Fix/clarify problems about mount namespaces

kernel/src/fs/fs_resolver.rs

@@ -56,6 +56,9 @@ impl FsResolver {
     ///
     /// Otherwise, the method will change both the `cwd` and `root` to their corresponding
     /// `Path`s within the new `MountNamespace`.
+    //
+    // FIXME: This cannot fail if we clone mount namespaces and update resolvers in an atomic way.
+    // We currently leak this error to userspace, which is not a correct behavior.
     pub fn switch_to_mnt_ns(&mut self, mnt_ns: &Arc<MountNamespace>) -> Result<()> {
         if mnt_ns.owns(self.root.mount_node()) && mnt_ns.owns(self.cwd.mount_node()) {
             return Ok(());

kernel/src/process/namespace/nsproxy.rs

@@ -169,7 +169,9 @@ impl ContextSetNsAdminApi for Context<'_> {
         // TODO: When setting a specific namespace,
         // other dependent fields of a posix thread may also need to be updated.
 
-        *self.thread_local.borrow_fs().resolver().write() = ns_proxy.mnt_ns.new_fs_resolver();
+        if !Arc::ptr_eq(&thread_local_ns_proxy.unwrap().mnt_ns, &ns_proxy.mnt_ns) {
+            *self.thread_local.borrow_fs().resolver().write() = ns_proxy.mnt_ns.new_fs_resolver();
+        }
 
         *pthread_ns_proxy = Some(ns_proxy.clone());
         thread_local_ns_proxy.replace(Some(ns_proxy));

kernel/src/process/namespace/unshare.rs

@@ -63,8 +63,7 @@ impl ContextUnshareAdminApi for Context<'_> {
                 .borrow_fs()
                 .resolver()
                 .write()
-                .switch_to_mnt_ns(new_ns_proxy.mnt_ns())
-                .unwrap();
+                .switch_to_mnt_ns(new_ns_proxy.mnt_ns())?;
         }
 
         *pthread_ns_proxy = Some(new_ns_proxy.clone());

test/src/apps/namespace/mnt_ns.c

@@ -2,15 +2,9 @@
 
 #define _GNU_SOURCE
 #include <sched.h>
-#include <stdio.h>
-#include <stdlib.h>
 #include <sys/mount.h>
-#include <unistd.h>
 #include <sys/wait.h>
-#include <errno.h>
 #include <sys/stat.h>
-#include <fcntl.h>
-#include <limits.h>
 #include <sys/syscall.h>
 
 #include "../test.h"
@@ -168,14 +162,21 @@ FN_TEST(setns_newns)
 
 	close(pipefd[1]);
 
-	char buf;
+	char buf[10];
 	TEST_SUCC(read(pipefd[0], &buf, 1));
 	close(pipefd[0]);
 
 	int pid_fd = TEST_SUCC(syscall(SYS_pidfd_open, child_pid, 0));
 
-	// Switch into the child's mount namespace using the pidfd.
+	// Switching into the child's UTS namespace will not change the CWD.
+	TEST_SUCC(chdir("/mnt"));
+	TEST_SUCC(setns(pid_fd, CLONE_NEWUTS));
+	TEST_RES(getcwd(buf, sizeof(buf)), strcmp(buf, "/mnt") == 0);
+
+	// Switching into the child's mount namespace will reset the CWD.
 	TEST_SUCC(setns(pid_fd, CLONE_NEWNS));
+	TEST_RES(getcwd(buf, sizeof(buf)), strcmp(buf, "/") == 0);
+
 	TEST_SUCC(close(pid_fd));
 
 	// Check if we can see the file created by the child in its namespace.
@@ -188,4 +189,4 @@ FN_TEST(setns_newns)
 	TEST_SUCC(umount(SETNS_MNT));
 	TEST_SUCC(rmdir(SETNS_MNT));
 }
-END_TEST()
+END_TEST()

test/src/apps/scripts/process.sh

@@ -34,7 +34,7 @@ mmap/mmap_beyond_the_file
 mmap/mmap_shared_filebacked
 mmap/mmap_readahead
 mmap/mmap_vmrss
-namepsace/mnt_ns
+namespace/mnt_ns
 namespace/setns
 namespace/unshare
 process/group_session