linux-kernel - Re: [PATCH] fbdev: fix possible NULL pointer derefernce

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Date:	Fri, 18 Apr 2014 19:31:37 +0900
From:	DaeSeok Youn <daeseok.youn@...il.com>
To:	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>
Cc:	tomi.valkeinen@...com, jg1.han@...sung.com,
	laurent.pinchart@...asonboard.com, Rob Clark <robdclark@...il.com>,
	linux-fbdev@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fbdev: fix possible NULL pointer derefernce

Hello,

2014-04-17 9:00 GMT+09:00 DaeSeok Youn <daeseok.youn@...il.com>:
> Hello,
>
> 2014-04-16 21:38 GMT+09:00 Jean-Christophe PLAGNIOL-VILLARD
> <plagnioj@...osoft.com>:
>>
>> On Apr 16, 2014, at 5:40 PM, Daeseok Youn <daeseok.youn@...il.com> wrote:
>>
>>>
>>> The spec->modedb can be NULL by fb_create_modedb().
>>>
>>> And also smatch says:
>>> drivers/video/fbdev/core/fbmon.c:975 fb_edid_to_monspecs() error:
>>> potential null dereference 'specs->modedb'.
>>> (fb_create_modedb returns null)
>>>
>>> Signed-off-by: Daeseok Youn <daeseok.youn@...il.com>
>>> ---
>>> drivers/video/fbdev/core/fbmon.c |    3 +++
>>> 1 files changed, 3 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
>>> index c204ebe..db274ca 100644
>>> --- a/drivers/video/fbdev/core/fbmon.c
>>> +++ b/drivers/video/fbdev/core/fbmon.c
>>> @@ -966,6 +966,9 @@ void fb_edid_to_monspecs(unsigned char *edid, struct fb_monspecs *specs)
>>>
>>>       specs->modedb = fb_create_modedb(edid, &specs->modedb_len);
>>>
>>> +     if (!specs->modedb)
>>> +             return;
>>> +
>>
>> we need to return an error and trace it
> Yes, you're right. I will change return type from void to int and add
> to handle an error when this function(fb_edid_to_monspecs) is called.
>
I try to look at the fbdev code related with fb_edid_to_monspecs().
And the result, I think it doesn't need to return an error and trace it.
My patch is also useless because modedb_len is zero when
fb_create_modedb() returns NULL so It cannot be dereferenced.

And callers of this function seem to need a log but I think they don't need to
handling an error. Some functions are handling error with checking
NULL of modedb variable,
they are just printing an error message.

If I am wrong or you have reasons for handling an error on that
function, please let me know.

Regards,
Daeseok Youn.

> I will send this patch as your comment.
>
> Thanks for review.
>
> Daeseok Youn.
>>
>> Best Regards,
>> J.
>>>       /*
>>>        * Workaround for buggy EDIDs that sets that the first
>>>        * detailed timing is preferred but has not detailed
>>> --
>>> 1.7.4.4
>>>
>>
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