build: syscalls: Add setns.

* guix/build/syscalls.scm (setns): New procedure.
* tests/syscalls.scm ("setns"): New test.

squash: setns

guix/build/syscalls.scm

@@ -54,6 +54,7 @@
             CLONE_NEWPID
             CLONE_NEWNET
             clone
+            setns
 
             IFF_UP
             IFF_BROADCAST
@@ -313,6 +314,21 @@ Unlike the fork system call, clone accepts FLAGS that specify which resources
 are shared between the parent and child processes."
       (proc syscall-id flags %null-pointer))))
 
+(define setns
+  (let* ((ptr  (dynamic-func "setns" (dynamic-link)))
+         (proc (pointer->procedure int ptr (list int int))))
+    (lambda (fdes nstype)
+      "Reassociate the current process with the namespace specified by FDES, a
+file descriptor obtained by opening a /proc/PID/ns/* file.  NSTYPE specifies
+which type of namespace the current process may be reassociated with, or 0 if
+there is no such limitation."
+      (let ((ret (proc fdes nstype))
+            (err (errno)))
+        (unless (zero? ret)
+          (throw 'system-error "setns" "~d ~d: ~A"
+                 (list fdes nstype (strerror err))
+                 (list err)))))))
+
 
 ;;;
 ;;; Packed structures.

tests/syscalls.scm

@@ -90,6 +90,32 @@
             ((_ . status)
              (= 42 (status:exit-val status))))))))
 
+(test-assert "setns"
+  (match (clone (logior CLONE_NEWUSER SIGCHLD))
+    (0 (primitive-exit 0))
+    (clone-pid
+     (match (pipe)
+       ((in . out)
+        (match (primitive-fork)
+          (0
+           (close in)
+           ;; Join the user namespace.
+           (call-with-input-file (user-namespace clone-pid)
+             (lambda (port)
+               (setns (port->fdes port) 0)))
+           (write 'done out)
+           (close out)
+           (primitive-exit 0))
+          (fork-pid
+           (close out)
+           ;; Wait for the child process to join the namespace.
+           (read in)
+           (let ((result (and (equal? (readlink (user-namespace clone-pid))
+                                      (readlink (user-namespace fork-pid))))))
+             ;; Clean up.
+             (waitpid clone-pid)
+             (waitpid fork-pid)
+             result))))))))
 
 (test-assert "all-network-interfaces"
   (match (all-network-interfaces)