linux-kernel - Re: [PATCH v2] opp: use list iterator only inside the loop

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Message-ID: <20220331024906.i7ucvoxrkkfpb6w5@vireshk-i7>
Date:   Thu, 31 Mar 2022 08:19:07 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Xiaomeng Tong <xiam0nd.tong@...il.com>
Cc:     vireshk@...nel.org, nm@...com, sboyd@...nel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] opp: use list iterator only inside the loop

On 31-03-22, 10:36, Xiaomeng Tong wrote:
> The list iterator 'new_dev' will point to a bogus position containing
> HEAD if any one of these conditions is possible: the list is empty or
> no element is found, thus can potentially lead to an invalid memory
> access in 'dev = new_dev->dev;'.

There is no such bug as I explained earlier, why you added this again
despite being discussed ?

> As discussed before,

I just told you not to use such language as this will go in logs, but
you still chose to add it :(

> we should avoid to use a list iterator variable
> outside the loop which is considered harmful[1].
> 
> In this case, use a new variable 'iter' as the list iterator, while
> use the old variable 'new_dev' as a dedicated pointer to point to the
> found entry. And BUG_ON(!new_dev);.

Please look at this on how to write the log, which fixes a very
similar problem.

https://lore.kernel.org/all/20220324071815.61405-1-jakobkoschel@gmail.com/

> 
> [1]:  https://lkml.org/lkml/2022/2/17/1032
> 
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@...il.com>
> ---
> 
> changes since v1:
>  - use BUG_ON(!new_dev); instead of return; (Viresh Kumar)
> 
> v1: https://lore.kernel.org/lkml/20220331015818.28045-1-xiam0nd.tong@gmail.com/
> 
> ---
>  drivers/opp/debugfs.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/opp/debugfs.c b/drivers/opp/debugfs.c
> index 596c185b5dda..81b2bc4b5f43 100644
> --- a/drivers/opp/debugfs.c
> +++ b/drivers/opp/debugfs.c
> @@ -187,14 +187,18 @@ void opp_debug_register(struct opp_device *opp_dev, struct opp_table *opp_table)
>  static void opp_migrate_dentry(struct opp_device *opp_dev,
>  			       struct opp_table *opp_table)
>  {
> -	struct opp_device *new_dev;
> +	struct opp_device *new_dev = NULL, *iter;
>  	const struct device *dev;
>  	struct dentry *dentry;
>  
>  	/* Look for next opp-dev */
> -	list_for_each_entry(new_dev, &opp_table->dev_list, node)
> -		if (new_dev != opp_dev)
> +	list_for_each_entry(iter, &opp_table->dev_list, node)
> +		if (iter != opp_dev) {
> +			new_dev = iter;
>  			break;
> +		}
> +
> +	BUG_ON(!new_dev);
>  
>  	/* new_dev is guaranteed to be valid here */
>  	dev = new_dev->dev;
> -- 
> 2.17.1

-- 
viresh