linux-kernel - Re: [PATCH] watchdog: avoid usage of iterator after loop

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Message-ID: <1570650d-ea9d-c3b5-801a-60e246947242@roeck-us.net>
Date:   Wed, 1 Mar 2023 10:31:38 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Jakob Koschel <jkl820.git@...il.com>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>
Cc:     linux-watchdog@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pietro Borrello <borrello@...g.uniroma1.it>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [PATCH] watchdog: avoid usage of iterator after loop

On 3/1/23 09:17, Jakob Koschel wrote:
> If potentially no valid element is found, 'p' would contain an invalid
> pointer past the iterator loop. To ensure 'p' is valid under any
> circumstances, the kfree() should be within the loop body.
> 
> Additionally, Linus proposed to avoid any use of the list iterator
> variable after the loop, in the attempt to move the list iterator
> variable declaration into the marcro to avoid any potential misuse after

macro

> the loop [1].
> 
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
> Signed-off-by: Jakob Koschel <jkl820.git@...il.com>
> ---
>   drivers/watchdog/watchdog_pretimeout.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/watchdog/watchdog_pretimeout.c b/drivers/watchdog/watchdog_pretimeout.c
> index 376a495ab80c..d8c78696eaf5 100644
> --- a/drivers/watchdog/watchdog_pretimeout.c
> +++ b/drivers/watchdog/watchdog_pretimeout.c
> @@ -207,10 +207,10 @@ void watchdog_unregister_pretimeout(struct watchdog_device *wdd)
>   	list_for_each_entry_safe(p, t, &pretimeout_list, entry) {
>   		if (p->wdd == wdd) {
>   			list_del(&p->entry);
> -			break;
> +			spin_unlock_irq(&pretimeout_lock);
> +			kfree(p);
> +			return;

Please just make it
			kfree(p);
			break;

There is no need to drop the spinlock here and/or to return
directly.

Thanks,
Guenter

>   		}
>   	}
>   	spin_unlock_irq(&pretimeout_lock);
> -
> -	kfree(p);
>   }
> 
> ---
> base-commit: c0927a7a5391f7d8e593e5e50ead7505a23cadf9
> change-id: 20230301-watchdog-avoid-iter-after-loop-a197bf201435
> 
> Best regards,