mm: introduce get_task_exe_file

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@redhat.com>
Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Richard Guy Briggs <rgb@redhat.com>
Cc: <stable@vger.kernel.org> # 4.3.x
Signed-off-by: Paul Moore <paul@paul-moore.com>

fs/proc/base.c

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 0d163a8..da8b194 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1552,18 +1552,13 @@
 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);

include/linux/mm.h

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8f468e0..004c73a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1987,6 +1987,7 @@
 
 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 bool may_expand_vm(struct mm_struct *, vm_flags_t, unsigned long npages);
 extern void vm_stat_account(struct mm_struct *, vm_flags_t, long npages);

kernel/fork.c

diff --git a/kernel/fork.c b/kernel/fork.c
index d277e83..42451ae 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -774,6 +774,29 @@
 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