[PATCH] kcmp: Comment get_file_raw_ptr() RCU usage

[PATCH] kcmp: Comment get_file_raw_ptr() RCU usage

[Jason Andryuk]

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();
-- 
2.34.1

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

[Cyrill Gorcunov]

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.
	 */

?

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

[Jason Andryuk]

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."

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?

Thanks,
Jason

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

[Cyrill Gorcunov]

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!