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Subject: Re: [PATCH] kernfs: fix potential null-ptr-deref in
 kernfs_path_from_node_locked()
To:     Tejun Heo <tj@kernel.org>
CC:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        <linux-kernel@vger.kernel.org>
References: <20221123020419.1867-1-thunder.leizhen@huawei.com>
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From:   "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Message-ID: <ba083b44-93d5-37c4-380c-8e0249b2333c@huawei.com>
Date:   Thu, 24 Nov 2022 10:24:21 +0800
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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.

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.

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