How to resolve the \"EINVAL\" error when entering the mount namespace in Go?

Entering the Mount Namespace in Go

Issue:

In Go, the setns call returns EINVAL (invalid argument) when attempting to enter the mount namespace for a container.

Explanation:

To enter a namespace using setns, the call must be made from a single-threaded context before the Go runtime threads start.

Solution:

There are two approaches to解决this issue:

1. Using Constructor Trick:

• Add the __attribute__((constructor)) macro to a C function that calls setns before Go starts.
• Modified Go code:

<code class="go">/*
#include <sched.h>
#include <stdio.h>
#include <fcntl.h>

__attribute__((constructor)) void enter_namespace(void) {
   setns(open("/proc/<PID>/ns/mnt", O_RDONLY, 0644), 0);
}
*/
import "C"</code>

2. Using syscall.RawSyscall:

• Ensure that the setns call is made in the main thread before starting the Go runtime threads.
• Use runtime.LockOSThread to lock the main thread if necessary.

<code class="go">package main

import (
    "fmt"
    "os"
    "path/filepath"
    "syscall"
    "runtime"
)

func main() {
    if syscall.Geteuid() != 0 {
        fmt.Println("abort: you want to run this as root")
        os.Exit(1)
    }

    if len(os.Args) != 2 {
        fmt.Println("abort: you must provide a PID as the sole argument")
        os.Exit(2)
    }

    // Lock the main thread to the OS thread
    runtime.LockOSThread()

    namespaces := []string{"ipc", "uts", "net", "pid", "mnt"}

    for i := range namespaces {
        fd, _ := syscall.Open(filepath.Join("/proc", os.Args[1], "ns", namespaces[i]), syscall.O_RDONLY, 0644)
        err, _, msg := syscall.RawSyscall(308, uintptr(fd), 0, 0) // 308 == setns

        if err != 0 {
            fmt.Println("setns on", namespaces[i], "namespace failed:", msg)
        } else {
            fmt.Println("setns on", namespaces[i], "namespace succeeded")
        }

    }
}</code>

Note:

• Make sure your kernel supports multiple namespaces by checking /proc/sys/kernel/namespaces_allowed.
• The kernel patch quoted in the documentation specifies that the process must have CAP_SYS_ADMIN in the target mount namespace. Ensure that these conditions are met before attempting to call setns.

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