lxc_user_nic: fix get_mtu() error handling

[mihalicyn]

get_mtu() returns int, but "mtu" variable has unsigned int type. It leads to logical error in error handling, which can end up with strange -EINVAL error in lxc_veth_create(), cause (mtu > 0) condition is met, but negative "mtu" value is too large when set as mtu for network device.

Issue #4232

Signed-off-by: Alexander Mikhalitsyn aleksandr.mikhalitsyn@canonical.com

[brauner]

Hm, maybe moving all of this into get_mtu() is nicer?

diff --git a/src/lxc/cmd/lxc_user_nic.c b/src/lxc/cmd/lxc_user_nic.c
index a91e2259d..3a015779f 100644
--- a/src/lxc/cmd/lxc_user_nic.c
+++ b/src/lxc/cmd/lxc_user_nic.c
@@ -489,20 +489,26 @@ static int instantiate_veth(char *veth1, char *veth2, pid_t pid, unsigned int mt
        return netdev_set_flag(veth1, IFF_UP);
 }

-static int get_mtu(char *name)
+#define NETDEV_MTU_DEFAULT 1500
+
+static unsigned int get_mtu(const char *name)
 {
-       int idx;
+       int idx, val;

        idx = if_nametoindex(name);
        if (idx < 0)
-               return -1;
+               return NETDEV_MTU_DEFAULT;
+
+       val = netdev_get_mtu(idx);
+       if (val < 0)
+               return NETDEV_MTU_DEFAULT;

-       return netdev_get_mtu(idx);
+       return val;
 }

 static int create_nic(char *nic, char *br, int pid, char **cnic)
 {
-       unsigned int mtu = 1500;
+       unsigned int mtu = NETDEV_MTU_DEFAULT;
        int ret;
        char veth1buf[IFNAMSIZ], veth2buf[IFNAMSIZ];

@@ -520,8 +526,6 @@ static int create_nic(char *nic, char *br, int pid, char **cnic)

        if (strcmp(br, "none"))
                mtu = get_mtu(br);
-       if (!mtu)
-               mtu = 1500;

        /* create the nics */
        ret = instantiate_veth(veth1buf, veth2buf, pid, mtu);

[lxc-jenkins]

Testsuite passed

[mihalicyn]

Thanks, Christian! Let's take this way :)

[mihalicyn]

Have done. I've also added error messages for convenience.

[lxc-jenkins]

Testsuite passed