8226919: attach in linux hangs due to permission denied accessing /proc/pid/root

Reviewed-by: sgehwolf, kevinw

Author: slovdahl

src/jdk.attach/linux/classes/sun/tools/attach/VirtualMachineImpl.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2005, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2005, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -210,11 +210,8 @@ protected void close(long fd) throws IOException {
     }
 
     // Return the socket file for the given process.
-    private File findSocketFile(int pid, int ns_pid) {
-        // A process may not exist in the same mount namespace as the caller.
-        // Instead, attach relative to the target root filesystem as exposed by
-        // procfs regardless of namespaces.
-        String root = "/proc/" + pid + "/root/" + tmpdir;
+    private File findSocketFile(int pid, int ns_pid) throws IOException {
+        String root = findTargetProcessTmpDirectory(pid, ns_pid);
         return new File(root, ".java_pid" + ns_pid);
     }
 
@@ -230,21 +227,34 @@ private File createAttachFile(int pid, int ns_pid) throws IOException {
             // Do not canonicalize the file path, or we will fail to attach to a VM in a container.
             f.createNewFile();
         } catch (IOException x) {
-            String root;
-            if (pid != ns_pid) {
-                // A process may not exist in the same mount namespace as the caller.
-                // Instead, attach relative to the target root filesystem as exposed by
-                // procfs regardless of namespaces.
-                root = "/proc/" + pid + "/root/" + tmpdir;
-            } else {
-                root = tmpdir;
-            }
+            String root = findTargetProcessTmpDirectory(pid, ns_pid);
             f = new File(root, fn);
             f.createNewFile();
         }
         return f;
     }
 
+    private String findTargetProcessTmpDirectory(int pid, int ns_pid) throws IOException {
+        String root;
+        if (pid != ns_pid) {
+            // A process may not exist in the same mount namespace as the caller, e.g.
+            // if we are trying to attach to a JVM process inside a container.
+            // Instead, attach relative to the target root filesystem as exposed by
+            // procfs regardless of namespaces.
+            String procRootDirectory = "/proc/" + pid + "/root";
+            if (!Files.isReadable(Path.of(procRootDirectory))) {
+                throw new IOException(
+                        String.format("Unable to access root directory %s " +
+                          "of target process %d", procRootDirectory, pid));
+            }
+
+            root = procRootDirectory + "/" + tmpdir;
+        } else {
+            root = tmpdir;
+        }
+        return root;
+    }
+
     /*
      * Write/sends the given to the target VM. String is transmitted in
      * UTF-8 encoding.