linux-kernel - Re: [PATCH] drivers/mmc/card/block.c: fix potential null dereference 'idata'

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Message-ID: <Pine.LNX.4.64.1105091713080.15483@pc-004.diku.dk>
Date:	Mon, 9 May 2011 17:14:22 +0200 (CEST)
From:	Julia Lawall <julia@...u.dk>
To:	Vladimir Motyka <vladimir.motyka@...il.com>
Cc:	cjb@...top.org, kernel-janitors@...r.kernel.org,
	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers/mmc/card/block.c: fix potential null dereference
 'idata'

On Mon, 9 May 2011, Vladimir Motyka wrote:

> On 05/09/2011 04:32 PM, Julia Lawall wrote:
> > On Mon, 9 May 2011, Vladimir Motyka wrote:
> > 
> >> When allocation of idata fails there was a null dereferece.
> > 
> > Why not have a different label for the two cases?  That would make the 
> > code easier to statically analyze, and perhaps be more understandable as 
> > well.
> > 
> > julia
> >
> I think You are right. So it could be better like this?
> 
> diff --git a/drivers/mmc/card/block.c b/drivers/mmc/card/block.c
> index 3dec493..a03cdc6 100644
> --- a/drivers/mmc/card/block.c
> +++ b/drivers/mmc/card/block.c
> @@ -237,7 +237,7 @@ static struct mmc_blk_ioc_data
> *mmc_blk_ioctl_copy_from_user(
>  	idata = kzalloc(sizeof(*idata), GFP_KERNEL);
>  	if (!idata) {
>  		err = -ENOMEM;
> -		goto copy_err;
> +		goto alloc_err;
>  	}
> 
>  	if (copy_from_user(&idata->ic, user, sizeof(idata->ic))) {
> @@ -266,9 +266,9 @@ static struct mmc_blk_ioc_data
> *mmc_blk_ioctl_copy_from_user(
>  	return idata;
> 
>  copy_err:
> -	if(idata)
> -		kfree(idata->buf);
> +	kfree(idata->buf);
>  	kfree(idata);
> +alloc_err:
>  	return ERR_PTR(err);
>  }
> 
> Or it could return right after allocation fails so there needn't be
> goto. It is simplier, but maybe worse looking and to read. What is your
> opinion?

Perhaps it is also pointless to call kfree on something that is known to 
be NULL.  But I think that there is quite some code that does that, so 
others might have another opinion.

julia


> 
> Vladimir Motyka
> 
> > 
> >> Signed-off-by: Vladimir Motyka <vladimir.motyka@...il.com>
> >>
> >> ---
> >> diff --git a/drivers/mmc/card/block.c b/drivers/mmc/card/block.c
> >> index 407836d..3dec493 100644
> >> --- a/drivers/mmc/card/block.c
> >> +++ b/drivers/mmc/card/block.c
> >> @@ -266,10 +266,10 @@ static struct mmc_blk_ioc_data
> >> *mmc_blk_ioctl_copy_from_user(
> >>  	return idata;
> >>
> >>  copy_err:
> >> -	kfree(idata->buf);
> >> +	if(idata)
> >> +		kfree(idata->buf);
> >>  	kfree(idata);
> >>  	return ERR_PTR(err);
> >> -
> >>  }
> >>
> >>  static int mmc_blk_ioctl_cmd(struct block_device *bdev,
> >> --
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> >>
> 
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