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

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Message-ID: <20230301224211.qz4tu7ayq5iwio4q@jm1p>
Date:   Wed, 1 Mar 2023 23:42:11 +0100
From:   Jakob Koschel <jkl820.git@...il.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        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 23/03/01 10:31AM, Guenter Roeck wrote:
> 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.

Ok great, I'll fix that in v2. I wasn't sure if something breaks if 'p' is released if the spinlock is still hold.

~ jakob

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