Re: [PATCH] kernfs: fix potential null-ptr-deref in kernfs_path_from_node_locked()

From: Leizhen (ThunderTown)
Date: Wed Nov 23 2022 - 21:24:30 EST




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@xxxxxxxxxx>
>
> 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