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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 17:49:50 +0800
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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?

>>>
>>> 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