[PATCH 4.4 019/118] mm: introduce get_task_exe_file

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mateusz Guzik <mguzik@xxxxxxxxxx>

commit cd81a9170e69e018bbaba547c1fd85a585f5697a upstream.

For more convenient access if one has a pointer to the task.

As a minor nit take advantage of the fact that only task lock + rcu are
needed to safely grab ->exe_file. This saves mm refcount dance.

Use the helper in proc_exe_link.

Signed-off-by: Mateusz Guzik <mguzik@xxxxxxxxxx>
Acked-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Acked-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
Signed-off-by: Paul Moore <paul@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/proc/base.c     |    7 +------
 include/linux/mm.h |    1 +
 kernel/fork.c      |   23 +++++++++++++++++++++++
 3 files changed, 25 insertions(+), 6 deletions(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1545,18 +1545,13 @@ static const struct file_operations proc
 static int proc_exe_link(struct dentry *dentry, struct path *exe_path)
 {
 	struct task_struct *task;
-	struct mm_struct *mm;
 	struct file *exe_file;
 
 	task = get_proc_task(d_inode(dentry));
 	if (!task)
 		return -ENOENT;
-	mm = get_task_mm(task);
+	exe_file = get_task_exe_file(task);
 	put_task_struct(task);
-	if (!mm)
-		return -ENOENT;
-	exe_file = get_mm_exe_file(mm);
-	mmput(mm);
 	if (exe_file) {
 		*exe_path = exe_file->f_path;
 		path_get(&exe_file->f_path);
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1898,6 +1898,7 @@ extern void mm_drop_all_locks(struct mm_
 
 extern void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file);
 extern struct file *get_mm_exe_file(struct mm_struct *mm);
+extern struct file *get_task_exe_file(struct task_struct *task);
 
 extern int may_expand_vm(struct mm_struct *mm, unsigned long npages);
 extern struct vm_area_struct *_install_special_mapping(struct mm_struct *mm,
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -764,6 +764,29 @@ struct file *get_mm_exe_file(struct mm_s
 EXPORT_SYMBOL(get_mm_exe_file);
 
 /**
+ * get_task_exe_file - acquire a reference to the task's executable file
+ *
+ * Returns %NULL if task's mm (if any) has no associated executable file or
+ * this is a kernel thread with borrowed mm (see the comment above get_task_mm).
+ * User must release file via fput().
+ */
+struct file *get_task_exe_file(struct task_struct *task)
+{
+	struct file *exe_file = NULL;
+	struct mm_struct *mm;
+
+	task_lock(task);
+	mm = task->mm;
+	if (mm) {
+		if (!(task->flags & PF_KTHREAD))
+			exe_file = get_mm_exe_file(mm);
+	}
+	task_unlock(task);
+	return exe_file;
+}
+EXPORT_SYMBOL(get_task_exe_file);
+
+/**
  * get_task_mm - acquire a reference to the task's mm
  *
  * Returns %NULL if the task has no mm.  Checks PF_KTHREAD (meaning