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>

3 files changed, 25 insertions, 6 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 0d163a84082d..da8b1943ba04 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1552,18 +1552,13 @@ static const struct file_operations proc_pid_set_comm_operations = {
 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);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8f468e0d2534..004c73a988b7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1987,6 +1987,7 @@ extern void mm_drop_all_locks(struct mm_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 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);
diff --git a/kernel/fork.c b/kernel/fork.c
index d277e83ed3e0..42451aeb245f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -774,6 +774,29 @@ struct file *get_mm_exe_file(struct mm_struct *mm)
 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