Subject: Re: [PATCH bpf-next V4 2/5] bpf: bpf_fib_lookup return MTU value as output when looked up

From: Dan Carpenter <[email-masked]>
Date: Wed, 28 Oct 2020 15:49:42 +0300

Hi Jesper,

url:    https://github.com/0day-ci/linux/commits/Jesper-Dangaard-Brouer/bpf-New-approach-for-BPF-MTU-handling/20201028-002919
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git master
config: i386-randconfig-m021-20201026 (attached as .config)
compiler: gcc-9 (Debian 9.3.0-15) 9.3.0

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <[email-masked]>
Reported-by: Dan Carpenter <[email-masked]>

New smatch warnings:
net/core/filter.c:5395 bpf_ipv4_fib_lookup() error: uninitialized symbol 'mtu'.

vim +/mtu +5395 net/core/filter.c

87f5fc7e48dd317 David Ahern            2018-05-09  5281  static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
4f74fede40df8db David Ahern            2018-05-21  5282  			       u32 flags, bool check_mtu)
87f5fc7e48dd317 David Ahern            2018-05-09  5283  {
eba618abacade71 David Ahern            2019-04-02  5284  	struct fib_nh_common *nhc;
87f5fc7e48dd317 David Ahern            2018-05-09  5285  	struct in_device *in_dev;
87f5fc7e48dd317 David Ahern            2018-05-09  5286  	struct neighbour *neigh;
87f5fc7e48dd317 David Ahern            2018-05-09  5287  	struct net_device *dev;
87f5fc7e48dd317 David Ahern            2018-05-09  5288  	struct fib_result res;
87f5fc7e48dd317 David Ahern            2018-05-09  5289  	struct flowi4 fl4;
87f5fc7e48dd317 David Ahern            2018-05-09  5290  	int err;
4f74fede40df8db David Ahern            2018-05-21  5291  	u32 mtu;
                                                                ^^^^^^^^

87f5fc7e48dd317 David Ahern            2018-05-09  5292  
87f5fc7e48dd317 David Ahern            2018-05-09  5293  	dev = dev_get_by_index_rcu(net, params->ifindex);
87f5fc7e48dd317 David Ahern            2018-05-09  5294  	if (unlikely(!dev))
87f5fc7e48dd317 David Ahern            2018-05-09  5295  		return -ENODEV;
87f5fc7e48dd317 David Ahern            2018-05-09  5296  
87f5fc7e48dd317 David Ahern            2018-05-09  5297  	/* verify forwarding is enabled on this interface */
87f5fc7e48dd317 David Ahern            2018-05-09  5298  	in_dev = __in_dev_get_rcu(dev);
87f5fc7e48dd317 David Ahern            2018-05-09  5299  	if (unlikely(!in_dev || !IN_DEV_FORWARD(in_dev)))
4c79579b44b1876 David Ahern            2018-06-26  5300  		return BPF_FIB_LKUP_RET_FWD_DISABLED;
87f5fc7e48dd317 David Ahern            2018-05-09  5301  
87f5fc7e48dd317 David Ahern            2018-05-09  5302  	if (flags & BPF_FIB_LOOKUP_OUTPUT) {
87f5fc7e48dd317 David Ahern            2018-05-09  5303  		fl4.flowi4_iif = 1;
87f5fc7e48dd317 David Ahern            2018-05-09  5304  		fl4.flowi4_oif = params->ifindex;
87f5fc7e48dd317 David Ahern            2018-05-09  5305  	} else {
87f5fc7e48dd317 David Ahern            2018-05-09  5306  		fl4.flowi4_iif = params->ifindex;
87f5fc7e48dd317 David Ahern            2018-05-09  5307  		fl4.flowi4_oif = 0;
87f5fc7e48dd317 David Ahern            2018-05-09  5308  	}
87f5fc7e48dd317 David Ahern            2018-05-09  5309  	fl4.flowi4_tos = params->tos & IPTOS_RT_MASK;
87f5fc7e48dd317 David Ahern            2018-05-09  5310  	fl4.flowi4_scope = RT_SCOPE_UNIVERSE;
87f5fc7e48dd317 David Ahern            2018-05-09  5311  	fl4.flowi4_flags = 0;
87f5fc7e48dd317 David Ahern            2018-05-09  5312  
87f5fc7e48dd317 David Ahern            2018-05-09  5313  	fl4.flowi4_proto = params->l4_protocol;
87f5fc7e48dd317 David Ahern            2018-05-09  5314  	fl4.daddr = params->ipv4_dst;
87f5fc7e48dd317 David Ahern            2018-05-09  5315  	fl4.saddr = params->ipv4_src;
87f5fc7e48dd317 David Ahern            2018-05-09  5316  	fl4.fl4_sport = params->sport;
87f5fc7e48dd317 David Ahern            2018-05-09  5317  	fl4.fl4_dport = params->dport;
1869e226a7b3ef7 David Ahern            2020-09-13  5318  	fl4.flowi4_multipath_hash = 0;
87f5fc7e48dd317 David Ahern            2018-05-09  5319  
87f5fc7e48dd317 David Ahern            2018-05-09  5320  	if (flags & BPF_FIB_LOOKUP_DIRECT) {
87f5fc7e48dd317 David Ahern            2018-05-09  5321  		u32 tbid = l3mdev_fib_table_rcu(dev) ? : RT_TABLE_MAIN;
87f5fc7e48dd317 David Ahern            2018-05-09  5322  		struct fib_table *tb;
87f5fc7e48dd317 David Ahern            2018-05-09  5323  
87f5fc7e48dd317 David Ahern            2018-05-09  5324  		tb = fib_get_table(net, tbid);
87f5fc7e48dd317 David Ahern            2018-05-09  5325  		if (unlikely(!tb))
4c79579b44b1876 David Ahern            2018-06-26  5326  			return BPF_FIB_LKUP_RET_NOT_FWDED;
87f5fc7e48dd317 David Ahern            2018-05-09  5327  
87f5fc7e48dd317 David Ahern            2018-05-09  5328  		err = fib_table_lookup(tb, &fl4, &res, FIB_LOOKUP_NOREF);
87f5fc7e48dd317 David Ahern            2018-05-09  5329  	} else {
87f5fc7e48dd317 David Ahern            2018-05-09  5330  		fl4.flowi4_mark = 0;
87f5fc7e48dd317 David Ahern            2018-05-09  5331  		fl4.flowi4_secid = 0;
87f5fc7e48dd317 David Ahern            2018-05-09  5332  		fl4.flowi4_tun_key.tun_id = 0;
87f5fc7e48dd317 David Ahern            2018-05-09  5333  		fl4.flowi4_uid = sock_net_uid(net, NULL);
87f5fc7e48dd317 David Ahern            2018-05-09  5334  
87f5fc7e48dd317 David Ahern            2018-05-09  5335  		err = fib_lookup(net, &fl4, &res, FIB_LOOKUP_NOREF);
87f5fc7e48dd317 David Ahern            2018-05-09  5336  	}
87f5fc7e48dd317 David Ahern            2018-05-09  5337  
4c79579b44b1876 David Ahern            2018-06-26  5338  	if (err) {
4c79579b44b1876 David Ahern            2018-06-26  5339  		/* map fib lookup errors to RTN_ type */
4c79579b44b1876 David Ahern            2018-06-26  5340  		if (err == -EINVAL)
4c79579b44b1876 David Ahern            2018-06-26  5341  			return BPF_FIB_LKUP_RET_BLACKHOLE;
4c79579b44b1876 David Ahern            2018-06-26  5342  		if (err == -EHOSTUNREACH)
4c79579b44b1876 David Ahern            2018-06-26  5343  			return BPF_FIB_LKUP_RET_UNREACHABLE;
4c79579b44b1876 David Ahern            2018-06-26  5344  		if (err == -EACCES)
4c79579b44b1876 David Ahern            2018-06-26  5345  			return BPF_FIB_LKUP_RET_PROHIBIT;
4c79579b44b1876 David Ahern            2018-06-26  5346  
4c79579b44b1876 David Ahern            2018-06-26  5347  		return BPF_FIB_LKUP_RET_NOT_FWDED;
4c79579b44b1876 David Ahern            2018-06-26  5348  	}
4c79579b44b1876 David Ahern            2018-06-26  5349  
4c79579b44b1876 David Ahern            2018-06-26  5350  	if (res.type != RTN_UNICAST)
4c79579b44b1876 David Ahern            2018-06-26  5351  		return BPF_FIB_LKUP_RET_NOT_FWDED;
87f5fc7e48dd317 David Ahern            2018-05-09  5352  
5481d73f81549e2 David Ahern            2019-06-03  5353  	if (fib_info_num_path(res.fi) > 1)
87f5fc7e48dd317 David Ahern            2018-05-09  5354  		fib_select_path(net, &res, &fl4, NULL);
87f5fc7e48dd317 David Ahern            2018-05-09  5355  
4f74fede40df8db David Ahern            2018-05-21  5356  	if (check_mtu) {
4f74fede40df8db David Ahern            2018-05-21  5357  		mtu = ip_mtu_from_fib_result(&res, params->ipv4_dst);
88ffc2c2e37ebb3 Jesper Dangaard Brouer 2020-10-27  5358  		if (params->tot_len > mtu) {
88ffc2c2e37ebb3 Jesper Dangaard Brouer 2020-10-27  5359  			params->mtu = mtu; /* union with tot_len */
4c79579b44b1876 David Ahern            2018-06-26  5360  			return BPF_FIB_LKUP_RET_FRAG_NEEDED;
4f74fede40df8db David Ahern            2018-05-21  5361  		}
88ffc2c2e37ebb3 Jesper Dangaard Brouer 2020-10-27  5362  	}

"mtu" not initialized on else path.

4f74fede40df8db David Ahern            2018-05-21  5363  
eba618abacade71 David Ahern            2019-04-02  5364  	nhc = res.nhc;
87f5fc7e48dd317 David Ahern            2018-05-09  5365  
87f5fc7e48dd317 David Ahern            2018-05-09  5366  	/* do not handle lwt encaps right now */
eba618abacade71 David Ahern            2019-04-02  5367  	if (nhc->nhc_lwtstate)
4c79579b44b1876 David Ahern            2018-06-26  5368  		return BPF_FIB_LKUP_RET_UNSUPP_LWT;
87f5fc7e48dd317 David Ahern            2018-05-09  5369  
eba618abacade71 David Ahern            2019-04-02  5370  	dev = nhc->nhc_dev;
87f5fc7e48dd317 David Ahern            2018-05-09  5371  
87f5fc7e48dd317 David Ahern            2018-05-09  5372  	params->rt_metric = res.fi->fib_priority;
d1c362e1dd68a42 Toke Høiland-Jørgensen 2020-10-09  5373  	params->ifindex = dev->ifindex;
87f5fc7e48dd317 David Ahern            2018-05-09  5374  
87f5fc7e48dd317 David Ahern            2018-05-09  5375  	/* xdp and cls_bpf programs are run in RCU-bh so
87f5fc7e48dd317 David Ahern            2018-05-09  5376  	 * rcu_read_lock_bh is not needed here
87f5fc7e48dd317 David Ahern            2018-05-09  5377  	 */
6f5f68d05ec0f64 David Ahern            2019-04-05  5378  	if (likely(nhc->nhc_gw_family != AF_INET6)) {
6f5f68d05ec0f64 David Ahern            2019-04-05  5379  		if (nhc->nhc_gw_family)
6f5f68d05ec0f64 David Ahern            2019-04-05  5380  			params->ipv4_dst = nhc->nhc_gw.ipv4;
6f5f68d05ec0f64 David Ahern            2019-04-05  5381  
6f5f68d05ec0f64 David Ahern            2019-04-05  5382  		neigh = __ipv4_neigh_lookup_noref(dev,
6f5f68d05ec0f64 David Ahern            2019-04-05  5383  						 (__force u32)params->ipv4_dst);
6f5f68d05ec0f64 David Ahern            2019-04-05  5384  	} else {
6f5f68d05ec0f64 David Ahern            2019-04-05  5385  		struct in6_addr *dst = (struct in6_addr *)params->ipv6_dst;
6f5f68d05ec0f64 David Ahern            2019-04-05  5386  
6f5f68d05ec0f64 David Ahern            2019-04-05  5387  		params->family = AF_INET6;
6f5f68d05ec0f64 David Ahern            2019-04-05  5388  		*dst = nhc->nhc_gw.ipv6;
6f5f68d05ec0f64 David Ahern            2019-04-05  5389  		neigh = __ipv6_neigh_lookup_noref_stub(dev, dst);
6f5f68d05ec0f64 David Ahern            2019-04-05  5390  	}
6f5f68d05ec0f64 David Ahern            2019-04-05  5391  
4c79579b44b1876 David Ahern            2018-06-26  5392  	if (!neigh)
4c79579b44b1876 David Ahern            2018-06-26  5393  		return BPF_FIB_LKUP_RET_NO_NEIGH;
87f5fc7e48dd317 David Ahern            2018-05-09  5394  
88ffc2c2e37ebb3 Jesper Dangaard Brouer 2020-10-27 @5395  	return bpf_fib_set_fwd_params(params, neigh, dev, mtu);
                                                                                                                  ^^^
Uninitialized variable warning.

87f5fc7e48dd317 David Ahern            2018-05-09  5396  }

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