Why Does \'setns\' Fail with EINVAL in a Go Multithreaded Environment?

Calling 'setns' from Go Fails with EINVAL for MNT Namespace

Problem

When attempting to call 'setns' from Go to enter the MNT namespace, the call consistently returns EINVAL. This issue is observed despite a C implementation of the same functionality working correctly.

Cause

The issue originates from the fact that Go is a multi-threaded language, while 'setns' expects to be called from a single-threaded context. When called from a multi-threaded environment, 'setns' fails to properly associate the current thread with the target namespace.

Solution - CGO Constructor Trick

A solution to this problem is to utilize the "CGO constructor trick." This technique allows you to specify a C function to be executed before the Go runtime initializes. By using this approach, your C function can call 'setns' before the Go threads are created, ensuring that the call is made in a single-threaded context.

1. Construct a C function that calls 'setns' with the desired parameters. Do this by decorating the function with the __attribute__((constructor)) macro.
2. Include the C function in your Go code by importing it using import "C".

Working Example

<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"

... rest of file is unchanged ...
</code>

Remember to replace the placeholder with the actual PID of the target process.

Limitations

This approach has some limitations:

• The PID must be hardcoded within the C function, as the Go code may not be available at the time of C function execution.
• It's not possible to call 'setns' multiple times from within the Go code, as the C code runs before Go runtime initialization.

Additional Notes

A patch to 'setns(2)' confirms the requirement for single-threaded execution in multi-threaded environments. Processes with capabilities must also possess CAP_SYS_CHROOT and CAP_SYS_ADMIN in their own user namespace and CAP_SYS_ADMIN in the target mount namespace.

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