Re: [PATCHv2 1/2] mm: introduce get_task_exe_file

[Mateusz Guzik]

On Tue, Aug 23, 2016 at 04:48:13PM +0200, Oleg Nesterov wrote:
> On 08/23, Mateusz Guzik wrote:
> >
> > +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;
> > +}
> 
> I can't believe I am going to comment the coding style but I can't resist ;)
> 
>   if (mm && !(task->flags & PF_KTHREAD)))
>   exe_file = get_mm_exe_file(mm);
> 
> looks a bit simpler to me. But this is purely cosmetic and subjective,
> both patches look good to me.
> 

Actually I did it for some consistency with get_task_mm.

The check can likely be done prior to taking the lock in both functions
and that would clean them up a little bit, but I wanted to avoid nit
picking... :>

> Acked-by: Oleg Nesterov 
> 

Thanks

-- 
Mateusz Guzik

Re: [PATCHv2 1/2] mm: introduce get_task_exe_file

[Oleg Nesterov]

On 08/23, Mateusz Guzik wrote:
>
> +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;
> +}

I can't believe I am going to comment the coding style but I can't resist ;)

if (mm && !(task->flags & PF_KTHREAD)))
exe_file = get_mm_exe_file(mm);

looks a bit simpler to me. But this is purely cosmetic and subjective,
both patches look good to me.

Acked-by: Oleg Nesterov 

[PATCHv2 1/2] mm: introduce get_task_exe_file

[Mateusz Guzik]

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

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 2ed41cb..ebccdc1 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1556,18 +1556,13 @@ 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 9d85402..f4e639e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2014,6 +2014,7 @@ 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 6fe775c..a4b2384 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -800,6 +800,29 @@ 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
-- 
1.8.3.1