Re: [PATCH] Typecasting required for comparing unlike datatypes

[Harsh Bora <harsh@linux.vnet.ibm.com>]

On 12/10/2010 06:23 AM, KAMEZAWA Hiroyuki wrote:
> On Wed,  8 Dec 2010 18:25:00 +0530
> Harsh Prateek Bora<harsh@linux.vnet.ibm.com>  wrote:
>
>> The existing code causes the if condition to pass when it should fail
>> on a *64-bit kernel* because of implicit data type conversions. It can
>> be observed by passing pos = -1 and count = some positive number.
>> This results in function returning EOVERFLOW instead of EINVAL.
>>
>> With this patch, the function returns EINVAL when pos is -1 and count
>> is a positive number. This can be tested by calling sendfile with
>> offset = -1 and count = some positive number on a 64-bit kernel.
>>
>
> Hmm, is this clearer ?
>
> ==
>
> commit 4a3956c790290efeb647bbb0c3a90476bb57800e adds support for
> negative (unsigned) page offset for very large files as /proc/<pid>/mem
> and /dev/mem.
>
> In that patch, overlap check routine is added but it was wrong.
>
> Considering 'pos' is loff_t, a signed value,
>
> In usual case, at comparing 'pos' and 'pos+count'
>
> 	(positive) / (positive)  OK
> 	(positive) / (nevative)  EOVERFLOW
> 	(negative) / (positive)  EINVAL
> 	(negative) / (negative)  EINVAL
>
> In FMODE_UNSIGNED_OFFSET case,
>
> 	(positive) / (positive)  OK
> 	(positive) / (nevative)  OK (ex. 0x7fff ->  0x8000)
> 	(nevative) / (negative)  OK
> 	(negative) / (positive)  EOVERFLOW (ex. 0xffff ->  0x1)
>
> Signed-off-by: KAMEZAWA Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com>
>
> ---
>   fs/read_write.c |   21 +++++++++++++++++----
>   1 file changed, 17 insertions(+), 4 deletions(-)
>
> Index: linux-2.6.37-rc5/fs/read_write.c
> ===================================================================
> --- linux-2.6.37-rc5.orig/fs/read_write.c
> +++ linux-2.6.37-rc5/fs/read_write.c
> @@ -37,11 +37,24 @@ __negative_fpos_check(struct file *file,
>   	 * pos or pos+count is negative here, check overflow.
>   	 * too big "count" will be caught in rw_verify_area().
>   	 */
> -	if ((pos<  0)&&  (pos + count<  pos))
> +	/* negative pos is allowed only when the flag is set */
> +	if (!(file->f_mode&  FMODE_UNSIGNED_OFFSET)) {
> +		if ((pos>  0)&&  (pos + count>  0))
> +			return 0;
> +		if ((pos>  0)&&  (pos + count<  0))
> +			return -EOVERFLOW;
> +		return -EINVAL;
> +	}
> +	/*
> +	 * The file supports 'unsigned long' offset. (but loff_t is signed)
> +	 * When pos is negative, -1 is the biggest number. So if pos + count
> +	 * is larger than pos, it's overflow.
> +	 * (ex) -1 + 10 = 9 ...means
> +	 *    0xffff + 0xa = 0x9 =>  overflow.
> +	 */
> +	if ((pos<  0)&&  (pos + count>  0))

Well, that works fine for what I am concerned but I think there is a 
mismatch in the code and the comment above. As per the comments above, 
it should be like:
			if ((pos < 0) && (pos + count > pos))

Regards,
Harsh.
>   		return -EOVERFLOW;
> -	if (file->f_mode&  FMODE_UNSIGNED_OFFSET)
> -		return 0;
> -	return -EINVAL;
> +	return 0;
>   }
>
>   /**
>
> --
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