linux-kernel - Re: [PATCH v6 1/3] Staging: zram: Fix access of NULL pointer

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Message-ID: <CAL1ERfOEurbxjy1r8fqGJBXT2Go+7BRM3u6Cg4XfCSdtjFzTYQ@mail.gmail.com>
Date:	Mon, 28 Oct 2013 23:09:36 +0800
From:	Weijie Yang <weijie.yang.kh@...il.com>
To:	Rashika Kheria <rashika.kheria@...il.com>
Cc:	opw-kernel <opw-kernel@...glegroups.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Jiang Liu <jiang.liu@...wei.com>,
	Nitin Gupta <ngupta@...are.org>,
	Jerome Marchand <jmarchand@...hat.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/3] Staging: zram: Fix access of NULL pointer

On Mon, Oct 28, 2013 at 10:33 PM, Rashika Kheria
<rashika.kheria@...il.com> wrote:
>
>
>
> On Mon, Oct 28, 2013 at 7:15 PM, Weijie Yang <weijie.yang.kh@...il.com>
> wrote:
>>
>> On Mon, Oct 28, 2013 at 8:21 PM, Rashika Kheria
>> <rashika.kheria@...il.com> wrote:
>> > This patch fixes the bug in reset_store caused by accessing NULL
>> > pointer.
>> > Hence, It introduces a check for bdev. It also removes unnecessary check
>> > of bdev for fsync_bdev().
>> >
>> > Signed-off-by: Rashika Kheria <rashika.kheria@...il.com>
>> > ---
>> >
>> > This revision fixes the following issues of the previous revision-
>> > Seperating patches into Bug fix and Smatch fix
>> >
>> >  drivers/staging/zram/zram_drv.c |    6 ++++--
>> >  1 file changed, 4 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/drivers/staging/zram/zram_drv.c
>> > b/drivers/staging/zram/zram_drv.c
>> > index 2c4ed52..d640a8f 100644
>> > --- a/drivers/staging/zram/zram_drv.c
>> > +++ b/drivers/staging/zram/zram_drv.c
>> > @@ -648,6 +648,9 @@ static ssize_t reset_store(struct device *dev,
>> >         zram = dev_to_zram(dev);
>> >         bdev = bdget_disk(zram->disk, 0);
>> >
>> > +       if (!bdev)
>> > +               return -EBUSY;
>> > +
>>
>> I am not sure how does it happen. Would you please make it clear to me?
>>
>
> Hi Weijie,
>
> The bdev gets its value from bdget_disk() which could fail when memory is
> less available and hence bdev can be NULL.
>
> If this check is not introduced then we might reference NULL pointer in the
> later part of the code.

I see. Thanks!

> I hope I am clear.
>
> Thanks,
>
> --
> Rashika Kheria
> B.Tech CSE
> IIIT Hyderabad
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