Re: [PATCH] rcu: kmemleak: Ignore kmemleak false positives when RCU-freeing objects

[Paul E. McKenney]

On Sat, Sep 30, 2023 at 05:46:56PM +0000, Joel Fernandes (Google) wrote:
> From: Catalin Marinas <catalin.marinas@xxxxxxx>
> 
> Since the actual slab freeing is deferred when calling kvfree_rcu(), so
> is the kmemleak_free() callback informing kmemleak of the object
> deletion. From the perspective of the kvfree_rcu() caller, the object is
> freed and it may remove any references to it. Since kmemleak does not
> scan RCU internal data storing the pointer, it will report such objects
> as leaks during the grace period.
> 
> Tell kmemleak to ignore such objects on the kvfree_call_rcu() path. Note
> that the tiny RCU implementation does not have such issue since the
> objects can be tracked from the rcu_ctrlblk structure.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Reported-by: Christoph Paasch <cpaasch@xxxxxxxxx>
> Closes: https://lore.kernel.org/all/F903A825-F05F-4B77-A2B5-7356282FBA2C@xxxxxxxxx/
> Cc: <stable@xxxxxxxxxxxxxxx>
> Tested-by: Christoph Paasch <cpaasch@xxxxxxxxx>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
>  kernel/rcu/tree.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index cb1caefa8bd0..24423877962c 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -31,6 +31,7 @@
>  #include <linux/bitops.h>
>  #include <linux/export.h>
>  #include <linux/completion.h>
> +#include <linux/kmemleak.h>
>  #include <linux/moduleparam.h>
>  #include <linux/panic.h>
>  #include <linux/panic_notifier.h>
> @@ -3388,6 +3389,14 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
>  		success = true;
>  	}
>  
> +	/*
> +	 * The kvfree_rcu() caller considers the pointer freed at this point
> +	 * and likely removes any references to it. Since the actual slab
> +	 * freeing (and kmemleak_free()) is deferred, tell kmemleak to ignore
> +	 * this object (no scanning or false positives reporting).
> +	 */
> +	kmemleak_ignore(ptr);
> +
>  	// Set timer to drain after KFREE_DRAIN_JIFFIES.
>  	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
>  		schedule_delayed_monitor_work(krcp);
> -- 
> 2.42.0.582.g8ccd20d70d-goog
>