linux-user: `Setsockopt` with IP_OPTIONS returns "Protocol not available" error

Host environment

• Operating system: Ubuntu 22.04.3 LTS
• OS/kernel version: Linux 5.15.0-101-generic
• Architecture: x86_64
• QEMU flavor: qemu-riscv64-static
• QEMU version: 6.2.0
• QEMU command line: qemu-riscv64-static ./setsockopt

Emulated/Virtualized environment

• Operating system: Ubuntu 22.04.3 LTS
• OS/kernel version: (userspace only)
• Architecture: RISC-V 64

Description of problem

It seems that call to setsockopt(sd, SOL_IP, IP_OPTIONS,_) behaves differently on RISC-V Qemu than on x64 Linux. On Linux syscall returns 0, but on Qemu it fails with Protocol not available. According man IP_OPTIONS on SOCK_STREAM socket "should work".

Steps to reproduce

1. Use below toy program setsockopt.c and compile it without optimizations like:

    gcc -Wall -W -Wextra -std=gnu17 -pedantic setsockopt.c -o setsockopt

#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
    {
        int sd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
        if(sd < 0) {
            perror("Opening stream socket error");
            exit(1);
        }
        else
            printf("Opening stream socket....OK.\n");

        struct sockaddr_in local_address = {AF_INET, htons(1234), {inet_addr("255.255.255.255")}, {0}};
        int err = connect(sd, (struct sockaddr*)&local_address, (socklen_t)16);

        if (err < 0) {
            perror("Connect error");
            close(sd);
        }
        else
            printf("Connect...OK.\n");
    }
    {
        int sd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
        if(sd < 0) {
            perror("Opening stream socket error");
            exit(1);
        }
        else
            printf("Opening stream socket....OK.\n");

        char option[4] = {0};
        if(setsockopt(sd, SOL_IP, IP_OPTIONS, (char *)option, sizeof(option)) < 0) {
            perror("setsockopt error");
            close(sd);
            exit(1);
        }
        else
            printf("setsockopt...OK.\n");

        struct sockaddr_in local_address = {AF_INET, htons(1234), {inet_addr("255.255.255.255")}, {0}};
        int err = connect(sd, (struct sockaddr*)&local_address, (socklen_t)16);

        if (err < 0) {
            perror("Connect error");
            close(sd);
        }
        else
            printf("Connect...OK.\n");
    }
    return 0;
}

2. Run program on Qemu and compare output with output from x64 build. In my case it looks like:

root@AMDC4705:~/runtime/connect$ ./setsockopt-x64
Opening stream socket....OK.
Connect error: Network is unreachable
Opening stream socket....OK.
setsockopt...OK.
Connect error: Network is unreachable

root@AMDC4705:/runtime/connect# ./setsockopt-riscv
Opening stream socket....OK.
Connect error: Network is unreachable
Opening stream socket....OK.
setsockopt error: Protocol not available

Additional information

In above demo option value is quite artificial. However I tried passing many different option arguments (with same SOL_IP + IP_OPTIONS combination) but always ended up with setsockopt failure. From the other hand on x64 it worked fine. Then I realized that appropriate path in Qemu was unimplemented: https://github.com/qemu/qemu/blob/master/linux-user/syscall.c#L2141