Re: [PATCH] kcmp: Comment get_file_raw_ptr() RCU usage

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Wed, Feb 02, 2022 at 02:48:48PM -0500, Jason Andryuk wrote:
> On Wed, Feb 2, 2022 at 12:44 PM Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> >
> > On Wed, Feb 02, 2022 at 10:17:34AM -0500, Jason Andryuk wrote:
> > > This usage of RCU appears wrong since the pointer is passed outside the
> > > RCU region.  However, it is not dereferenced, so it is "okay".  Leave a
> > > comment for the next reader.
> > >
> > > Without a reference, these comparisons are racy, but even with their use
> > > inside an RCU region, the result could go stale.
> > >
> > > Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
> > > ---
> > > I was looking for examples of task_lookup_fd_rcu()/files_lookup_fd_rcu()
> > > and found this.  It differed from the example given in
> > > Documentation/filesystems/files.rst, so I was initially confused.  A
> > > comment seemed appropriate to avoid confusion.
> > >
> > >  kernel/kcmp.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/kernel/kcmp.c b/kernel/kcmp.c
> > > index 5353edfad8e1..4fb23f242e0f 100644
> > > --- a/kernel/kcmp.c
> > > +++ b/kernel/kcmp.c
> > > @@ -63,6 +63,9 @@ get_file_raw_ptr(struct task_struct *task, unsigned int idx)
> > >  {
> > >       struct file *file;
> > >
> > > +     /* This RCU locking is only present to silence warnings.  The pointer
> > > +      * value is only used for comparison and not dereferenced, so it is
> > > +      * acceptable. */
> > >       rcu_read_lock();
> > >       file = task_lookup_fd_rcu(task, idx);
> > >       rcu_read_unlock();
> >
> > They are not wrong, this is just such a bit weird semantics where
> > we fetch the pointers and strictly speaking map them into numbers
> > set to compare. But I agree that such tricks might confuse. How about
> >
> >         /*
> >          * Fetching file pointers inside RCU read-lock section
> >          * and reuse them as plain numbers is done in a sake
> >          * of speed. But make sure never dereference them after.
> >          */
> 
> I would tweak it a little to "Fetch file pointers inside RCU read-lock
> section, but skip additional locking for speed.  The pointer values
> will be used as integers, and must not be dereferenced."

Up to you. Initially I use _raw_ptr suffix in function name trying to
point out that the pointer obtained should not be considered in any
way except a natural number. So I'm fine with any comment which helps
readability.

> 
> One other idea I had was to switch the return value to "void *".  That
> way it isn't a struct file, and it isn't readily dereference-able.
> But I wasn't sure if that would be overkill.  What do you think?

Lets better stick with a comment I think. Thanks!