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Subject: Re: [PATCH] kernfs: fix potential null-ptr-deref in
 kernfs_path_from_node_locked()
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
To:     Tejun Heo <tj@kernel.org>
CC:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        <linux-kernel@vger.kernel.org>
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Date:   Sat, 26 Nov 2022 18:46:09 +0800
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On 2022/11/26 17:49, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/11/24 10:52, Leizhen (ThunderTown) wrote:
>>
>>
>> On 2022/11/24 10:28, Leizhen (ThunderTown) wrote:
>>>
>>>
>>> On 2022/11/24 10:24, Leizhen (ThunderTown) wrote:
>>>>
>>>>
>>>> On 2022/11/24 0:55, Tejun Heo wrote:
>>>>> On Wed, Nov 23, 2022 at 10:04:19AM +0800, Zhen Lei wrote:
>>>>>> Ensure that the 'buf' is not empty before strlcpy() uses it.
>>>>>>
>>>>>> Commit bbe70e4e4211 ("fs: kernfs: Fix possible null-pointer dereferences
>>>>>> in kernfs_path_from_node_locked()") first noticed this, but it didn't
>>>>>> fix it completely.
>>>>>>
>>>>>> Fixes: 9f6df573a404 ("kernfs: Add API to generate relative kernfs path")
>>>>>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>>>>>
>>>>> I think the right thing to do is removing that if. It makes no sense to call
>>>>> that function with NULL buf and the fact that nobody reported crashes on
>>>>> NULL buf indicates that we in fact never do.
> 
> kernfs_path_from_node
>     -->kernfs_path_from_node_locked
> 
> EXPORT_SYMBOL_GPL(kernfs_path_from_node)
> 
> I've rethought it. The export APIs need to do null pointer check, right?

Sorry，I looked at some of the other functions and they didn't check either.

> 
>>>>
>>>> OK.
>>>>
>>>> How about I remove "buf[0] = '\0';" too? It seems to be a useless operation.
>>>> When 'kn_from' and 'kn_to' have a common ancestor, there must be a path from
>>>> 'kn_from' to 'kn_to', and strlcpy() always fills in the terminator correctly,
>>>> even if the buf is too small to save the first path node.
>>>
>>> Sorry, I misanalyzed. The length used by "len < buflen ? buflen - len : 0" may
>>> be zero.
>>
>> Ah, my brain is unstable today. The initial value of len is 0. So "buf[0] = '\0';"
>> can still be safely removed.
>>
>>>
>>>>
>>>> static void test(void)
>>>> {
>>>>         char buf[4];
>>>>         int i, n, buflen;
>>>>
>>>>         buflen = 1;
>>>>         n = strlcpy(buf, "string", buflen);
>>>>         for (i = 0; i < buflen; i++)
>>>>                 printk("%d: %02x\n", i, buf[i]);
>>>>         printk("n=%d\n\n", n);
>>>>
>>>>         buflen = sizeof(buf);
>>>>         n = strlcpy(buf, "string", buflen);
>>>>         for (i = 0; i < buflen; i++)
>>>>                 printk("%d: %02x\n", i, buf[i]);
>>>>         printk("n=%d\n", n);
>>>> }
>>>>
>>>> Output:
>>>> [   33.691497] 0: 00
>>>> [   33.691569] n=6
>>>>
>>>> [   33.691595] 0: 73
>>>> [   33.691622] 1: 74
>>>> [   33.691630] 2: 72
>>>> [   33.691637] 3: 00
>>>> [   33.691650] n=6
>>>>
>>>>>
>>>>> Thanks.
>>>>>
>>>>
>>>
>>
> 

-- 
Regards,
  Zhen Lei