ? : dest_net, ifname,
-				       name_assign_type, ops, tb, extack);
-		if (IS_ERR(dev)) {
-			err = PTR_ERR(dev);
+		link_net = get_net_ns_by_id(dest_net, id);
+		if (!link_net) {
+			NL_SET_ERR_MSG(extack, "Unknown network namespace id");
+			err =  -EINVAL;
 			goto out;
 		}
+		err = -EPERM;
+		if (!netlink_ns_capable(skb, link_net->user_ns, CAP_NET_ADMIN))
+			goto out;
+	} else {
+		link_net = NULL;
+	}
 
-		dev->ifindex = ifm->ifi_index;
+	dev = rtnl_create_link(link_net ? : dest_net, ifname,
+			       name_assign_type, ops, tb, extack);
+	if (IS_ERR(dev)) {
+		err = PTR_ERR(dev);
+		goto out;
+	}
 
-		if (ops->newlink) {
-			err = ops->newlink(link_net ? : net, dev, tb, data,
-					   extack);
-			/* Drivers should call free_netdev() in ->destructor
-			 * and unregister it on failure after registration
-			 * so that device could be finally freed in rtnl_unlock.
-			 */
-			if (err < 0) {
-				/* If device is not registered at all, free it now */
-				if (dev->reg_state == NETREG_UNINITIALIZED)
-					free_netdev(dev);
-				goto out;
-			}
-		} else {
-			err = register_netdevice(dev);
-			if (err < 0) {
+	dev->ifindex = ifm->ifi_index;
+
+	if (ops->newlink) {
+		err = ops->newlink(link_net ? : net, dev, tb, data, extack);
+		/* Drivers should call free_netdev() in ->destructor
+		 * and unregister it on failure after registration
+		 * so that device could be finally freed in rtnl_unlock.
+		 */
+		if (err < 0) {
+			/* If device is not registered at all, free it now */
+			if (dev->reg_state == NETREG_UNINITIALIZED)
 				free_netdev(dev);
-				goto out;
-			}
+			goto out;
+		}
+	} else {
+		err = register_netdevice(dev);
+		if (err < 0) {
+			free_netdev(dev);
+			goto out;
 		}
-		err = rtnl_configure_link(dev, ifm);
+	}
+	err = rtnl_configure_link(dev, ifm);
+	if (err < 0)
+		goto out_unregister;
+	if (link_net) {
+		err = dev_change_net_namespace(dev, dest_net, ifname);
 		if (err < 0)
 			goto out_unregister;
-		if (link_net) {
-			err = dev_change_net_namespace(dev, dest_net, ifname);
-			if (err < 0)
-				goto out_unregister;
-		}
-		if (tb[IFLA_MASTER]) {
-			err = do_set_master(dev, nla_get_u32(tb[IFLA_MASTER]),
-					    extack);
-			if (err)
-				goto out_unregister;
-		}
+	}
+	if (tb[IFLA_MASTER]) {
+		err = do_set_master(dev, nla_get_u32(tb[IFLA_MASTER]), extack);
+		if (err)
+			goto out_unregister;
+	}
 out:
-		if (link_net)
-			put_net(link_net);
-		put_net(dest_net);
-		return err;
+	if (link_net)
+		put_net(link_net);
+	put_net(dest_net);
+	return err;
 out_unregister:
-		if (ops->newlink) {
-			LIST_HEAD(list_kill);
+	if (ops->newlink) {
+		LIST_HEAD(list_kill);
 
-			ops->dellink(dev, &list_kill);
-			unregister_netdevice_many(&list_kill);
-		} else {
-			unregister_netdevice(dev);
-		}
-		goto out;
+		ops->dellink(dev, &list_kill);
+		unregister_netdevice_many(&list_kill);
+	} else {
+		unregister_netdevice(dev);
 	}
+	goto out;
 }
 
 static int rtnl_getlink(struct sk_buff *skb, struct nlmsghdr *nlh,
-- 
2.17.1

[PATCH net-next 2/2] rtnetlink: avoid frame size warning in rtnl_newlink()

[Jakub Kicinski]

Standard kernel compilation produces the following warning:

net/core/rtnetlink.c: In function ‘rtnl_newlink’:
net/core/rtnetlink.c:3232:1: warning: the frame size of 1288 bytes is larger than 1024 bytes [-Wframe-larger-than=]
 }
  ^

This should not really be an issue, as rtnl_newlink() stack is
generally quite shallow.

Fix the warning by allocating attributes with kmalloc() in a wrapper
and passing it down to rtnl_newlink(), avoiding complexities on error
paths.

Alternatively we could kmalloc() some structure within rtnl_newlink(),
slave attributes look like a good candidate.  In practice it adds to
already rather high complexity and length of the function.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
---
 net/core/rtnetlink.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 8cbdc8398b8c..1699f8540d58 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -2971,14 +2971,13 @@ static int rtnl_group_changelink(const struct sk_buff *skb,
 	return 0;
 }
 
-static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
-			struct netlink_ext_ack *extack)
+static int __rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
+			  struct nlattr **attr, struct netlink_ext_ack *extack)
 {
 	struct nlattr *slave_attr[RTNL_SLAVE_MAX_TYPE + 1];
 	unsigned char name_assign_type = NET_NAME_USER;
 	struct nlattr *linkinfo[IFLA_INFO_MAX + 1];
 	const struct rtnl_link_ops *m_ops = NULL;
-	struct nlattr *attr[RTNL_MAX_TYPE + 1];
 	struct net_device *master_dev = NULL;
 	struct net *net = sock_net(skb->sk);
 	const struct rtnl_link_ops *ops;
@@ -3226,6 +3225,21 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 	goto out;
 }
 
+static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
+			struct netlink_ext_ack *extack)
+{
+	struct nlattr **attr;
+	int ret;
+
+	attr = kmalloc_array(RTNL_MAX_TYPE + 1, sizeof(*attr), GFP_KERNEL);
+	if (!attr)
+		return -ENOMEM;
+
+	ret = __rtnl_newlink(skb, nlh, attr, extack);
+	kfree(attr);
+	return ret;
+}
+
 static int rtnl_getlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 			struct netlink_ext_ack *extack)
 {
-- 
2.17.1

Re: [PATCH net-next 0/2] rtnetlink: avoid a warning in rtnl_newlink()

[David Ahern]

On 11/27/18 11:32 PM, Jakub Kicinski wrote:
> Hi!
> 
> I've been hoping for some time that someone more competent would fix
> the stack frame size warning in rtnl_newlink(), but looks like I'll
> have to take a stab at it myself :)  That's the only warning I see
> in most of my builds.

Somehow my CONFIG_FRAME_WARN got set to 2048 in all of my config files,
so I don't see the warning.

> 
> First patch refactors away a somewhat surprising if (1) code block.
> Reindentation will most likely cause cherry-pick problems but OTOH
> rtnl_newlink() doesn't seem to be changed often, so perhaps we can
> risk it in the name of cleaner code?

The unnecessary indentation with the if(1) has always annoyed me. I like
the cleanup, but strictly speaking if Dave objects patch 2 can be done
without it.

> 
> Second patch fixes the warning in simplest possible way.  I was
> pondering if there is any more clever solution, but I can't see it..
> rtnl_newlink() is quite long with a lot of possible execution paths
> so doing memory allocations half way through leads to very ugly
> results.

Seems like a reasonable first step and in time slave_attr can follow
suit. Those are the 2 high runners for stack usage.

Re: [PATCH net-next 1/2] rtnetlink: remove a level of indentation in rtnl_newlink()

[David Ahern]

On 11/27/18 11:32 PM, Jakub Kicinski wrote:
> rtnl_newlink() used to create VLAs based on link kind.  Since
> commit ccf8dbcd062a ("rtnetlink: Remove VLA usage") statically
> sized array is created on the stack, so there is no more use
> for a separate code block that used to be the VLA's live range.
> 
> While at it christmas tree the variables.  Note that there is
> a goto-based retry so to be on the safe side the variables can
> no longer be initialized in place.  It doesn't seem to matter,
> logically, but why make the code harder to read..
> 
> Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
> ---
>  net/core/rtnetlink.c | 313 +++++++++++++++++++++----------------------
>  1 file changed, 154 insertions(+), 159 deletions(-)

Reviewed-by: David Ahern <dsahern@gmail.com>

Re: [PATCH net-next 2/2] rtnetlink: avoid frame size warning in rtnl_newlink()

[David Ahern]

On 11/27/18 11:32 PM, Jakub Kicinski wrote:
> Standard kernel compilation produces the following warning:
> 
> net/core/rtnetlink.c: In function ‘rtnl_newlink’:
> net/core/rtnetlink.c:3232:1: warning: the frame size of 1288 bytes is larger than 1024 bytes [-Wframe-larger-than=]
>  }
>   ^
> 
> This should not really be an issue, as rtnl_newlink() stack is
> generally quite shallow.
> 
> Fix the warning by allocating attributes with kmalloc() in a wrapper
> and passing it down to rtnl_newlink(), avoiding complexities on error
> paths.
> 
> Alternatively we could kmalloc() some structure within rtnl_newlink(),
> slave attributes look like a good candidate.  In practice it adds to
> already rather high complexity and length of the function.
> 
> Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
> ---
>  net/core/rtnetlink.c | 20 +++++++++++++++++---
>  1 file changed, 17 insertions(+), 3 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@gmail.com>

Re: [PATCH net-next 0/2] rtnetlink: avoid a warning in rtnl_newlink()

[David Miller]

From: Jakub Kicinski <jakub.kicinski@netronome.com>
Date: Tue, 27 Nov 2018 22:32:29 -0800

> I've been hoping for some time that someone more competent would fix
> the stack frame size warning in rtnl_newlink(), but looks like I'll
> have to take a stab at it myself :)  That's the only warning I see
> in most of my builds.
> 
> First patch refactors away a somewhat surprising if (1) code block.
> Reindentation will most likely cause cherry-pick problems but OTOH
> rtnl_newlink() doesn't seem to be changed often, so perhaps we can
> risk it in the name of cleaner code?
> 
> Second patch fixes the warning in simplest possible way.  I was
> pondering if there is any more clever solution, but I can't see it..
> rtnl_newlink() is quite long with a lot of possible execution paths
> so doing memory allocations half way through leads to very ugly
> results.

Series applied, thanks for tackling this Jakub.

That whole "if (1) {" was probably just a construct used in order
to create an inner basic block for local variables, nothing more.