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net: AF_NETLINK, so getifaddrs() works for everything and not just one program

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fastfetch reported "getifaddrs(&ifAddrStruct) failed", and the right answer to
that is not a patch to fastfetch. Asking what interfaces a machine has is
something every networked program does, and on Linux there is exactly one way:
a NETLINK_ROUTE socket. musl implements getifaddrs() over it, so a libc call
that looks like a syscall is really a small protocol conversation -- and without
this side of it every caller fails the same way.

kernel/net/netlink.c is a responder rather than a general socket family. No
event stream, no multicast groups, nothing subscribes to link state: a request
arrives, the whole answer is generated into the socket's buffer, and reads drain
it. That shape is chosen, not a shortcut -- getifaddrs sends a dump and reads
until NLMSG_DONE with MSG_DONTWAIT, so generating at send time means a read
never blocks and the non-blocking flag is honoured for free.

RTM_GETLINK returns one RTM_NEWLINK per interface with IFLA_IFNAME and
IFLA_ADDRESS; RTM_GETADDR returns one RTM_NEWADDR per address with IFA_ADDRESS,
IFA_LOCAL and IFA_LABEL. Anything else gets a bare NLMSG_DONE, because the
caller's next move is a read either way and an unexpected error message is worse
than nothing to read.

The data is not netlink's. Interfaces come from the netdev class and addresses
from lwIP's netifs, exactly as /proc/netif reads them; nothing in the file knows
what an e1000 is. Interface indices matter more than they look -- getifaddrs
matches address records to link records by index, so the two dumps have to
agree. Loopback is 1, netdevs follow from 2.

Verified by reading it back through musl rather than by inspecting the bytes:

    lo     flags=0x49    fam=-1
    eth0   flags=0x1043  fam=17                    (AF_PACKET, the MAC)
    lo     flags=0x49    fam=2  addr=127.0.0.1  mask=255.0.0.0
    eth0   flags=0x1043  fam=2  addr=10.0.2.15  mask=255.255.255.0

Correct flags, addresses and netmasks, and the link records pair with the
address records.

One thing this does not close: fastfetch's Local IP module defaults to
default-route-only, which calls ffNetifGetDefaultRouteIfName() and reads
/proc/net/route. SchOS has no routing table and no such file, so the module
still shows nothing -- now for a reason that is about routing rather than about
enumeration. That is the next file to provide.

socktest still passes; the AF_UNIX and AF_INET paths are untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
This commit is contained in:
Nexoniarz 2026-09-12 19:23:34 +02:00
commit f395c39704
9 changed files with 462 additions and 6 deletions
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81
include/socket.h
View file

@ -18,9 +18,16 @@
#define AF_UNIX 1
#define AF_LOCAL 1
#define AF_INET 2
// Linux's number, because a musl program passes Linux's constant. getifaddrs()
// is implemented over this, which is why it exists here: enumerating interfaces
// is something every networked program does and nothing else offers.
#define AF_NETLINK 16
#define PF_NETLINK AF_NETLINK
#define NETLINK_ROUTE 0
#define SOCK_STREAM 1
#define SOCK_DGRAM 2
#define SOCK_RAW 3
// Flags ORed into socket()'s and accept4()'s `type`/`flags` argument. Linux's
// values, because a musl program passes Linux's constants.
@ -77,6 +84,80 @@ struct sockaddr_in {
char sin_zero[8];
};
// ---------------------------------------------------------------------------
// rtnetlink, enough of it for getifaddrs(3)
//
// Every structure below is Linux's, byte for byte, because musl parses what it
// is handed with Linux's own headers. They are not ours to design.
//
// What musl actually does (src/network/netlink.c) is the whole specification
// this has to meet: open PF_NETLINK/SOCK_RAW/NETLINK_ROUTE, send an
// nlmsghdr+rtgenmsg asking RTM_GETLINK with NLM_F_DUMP, read replies until
// NLMSG_DONE, then do it again for RTM_GETADDR on the same socket.
// ---------------------------------------------------------------------------
struct nlmsghdr {
uint32 nlmsg_len;
uint16 nlmsg_type;
uint16 nlmsg_flags;
uint32 nlmsg_seq;
uint32 nlmsg_pid;
};
struct rtgenmsg { uchar rtgen_family; };
struct ifinfomsg {
uchar ifi_family;
uchar __ifi_pad;
uint16 ifi_type; // ARPHRD_*
int32 ifi_index;
uint32 ifi_flags; // IFF_*
uint32 ifi_change;
};
struct ifaddrmsg {
uchar ifa_family;
uchar ifa_prefixlen;
uchar ifa_flags;
uchar ifa_scope;
uint32 ifa_index;
};
struct rtattr {
uint16 rta_len;
uint16 rta_type;
};
#define NLMSG_ERROR 2
#define NLMSG_DONE 3
#define NLM_F_REQUEST 0x001
#define NLM_F_MULTI 0x002
#define NLM_F_DUMP 0x300
#define RTM_NEWLINK 16
#define RTM_GETLINK 18
#define RTM_NEWADDR 20
#define RTM_GETADDR 22
#define IFLA_ADDRESS 1
#define IFLA_IFNAME 3
#define IFA_ADDRESS 1
#define IFA_LOCAL 2
#define IFA_LABEL 3
#define ARPHRD_ETHER 1
#define ARPHRD_LOOPBACK 772
#define IFF_UP 0x1
#define IFF_LOOPBACK 0x8
#define IFF_RUNNING 0x40
#define IFF_BROADCAST 0x2
#define IFF_MULTICAST 0x1000
// Both are four-byte aligned on every architecture Linux supports.
#define NLMSG_ALIGN(n) (((n) + 3) & ~3)
#define RTA_ALIGN(n) (((n) + 3) & ~3)
#ifdef _KERNEL_SOCKET_INTERNALS
#define NSOCK 32 // sockets in the system