proc/maps: make vm_is_stack() logic namespace-friendly

- Rename vm_is_stack() to task_of_stack() and change it to return
"struct task_struct *" rather than the global (and thus wrong in
general) pid_t.

- Add the new pid_of_stack() helper which calls task_of_stack() and
uses the right namespace to report the correct pid_t.

Unfortunately we need to define this helper twice, in task_mmu.c
and in task_nommu.c. perhaps it makes sense to add fs/proc/util.c
and move at least pid_of_stack/task_of_stack there to avoid the
code duplication.

- Change show_map_vma() and show_numa_map() to use the new helper.

Signed-off-by: Oleg Nesterov <[e-mail]>
Cc: Alexander Viro <[e-mail]>
Cc: Cyrill Gorcunov <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Cc: Greg Ungerer <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/task_mmu.c

···
 				sizeof(struct proc_maps_private));
 }
 
+static pid_t pid_of_stack(struct proc_maps_private *priv,
+				struct vm_area_struct *vma, bool is_pid)
+{
+	struct inode *inode = priv->inode;
+	struct task_struct *task;
+	pid_t ret = 0;
+
+	rcu_read_lock();
+	task = pid_task(proc_pid(inode), PIDTYPE_PID);
+	if (task) {
+		task = task_of_stack(task, vma, is_pid);
+		if (task)
+			ret = task_pid_nr_ns(task, inode->i_sb->s_fs_info);
+	}
+	rcu_read_unlock();
+
+	return ret;
+}
+
 static void
 show_map_vma(struct seq_file *m, struct vm_area_struct *vma, int is_pid)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	struct file *file = vma->vm_file;
 	struct proc_maps_private *priv = m->private;
-	struct task_struct *task = priv->task;
 	vm_flags_t flags = vma->vm_flags;
 	unsigned long ino = 0;
 	unsigned long long pgoff = 0;
···
 			goto done;
 		}
 
-		tid = vm_is_stack(task, vma, is_pid);
-
+		tid = pid_of_stack(priv, vma, is_pid);
 		if (tid != 0) {
 			/*
 			 * Thread stack in /proc/PID/task/TID/maps or
···
 	} else if (vma->vm_start <= mm->brk && vma->vm_end >= mm->start_brk) {
 		seq_puts(m, " heap");
 	} else {
-		pid_t tid = vm_is_stack(task, vma, is_pid);
+		pid_t tid = pid_of_stack(proc_priv, vma, is_pid);
 		if (tid != 0) {
 			/*
 			 * Thread stack in /proc/PID/task/TID/maps or

fs/proc/task_nommu.c

···
 	return size;
 }
 
+static pid_t pid_of_stack(struct proc_maps_private *priv,
+				struct vm_area_struct *vma, bool is_pid)
+{
+	struct inode *inode = priv->inode;
+	struct task_struct *task;
+	pid_t ret = 0;
+
+	rcu_read_lock();
+	task = pid_task(proc_pid(inode), PIDTYPE_PID);
+	if (task) {
+		task = task_of_stack(task, vma, is_pid);
+		if (task)
+			ret = task_pid_nr_ns(task, inode->i_sb->s_fs_info);
+	}
+	rcu_read_unlock();
+
+	return ret;
+}
+
 /*
  * display a single VMA to a sequenced file
  */
···
 		seq_pad(m, ' ');
 		seq_path(m, &file->f_path, "");
 	} else if (mm) {
-		pid_t tid = vm_is_stack(priv->task, vma, is_pid);
+		pid_t tid = pid_of_stack(priv, vma, is_pid);
 
 		if (tid != 0) {
 			seq_pad(m, ' ');

include/linux/mm.h

···
 		!vma_growsup(vma->vm_next, addr);
 }
 
-extern pid_t
-vm_is_stack(struct task_struct *task, struct vm_area_struct *vma, int in_group);
+extern struct task_struct *task_of_stack(struct task_struct *task,
+				struct vm_area_struct *vma, bool in_group);
 
 extern unsigned long move_page_tables(struct vm_area_struct *vma,
 		unsigned long old_addr, struct vm_area_struct *new_vma,

mm/util.c

···
 /*
  * Check if the vma is being used as a stack.
  * If is_group is non-zero, check in the entire thread group or else
- * just check in the current task. Returns the pid of the task that
- * the vma is stack for.
+ * just check in the current task. Returns the task_struct of the task
+ * that the vma is stack for. Must be called under rcu_read_lock().
  */
-pid_t vm_is_stack(struct task_struct *task,
-		  struct vm_area_struct *vma, int in_group)
+struct task_struct *task_of_stack(struct task_struct *task,
+				struct vm_area_struct *vma, bool in_group)
 {
-	pid_t ret = 0;
-
 	if (vm_is_stack_for_task(task, vma))
-		return task->pid;
+		return task;
 
 	if (in_group) {
 		struct task_struct *t;
 
-		rcu_read_lock();
 		for_each_thread(task, t) {
-			if (vm_is_stack_for_task(t, vma)) {
-				ret = t->pid;
-				goto done;
-			}
+			if (vm_is_stack_for_task(t, vma))
+				return t;
 		}
-done:
-		rcu_read_unlock();
 	}
 
-	return ret;
+	return NULL;
 }
 
 #if defined(CONFIG_MMU) && !defined(HAVE_ARCH_PICK_MMAP_LAYOUT)