Fix some mount-related bugs

kernel/src/fs/path/dentry.rs

@@ -101,16 +101,18 @@ impl Dentry {
         DentryFlags::from_bits(flags).unwrap()
     }
 
-    /// Checks if this dentry is a descendant (child, grandchild, or
-    /// great-grandchild, etc.) of another dentry.
-    pub(super) fn is_descendant_of(&self, ancestor: &Arc<Self>) -> bool {
-        let mut parent = self.parent();
-        while let Some(p) = parent {
-            if Arc::ptr_eq(&p, ancestor) {
+    /// Checks if this dentry is a descendant of or the same as the given
+    /// ancestor dentry.
+    pub(super) fn is_equal_or_descendant_of(&self, ancestor: &Arc<Self>) -> bool {
+        let mut current = Some(self.this());
+
+        while let Some(node) = current {
+            if Arc::ptr_eq(&node, ancestor) {
                 return true;
             }
-            parent = p.parent();
+            current = node.parent();
         }
+
         false
     }
 

kernel/src/fs/path/mount.rs

@@ -326,7 +326,7 @@ impl Mount {
             let old_children = old_mount.children.read();
             for old_child_mount in old_children.values() {
                 let mountpoint = old_child_mount.mountpoint().unwrap();
-                if !mountpoint.is_descendant_of(root_dentry) {
+                if !mountpoint.is_equal_or_descendant_of(old_mount.root_dentry()) {
                     continue;
                 }
                 let new_child_mount =

test/src/apps/namespace/mnt_ns.c

@@ -190,3 +190,71 @@ FN_TEST(setns_newns)
 	TEST_SUCC(rmdir(SETNS_MNT));
 }
 END_TEST()
+
+#define COMPLEX_BASE "/test0"
+#define COMPLEX_CONTENT "/test0/content"
+#define COMPLEX_FILE "/test0/content/hello.txt"
+
+FN_TEST(complex_mount_tree_unshare)
+{
+	// Setup: Create directory structure
+	CHECK_WITH(mkdir(COMPLEX_BASE, 0755), _ret >= 0 || errno == EEXIST);
+
+	// Mount tmpfs on test0 (first layer)
+	TEST_SUCC(mount("none", COMPLEX_BASE, "tmpfs", 0, ""));
+
+	// Mount tmpfs on test0 again (second layer, stacked mount)
+	TEST_SUCC(mount("none", COMPLEX_BASE, "tmpfs", 0, ""));
+
+	// Recreate content directory on the new mount
+	TEST_SUCC(mkdir(COMPLEX_CONTENT, 0755));
+
+	// Mount tmpfs on test0/content (third layer)
+	TEST_SUCC(mount("none", COMPLEX_CONTENT, "tmpfs", 0, ""));
+
+	// Create file with content
+	int fd = TEST_SUCC(open(COMPLEX_FILE, O_CREAT | O_WRONLY, 0644));
+	TEST_SUCC(write(fd, "hello world\n", 12));
+	TEST_SUCC(close(fd));
+
+	// Verify file exists in parent
+	TEST_SUCC(access(COMPLEX_FILE, F_OK));
+
+	// Fork and unshare in child
+	pid_t pid = TEST_SUCC(fork());
+
+	if (pid == 0) {
+		TEST_SUCC(unshare(CLONE_NEWNS));
+
+		// Try to read the file created in the parent's mount tree
+		int fd = CHECK(open(COMPLEX_FILE, O_RDONLY));
+
+		char buf[32] = { 0 };
+		TEST_SUCC(read(fd, buf, sizeof(buf) - 1));
+		TEST_SUCC(close(fd));
+
+		// Verify the content
+		CHECK_WITH(strcmp(buf, "hello world\n"), _ret == 0);
+
+		// Verify we can still access the nested mount structure
+		TEST_SUCC(access(COMPLEX_CONTENT, F_OK));
+		TEST_SUCC(access(COMPLEX_BASE, F_OK));
+
+		exit(0);
+	} else {
+		int status;
+		TEST_SUCC(waitpid(pid, &status, 0));
+		TEST_RES(WIFEXITED(status) && WEXITSTATUS(status), _ret == 0);
+
+		// Verify parent's mount tree is unchanged
+		TEST_SUCC(access(COMPLEX_FILE, F_OK));
+
+		// Teardown: Unmount in reverse order
+		TEST_SUCC(umount(COMPLEX_CONTENT));
+		TEST_SUCC(rmdir(COMPLEX_CONTENT));
+		TEST_SUCC(umount(COMPLEX_BASE));
+		TEST_SUCC(umount(COMPLEX_BASE));
+		TEST_SUCC(rmdir(COMPLEX_BASE));
+	}
+}
+END_TEST()