linux-ext4 - Re: [PATCH v2] ext3, ext4: do_split() fix loop, with obvious unsigned wrap

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Date:	Tue, 02 Dec 2008 12:08:38 -0500
From:	Bill Davidsen <davidsen@....com>
To:	Theodore Tso <tytso@....edu>, roel kluin <roel.kluin@...il.com>,
	davidsen@....com, adilger@....com, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ext3, ext4: do_split() fix loop, with obvious	unsigned
 wrap

Theodore Tso wrote:
> On Mon, Dec 01, 2008 at 02:28:25PM -0500, roel kluin wrote:
>   
>> Fix loop, with obvious unsigned wrap
>>
>> Signed-off-by: Roel Kluin <roel.kluin@...il.com>
>>     
>
> Um, no.  Sorry, I didn't have a chance to reply earlier but this is
> obviously wrong.
>
>   
Sorry, you are reading it wrong, the i values inside the loop are 
identical to those in the original. The value of i starts at count, and 
the test comes *before* the value is used inside the loop. The values of 
i inside the loop start at count-1 and go to zero, just as it did in the 
original. That's why the "i--" is there, the test is on the 
unincremented value range count to one, but the value inside the loop is 
correct (or at least is the same as the original patch).
>> ---
>> diff --git a/fs/ext3/namei.c b/fs/ext3/namei.c
>> index 3e5edc9..b0dcfb3 100644
>> --- a/fs/ext3/namei.c
>> +++ b/fs/ext3/namei.c
>> @@ -1188,7 +1188,7 @@ static struct ext3_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
>>  	/* Split the existing block in the middle, size-wise */
>>  	size = 0;
>>  	move = 0;
>> -	for (i = count-1; i >= 0; i--) {
>> +	for (i = count; i--; ) {
>>  		/* is more than half of this entry in 2nd half of the block? */
>>  		if (size + map[i].size/2 > blocksize/2)
>>  			break;
>>     
>
> Note that i is actually **used** in the loop?  So changing the
> starting value of the counter without also adjusting all of the places
> where i is used will cause the code to break, and in hard to find
> ways...
>
>   
As I said, the values used are identical, and the code works correctly.
> Given that there are two loop termination conditions, and in fact the
> one in the loop is the one that actually gets used 99% of the time
> (which is why we've never noticed the problem in real life), probably
> the best way of handling this is to recast it not as a for loop, but
> as a while loop.
>
> 						- Ted
>
>   


-- 
Bill Davidsen <davidsen@....com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark 


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