Start factoring out Unix assumptions

This splits files and adds new identifiers in preperation for supporting
windows, but no Windows-specific code is actually added yet.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libutil/linux/namespaces.cc

@@ -0,0 +1,146 @@
+#include "current-process.hh"
+#include "util.hh"
+#include "finally.hh"
+#include "file-system.hh"
+#include "processes.hh"
+#include "signals.hh"
+
+#include <mutex>
+#include <sys/resource.h>
+#include "cgroup.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;
+}
+
+
+//////////////////////////////////////////////////////////////////////
+
+static AutoCloseFD fdSavedMountNamespace;
+static AutoCloseFD fdSavedRoot;
+
+void saveMountNamespace()
+{
+    static std::once_flag done;
+    std::call_once(done, []() {
+        fdSavedMountNamespace = open("/proc/self/ns/mnt", O_RDONLY);
+        if (!fdSavedMountNamespace)
+            throw SysError("saving parent mount namespace");
+
+        fdSavedRoot = open("/proc/self/root", O_RDONLY);
+    });
+}
+
+void restoreMountNamespace()
+{
+    try {
+        auto savedCwd = absPath(".");
+
+        if (fdSavedMountNamespace && setns(fdSavedMountNamespace.get(), CLONE_NEWNS) == -1)
+            throw SysError("restoring parent mount namespace");
+
+        if (fdSavedRoot) {
+            if (fchdir(fdSavedRoot.get()))
+                throw SysError("chdir into saved root");
+            if (chroot("."))
+                throw SysError("chroot into saved root");
+        }
+
+        if (chdir(savedCwd.c_str()) == -1)
+            throw SysError("restoring cwd");
+    } catch (Error & e) {
+        debug(e.msg());
+    }
+}
+
+void unshareFilesystem()
+{
+    if (unshare(CLONE_FS) != 0 && errno != EPERM)
+        throw SysError("unsharing filesystem state in download thread");
+}
+
+}

src/libutil/linux/namespaces.hh

@@ -0,0 +1,33 @@
+#pragma once
+///@file
+
+#include <optional>
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * Save the current mount namespace. Ignored if called more than
+ * once.
+ */
+void saveMountNamespace();
+
+/**
+ * Restore the mount namespace saved by saveMountNamespace(). Ignored
+ * if saveMountNamespace() was never called.
+ */
+void restoreMountNamespace();
+
+/**
+ * Cause this thread to not share any FS attributes with the main
+ * thread, because this causes setns() in restoreMountNamespace() to
+ * fail.
+ */
+void unshareFilesystem();
+
+bool userNamespacesSupported();
+
+bool mountAndPidNamespacesSupported();
+
+}