From e2ec9c4d3abf49e95565574214f8979d819e3f48 Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Sun, 2 Feb 2020 18:56:12 +0100
Subject: [PATCH] namespace-util: introduce helper for combining unshare() +
 MS_SLAVE remount

We have multiple places we do these two non-trivial operations together,
let's introduce a unified helper for doing both at once.
---
 src/basic/namespace-util.c  | 14 ++++++++++++++
 src/basic/namespace-util.h  |  2 ++
 src/core/machine-id-setup.c |  8 +++-----
 src/shared/tests.c          |  6 ++----
 src/test/test-udev.c        | 13 ++++++-------
 5 files changed, 27 insertions(+), 16 deletions(-)

diff --git a/src/basic/namespace-util.c b/src/basic/namespace-util.c
index b0168ae227..b34c532604 100644
--- a/src/basic/namespace-util.c
+++ b/src/basic/namespace-util.c
@@ -2,6 +2,7 @@
 
 #include <fcntl.h>
 #include <sys/ioctl.h>
+#include <sys/mount.h>
 
 #include "fd-util.h"
 #include "missing_fs.h"
@@ -169,3 +170,16 @@ int fd_is_network_ns(int fd) {
 
         return r == CLONE_NEWNET;
 }
+
+int detach_mount_namespace(void) {
+
+        /* Detaches the mount namespace, disabling propagation from our namespace to the host */
+
+        if (unshare(CLONE_NEWNS) < 0)
+                return -errno;
+
+        if (mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) < 0)
+                return -errno;
+
+        return 0;
+}
diff --git a/src/basic/namespace-util.h b/src/basic/namespace-util.h
index 8c17ce91b2..99d9b977ed 100644
--- a/src/basic/namespace-util.h
+++ b/src/basic/namespace-util.h
@@ -7,3 +7,5 @@ int namespace_open(pid_t pid, int *pidns_fd, int *mntns_fd, int *netns_fd, int *
 int namespace_enter(int pidns_fd, int mntns_fd, int netns_fd, int userns_fd, int root_fd);
 
 int fd_is_network_ns(int fd);
+
+int detach_mount_namespace(void);
diff --git a/src/core/machine-id-setup.c b/src/core/machine-id-setup.c
index 284b77c1fc..f76b82a8a4 100644
--- a/src/core/machine-id-setup.c
+++ b/src/core/machine-id-setup.c
@@ -223,11 +223,9 @@ int machine_id_commit(const char *root) {
                 return log_error_errno(r, "Can't fetch current mount namespace: %m");
 
         /* Switch to a new mount namespace, isolate ourself and unmount etc_machine_id in our new namespace */
-        if (unshare(CLONE_NEWNS) < 0)
-                return log_error_errno(errno, "Failed to enter new namespace: %m");
-
-        if (mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) < 0)
-                return log_error_errno(errno, "Couldn't make-rslave / mountpoint in our private namespace: %m");
+        r = detach_mount_namespace();
+        if (r < 0)
+                return log_error_errno(r, "Failed to set up new mount namespace: %m");
 
         if (umount(etc_machine_id) < 0)
                 return log_error_errno(errno, "Failed to unmount transient %s file in our private namespace: %m", etc_machine_id);
diff --git a/src/shared/tests.c b/src/shared/tests.c
index ecf8e8f623..ff662ecfe0 100644
--- a/src/shared/tests.c
+++ b/src/shared/tests.c
@@ -21,6 +21,7 @@
 #include "env-util.h"
 #include "fs-util.h"
 #include "log.h"
+#include "namespace-util.h"
 #include "path-util.h"
 #include "random-util.h"
 #include "strv.h"
@@ -137,10 +138,7 @@ bool have_namespaces(void) {
 
         if (pid == 0) {
                 /* child */
-                if (unshare(CLONE_NEWNS) < 0)
-                        _exit(EXIT_FAILURE);
-
-                if (mount(NULL, "/", NULL, MS_SLAVE|MS_REC, NULL) < 0)
+                if (detach_mount_namespace() < 0)
                         _exit(EXIT_FAILURE);
 
                 _exit(EXIT_SUCCESS);
diff --git a/src/test/test-udev.c b/src/test/test-udev.c
index 208e7a0e96..c0b215dadc 100644
--- a/src/test/test-udev.c
+++ b/src/test/test-udev.c
@@ -17,6 +17,7 @@
 #include "log.h"
 #include "main-func.h"
 #include "mkdir.h"
+#include "namespace-util.h"
 #include "selinux-util.h"
 #include "signal-util.h"
 #include "string-util.h"
@@ -36,15 +37,13 @@ static int fake_filesystems(void) {
                 { "test/run",       "/etc/udev/rules.d",       "Failed to mount empty /etc/udev/rules.d",          true },
                 { "test/run",       UDEVLIBEXECDIR "/rules.d", "Failed to mount empty " UDEVLIBEXECDIR "/rules.d", true },
         };
-        unsigned i;
+        int r;
 
-        if (unshare(CLONE_NEWNS) < 0)
-                return log_error_errno(errno, "Failed to call unshare(): %m");
+        r = detach_mount_namespace();
+        if (r < 0)
+                return log_error_errno(r, "Failed to detach mount namespace: %m");
 
-        if (mount(NULL, "/", NULL, MS_SLAVE|MS_REC, NULL) < 0)
-                return log_error_errno(errno, "Failed to mount / as private: %m");
-
-        for (i = 0; i < ELEMENTSOF(fakefss); i++)
+        for (size_t i = 0; i < ELEMENTSOF(fakefss); i++)
                 if (mount(fakefss[i].src, fakefss[i].target, NULL, MS_BIND, NULL) < 0) {
                         log_full_errno(fakefss[i].ignore_mount_error ? LOG_DEBUG : LOG_ERR, errno, "%s: %m", fakefss[i].error);
                         if (!fakefss[i].ignore_mount_error)