linux-kernel - Re: [slabinfo PATCH] Fix off-by-one after readlink() call

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Date:	Fri, 14 Oct 2011 19:26:00 +0200
From:	Thomas Jarosch <thomas.jarosch@...ra2net.com>
To:	Christoph Lameter <cl@...two.org>
CC:	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [slabinfo PATCH] Fix off-by-one after readlink() call

On 10/14/2011 07:16 PM, Christoph Lameter wrote:
>> index 868cc93..cc1a378 100644
>> --- a/tools/slub/slabinfo.c
>> +++ b/tools/slub/slabinfo.c
>> @@ -1145,7 +1145,7 @@ static void read_slab_dir(void)
>>  		switch (de->d_type) {
>>  		   case DT_LNK:
>>  			alias->name = strdup(de->d_name);
>> -			count = readlink(de->d_name, buffer, sizeof(buffer));
>> +			count = readlink(de->d_name, buffer, sizeof(buffer)-1);
>>
> 
> DESCRIPTION
>        readlink()  places  the  contents  of the symbolic link path in the buffer buf, which has size bufsiz.  readlink() does not append a
>        null byte to buf.  It will truncate the contents (to a length of bufsiz characters), in case the buffer is too small to hold all  of
>        the contents.

The problem is the line after the readlink() call:

buffer[count] = '\0';

The common technique is to reduce the buffer size by one.
Another fix would be to check

"
if (count < 0 || count == sizeof(buffer))
    fatal();
"

Reducing the buffer size by one is easier IMHO.

Cheers,
Thomas
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