linux-kernel - Re: [PATCH] fs: sysfs: don't pass count == 0 to bin file readers

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Date:	Fri, 22 May 2015 02:04:31 +0300
From:	Vladimir Zapolskiy <vz@...ia.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: sysfs: don't pass count == 0 to bin file readers

Hello Tejun,

On 22.05.2015 01:14, Tejun Heo wrote:
> Hello,
> 
> On Fri, May 22, 2015 at 12:21:16AM +0300, Vladimir Zapolskiy wrote:
>> If count == 0 bytes are requested by a reader, sysfs_kf_bin_read()
>> deliberately returns 0 without passing a potentially harmful value to
>> some externally defined underlying battr->read() function.
>>
>> However in case of (pos == size && count) the next clause always sets
>> count to 0 and this value is handed over to battr->read().
>>
>> The change intends to make obsolete (and remove later) a redundant
>> sanity check in battr->read(), if it is present, or add more
>> protection to struct bin_attribute users, who does not care about
>> input arguments.
>>
>> Signed-off-by: Vladimir Zapolskiy <vz@...ia.com>
>> ---
>>  fs/sysfs/file.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
>> index 7c2867b..6c95628 100644
>> --- a/fs/sysfs/file.c
>> +++ b/fs/sysfs/file.c
>> @@ -90,7 +90,7 @@ static ssize_t sysfs_kf_bin_read(struct kernfs_open_file *of, char *buf,
>>  		return 0;
>>  
>>  	if (size) {
>> -		if (pos > size)
>> +		if (pos >= size)
>>  			return 0;
>>  		if (pos + count > size)
>>  			count = size - pos;
> 
> Hmmm... maybe just move that test upwards?
> 
> 	if (!count || pos >= size)
> 		return 0;
> 
> 	count = min(count, size - pos);
> 

If the code block stays within if (size && count) { ... }, then !count
check is redundant (you may notice that !count check is already present
above but not shown in diff's 3 lines context), and I agree that

	if (pos >= size)
		return 0;

	if (pos + count > size)
		count = size - pos;

and

	if (pos >= size)
		return 0;

	count = min(count, size - pos);

are equal.

But "!size" is a special case,

	if (!count || pos >= size)
		return 0;

seems to be incorrect in case of !size ===> (pos >= size) == true.

To the sent change I may add a replacement of "if (pos + count > size)
..." with min_t (ssize_t, count, size - pos), if you wish.

--
With best wishes,
Vladimir
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