1 files changed, 97 insertions, 0 deletions
diff --git a/src/libutil/namespaces.cc b/src/libutil/namespaces.cc
new file mode 100644
index 000000000..f66accb10
--- /dev/null
+++ b/src/libutil/namespaces.cc
@@ -0,0 +1,97 @@
+#if __linux__
+
+#include "namespaces.hh"
+#include "util.hh"
+#include "finally.hh"
+
+#include <sys/mount.h>
+
+namespace nix {
+
+bool userNamespacesSupported()
+{
+    static auto res = [&]() -> bool
+    {
+        if (!pathExists("/proc/self/ns/user")) {
+            debug("'/proc/self/ns/user' does not exist; your kernel was likely built without CONFIG_USER_NS=y");
+            return false;
+        }
+
+        Path maxUserNamespaces = "/proc/sys/user/max_user_namespaces";
+        if (!pathExists(maxUserNamespaces) ||
+            trim(readFile(maxUserNamespaces)) == "0")
+        {
+            debug("user namespaces appear to be disabled; check '/proc/sys/user/max_user_namespaces'");
+            return false;
+        }
+
+        Path procSysKernelUnprivilegedUsernsClone = "/proc/sys/kernel/unprivileged_userns_clone";
+        if (pathExists(procSysKernelUnprivilegedUsernsClone)
+            && trim(readFile(procSysKernelUnprivilegedUsernsClone)) == "0")
+        {
+            debug("user namespaces appear to be disabled; check '/proc/sys/kernel/unprivileged_userns_clone'");
+            return false;
+        }
+
+        try {
+            Pid pid = startProcess([&]()
+            {
+                _exit(0);
+            }, {
+                .cloneFlags = CLONE_NEWUSER
+            });
+
+            auto r = pid.wait();
+            assert(!r);
+        } catch (SysError & e) {
+            debug("user namespaces do not work on this system: %s", e.msg());
+            return false;
+        }
+
+        return true;
+    }();
+    return res;
+}
+
+bool mountAndPidNamespacesSupported()
+{
+    static auto res = [&]() -> bool
+    {
+        try {
+
+            Pid pid = startProcess([&]()
+            {
+                /* Make sure we don't remount the parent's /proc. */
+                if (mount(0, "/", 0, MS_PRIVATE | MS_REC, 0) == -1)
+                    _exit(1);
+
+                /* Test whether we can remount /proc. The kernel disallows
+                   this if /proc is not fully visible, i.e. if there are
+                   filesystems mounted on top of files inside /proc.  See
+                   https://lore.kernel.org/lkml/87tvsrjai0.fsf@xmission.com/T/. */
+                if (mount("none", "/proc", "proc", 0, 0) == -1)
+                    _exit(2);
+
+                _exit(0);
+            }, {
+                .cloneFlags = CLONE_NEWNS | CLONE_NEWPID | (userNamespacesSupported() ? CLONE_NEWUSER : 0)
+            });
+
+            if (pid.wait()) {
+                debug("PID namespaces do not work on this system: cannot remount /proc");
+                return false;
+            }
+
+        } catch (SysError & e) {
+            debug("mount namespaces do not work on this system: %s", e.msg());
+            return false;
+        }
+
+        return true;
+    }();
+    return res;
+}
+
+}
+
+#endif