linux: add loopback carrier for operstate up

[troglobit]

This is a proposal to fix the issue of loopback having operstate UNKNOWN, which otherwise propagates over NETCONF as well.

See separate discussion thread in Projects.

[wkz]

Supplying a known operstate when we bring up lo should be sufficient to not have to carry this kernel patch.

I.e. doing the equivalent of ip link set dev lo state up accomplishes the same thing, me thinks. NOTE: This is (confusingly) not the same thing as ip link set dev lo up. The former sets IFF_UP in the device flags, while the latter sends an updated IFLA_OPERSTATE.

[troglobit]

Awesome! And yeah, you're right of course, just verified locally. Unsurprisingly this command isn't documented in ip link help nor anywhere in the manpage ip-link(8), which explains why I didn't find it while looking.

So something like this, right?

diff --git a/src/confd/src/confd/ietf-interfaces.c b/src/confd/src/confd/ietf-interfaces.c
index 0cc1bff..c29e54d 100644
--- a/src/confd/src/confd/ietf-interfaces.c
+++ b/src/confd/src/confd/ietf-interfaces.c
@@ -1020,8 +1020,12 @@ static sr_error_t netdag_gen_iface(struct dagger *net,
 
        /* Bring interface back up, if enabled */
        attr = lydx_get_cattr(cif, "enabled");
-       if (!attr || !strcmp(attr, "true"))
+       if (!attr || !strcmp(attr, "true")) {
                fprintf(ip, "link set dev %s up\n", ifname);
+               /* Ensure loopback has operstate UP, not UNKNOWN */
+               if (!strcmp(ifname, "lo"))
+                       fprintf(ip, "link set dev %s state up\n", ifname);
+       }
 
        err = err ? : netdag_gen_sysctl(net, dif);

[wkz]

I think we can even get away with just this:

diff --git a/src/confd/src/confd/ietf-interfaces.c b/src/confd/src/confd/ietf-interfaces.c
index 7299ede..142ae3c 100644
--- a/src/confd/src/confd/ietf-interfaces.c
+++ b/src/confd/src/confd/ietf-interfaces.c
@@ -1021,7 +1021,7 @@ static sr_error_t netdag_gen_iface(struct dagger *net,
        /* Bring interface back up, if enabled */
        attr = lydx_get_cattr(cif, "enabled");
        if (!attr || !strcmp(attr, "true"))
-               fprintf(ip, "link set dev %s up\n", ifname);
+               fprintf(ip, "link set dev %s up state up\n", ifname);

        err = err ? : netdag_gen_sysctl(net, dif);

This would ensure that we get the same behavior for other kinds of virtual interfaces (dummy interfaces, tunnels, etc.). When tried on a physical interface, the attribute has no effect.

[troglobit]

Great stuff, lgtm 👍