linux-kernel - Re: [PATCH] kcmp: Comment get_file_raw

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Date:   Wed, 2 Feb 2022 20:44:29 +0300
From:   Cyrill Gorcunov <gorcunov@...il.com>
To:     Jason Andryuk <jandryuk@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        "Eric W . Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] kcmp: Comment get_file_raw_ptr() RCU usage

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@...il.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.
	 */

?