Network hangs when communicating with host

[Dmitry Vyukov <dvyukov@xxxxxxxxxx>]

Hello,

I am trying to run a program in lkvm sandbox so that it communicates
with a program on host. I run lkvm as:

./lkvm sandbox --disk sandbox-test --mem=2048 --cpus=4 --kernel
/arch/x86/boot/bzImage --network mode=user -- /my_prog

/my_prog then connects to a program on host over a tcp socket.
I see that host receives some data, sends some data back, but then
my_prog hangs on network read.

To localize this I wrote 2 programs (attached). ping is run on host
and pong is run from lkvm sandbox. They successfully establish tcp
connection, but after some iterations both hang on read.

Networking code in Go runtime is there for more than 3 years, widely
used in production and does not have any known bugs. However, it uses
epoll edge-triggered readiness notifications that known to be tricky.
Is it possible that lkvm contains some networking bug? Can it be
related to the data races in lkvm I reported earlier today?

I am on commit 3695adeb227813d96d9c41850703fb53a23845eb.

Thank you

package main

import (
	"log"
	"net"
)

func main() {
	c, err := net.Dial("tcp", "192.168.33.1:39921")
	if err != nil {
		log.Fatalf("failed to dial: %v", err)
	}
	log.Printf("connected")
	for {
		var buf [1]byte
		n, err := c.Write(buf[:])
		if err != nil || n != 1 {
			log.Fatalf("failed to write: %v", err)
		}
		log.Printf("write")
		n, err = c.Read(buf[:])
		if err != nil || n != 1 {
			log.Fatalf("failed to read: %v", err)
		}
		log.Printf("read")
	}
}

package main

import (
	"log"
	"net"
)

func main() {
	ln, err := net.Listen("tcp", "127.0.0.1:39921")
	if err != nil {
		log.Fatalf("failed to listen: %v", err)
	}
	log.Printf("listening")
	c, err := ln.Accept()
	if err != nil {
		log.Fatalf("failed to accept: %v", err)
	}
	log.Printf("connected")
	for {
		var buf [1]byte
		n, err := c.Read(buf[:])
		if err != nil || n != 1 {
			log.Fatalf("failed to read: %v", err)
		}
		log.Printf("read")
		n, err = c.Write(buf[:])
		if err != nil || n != 1 {
			log.Fatalf("failed to write: %v", err)
		}
		log.Printf("write")
	}
}