netdev - Re: [Patch] net: fix incorrect counting in __scm

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Date:	Wed, 04 Nov 2009 11:29:05 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Amerigo Wang <amwang@...hat.com>
CC:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [Patch] net: fix incorrect counting in __scm_destroy()

Amerigo Wang a écrit :
> It seems that in __scm_destroy() we forgot to decrease
> the ->count after fput(->fp[i]), this may cause some
> problem when we recursively call fput() again.
> 
> Signed-off-by: WANG Cong <amwang@...hat.com>
> Cc: David S. Miller <davem@...emloft.net>
> 
> ---
> diff --git a/net/core/scm.c b/net/core/scm.c
> index b7ba91b..fa53219 100644
> --- a/net/core/scm.c
> +++ b/net/core/scm.c
> @@ -120,8 +120,10 @@ void __scm_destroy(struct scm_cookie *scm)
>  				fpl = list_first_entry(&work_list, struct scm_fp_list, list);
>  
>  				list_del(&fpl->list);
> -				for (i=fpl->count-1; i>=0; i--)
> +				for (i = fpl->count-1; i >= 0; i--) {
>  					fput(fpl->fp[i]);
> +					fpl->count--;
> +				}
>  				kfree(fpl);
>  			}
>  

Hmm, your patch seems suspicious.

Are you fixing a real crash/bug, or is it something you discovered in a code review ?

Given we kfree(fpl) at the end of loop, we cannot recursively call __scm_destroy()
on same fpl, it would be a bug anyway ?

So you probably need something better, like testing fpl->list being not re-included
in current->scm_work_list before kfree() it 

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