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Date:   Tue, 5 May 2020 19:59:55 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Martin Varghese <martin.varghese@...ia.com>,
        Taehee Yoo <ap420073@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] net: bareudp: avoid uninitialized variable warning

On Tue, May 05, 2020 at 07:22:14PM +0200, Arnd Bergmann wrote:
> clang points out that building without IPv6 would lead to returning
> an uninitialized variable if a packet with family!=AF_INET is
> passed into bareudp_udp_encap_recv():
> 
> drivers/net/bareudp.c:139:6: error: variable 'err' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
>         if (family == AF_INET)
>             ^~~~~~~~~~~~~~~~~
> drivers/net/bareudp.c:146:15: note: uninitialized use occurs here
>         if (unlikely(err)) {
>                      ^~~
> include/linux/compiler.h:78:42: note: expanded from macro 'unlikely'
>  # define unlikely(x)    __builtin_expect(!!(x), 0)
>                                             ^
> drivers/net/bareudp.c:139:2: note: remove the 'if' if its condition is always true
>         if (family == AF_INET)
>         ^~~~~~~~~~~~~~~~~~~~~~
> 
> This cannot happen in practice, so change the condition in a way that
> gcc sees the IPv4 case as unconditionally true here.
> For consistency, change all the similar constructs in this file the
> same way, using "if(IS_ENABLED())" instead of #if IS_ENABLED()".
> 
> Fixes: 571912c69f0e ("net: UDP tunnel encapsulation module for tunnelling different protocols like MPLS, IP, NSH etc.")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

> ---
>  drivers/net/bareudp.c    | 18 ++++--------------
>  include/net/udp_tunnel.h |  2 --
>  2 files changed, 4 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/net/bareudp.c b/drivers/net/bareudp.c
> index cc0703c3d57f..efd1a1d1f35e 100644
> --- a/drivers/net/bareudp.c
> +++ b/drivers/net/bareudp.c
> @@ -136,25 +136,21 @@ static int bareudp_udp_encap_recv(struct sock *sk, struct sk_buff *skb)
>  	oiph = skb_network_header(skb);
>  	skb_reset_network_header(skb);
>  
> -	if (family == AF_INET)
> +	if (!IS_ENABLED(CONFIG_IPV6) || family == AF_INET)
>  		err = IP_ECN_decapsulate(oiph, skb);
> -#if IS_ENABLED(CONFIG_IPV6)
>  	else
>  		err = IP6_ECN_decapsulate(oiph, skb);
> -#endif
>  
>  	if (unlikely(err)) {
>  		if (log_ecn_error) {
> -			if  (family == AF_INET)
> +			if  (!IS_ENABLED(CONFIG_IPV6) || family == AF_INET)
>  				net_info_ratelimited("non-ECT from %pI4 "
>  						     "with TOS=%#x\n",
>  						     &((struct iphdr *)oiph)->saddr,
>  						     ((struct iphdr *)oiph)->tos);
> -#if IS_ENABLED(CONFIG_IPV6)
>  			else
>  				net_info_ratelimited("non-ECT from %pI6\n",
>  						     &((struct ipv6hdr *)oiph)->saddr);
> -#endif
>  		}
>  		if (err > 1) {
>  			++bareudp->dev->stats.rx_frame_errors;
> @@ -350,7 +346,6 @@ static int bareudp_xmit_skb(struct sk_buff *skb, struct net_device *dev,
>  	return err;
>  }
>  
> -#if IS_ENABLED(CONFIG_IPV6)
>  static int bareudp6_xmit_skb(struct sk_buff *skb, struct net_device *dev,
>  			     struct bareudp_dev *bareudp,
>  			     const struct ip_tunnel_info *info)
> @@ -411,7 +406,6 @@ static int bareudp6_xmit_skb(struct sk_buff *skb, struct net_device *dev,
>  	dst_release(dst);
>  	return err;
>  }
> -#endif
>  
>  static netdev_tx_t bareudp_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
> @@ -435,11 +429,9 @@ static netdev_tx_t bareudp_xmit(struct sk_buff *skb, struct net_device *dev)
>  	}
>  
>  	rcu_read_lock();
> -#if IS_ENABLED(CONFIG_IPV6)
> -	if (info->mode & IP_TUNNEL_INFO_IPV6)
> +	if (IS_ENABLED(CONFIG_IPV6) && info->mode & IP_TUNNEL_INFO_IPV6)
>  		err = bareudp6_xmit_skb(skb, dev, bareudp, info);
>  	else
> -#endif
>  		err = bareudp_xmit_skb(skb, dev, bareudp, info);
>  
>  	rcu_read_unlock();
> @@ -467,7 +459,7 @@ static int bareudp_fill_metadata_dst(struct net_device *dev,
>  
>  	use_cache = ip_tunnel_dst_cache_usable(skb, info);
>  
> -	if (ip_tunnel_info_af(info) == AF_INET) {
> +	if (!IS_ENABLED(CONFIG_IPV6) || ip_tunnel_info_af(info) == AF_INET) {
>  		struct rtable *rt;
>  		__be32 saddr;
>  
> @@ -478,7 +470,6 @@ static int bareudp_fill_metadata_dst(struct net_device *dev,
>  
>  		ip_rt_put(rt);
>  		info->key.u.ipv4.src = saddr;
> -#if IS_ENABLED(CONFIG_IPV6)
>  	} else if (ip_tunnel_info_af(info) == AF_INET6) {
>  		struct dst_entry *dst;
>  		struct in6_addr saddr;
> @@ -492,7 +483,6 @@ static int bareudp_fill_metadata_dst(struct net_device *dev,
>  
>  		dst_release(dst);
>  		info->key.u.ipv6.src = saddr;
> -#endif
>  	} else {
>  		return -EINVAL;
>  	}
> diff --git a/include/net/udp_tunnel.h b/include/net/udp_tunnel.h
> index 4b1f95e08307..e7312ceb2794 100644
> --- a/include/net/udp_tunnel.h
> +++ b/include/net/udp_tunnel.h
> @@ -143,14 +143,12 @@ void udp_tunnel_xmit_skb(struct rtable *rt, struct sock *sk, struct sk_buff *skb
>  			 __be16 df, __be16 src_port, __be16 dst_port,
>  			 bool xnet, bool nocheck);
>  
> -#if IS_ENABLED(CONFIG_IPV6)
>  int udp_tunnel6_xmit_skb(struct dst_entry *dst, struct sock *sk,
>  			 struct sk_buff *skb,
>  			 struct net_device *dev, struct in6_addr *saddr,
>  			 struct in6_addr *daddr,
>  			 __u8 prio, __u8 ttl, __be32 label,
>  			 __be16 src_port, __be16 dst_port, bool nocheck);
> -#endif
>  
>  void udp_tunnel_sock_release(struct socket *sock);
>  
> -- 
> 2.26.0
> 

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