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Calling Setns from Go Returns EINVAL for Mnt Namespace
Background
The goal is to enter the mnt namespace of a container using either C or Go code. However, the Go code consistently returns EINVAL from the setns call when attempting to enter the mnt namespace.
Working C Code
The following C code successfully enters all specified namespaces:
<code class="c">#include <sched.h> main() { // ... for (i=0; i<5; i++) { setns(fd, 0); // Join the provided namespace } }</code>
Failing Go Code
On the other hand, the equivalent Go code returns an EINVAL error for the mnt namespace:
<code class="go">import ( "syscall" ) func main() { // ... err := syscall.RawSyscall(308, fd, 0, 0) // Calling setns if err != 0 { fmt.Println("setns on", namespaces[i], "namespace failed") } }</code>
Answer
The issue lies in the multi-threaded nature of Go. When Go calls setns from a multi-threaded context, it fails for the mnt namespace because it requires a single-threaded caller. To resolve this, the setns call must be made before the Go runtime creates any additional threads.
Solution: Single-Threaded Constructor Trick
One way to achieve this is to use the CGO constructor trick, where a C function is executed before Go starts up:
<code class="c">__attribute__((constructor)) void enter_namespace(void) { setns(...); }</code>
Adding this constructor to the Go code, along with hardcoding the PID, allows successful entry into the mnt namespace:
<code class="go">/* __attribute__((constructor)) void enter_namespace(void) { ... } */ import "C"</code>
However, this approach requires hardcoding the PID, which is not ideal.
Alternative: Linux Kernel 4.16 and Above
In Linux kernel 4.16 and above, a new mode has been introduced for setns that allows it to be safely called from multi-threaded contexts. By using a modified version of the setns call that takes an additional argument (CLONE_IOCLONE_COMMON), it is possible to enter the mnt namespace from Go, even if it is multi-threaded.
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