netdev - Re: [PATCH net-next 3/9] s390/qeth: clean up string ops in qeth_l3_parse

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Date:   Wed, 23 Sep 2020 13:43:04 +0300
From:   Julian Wiedmann <jwi@...ux.ibm.com>
To:     David Laight <David.Laight@...LAB.COM>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Ursula Braun <ubraun@...ux.ibm.com>,
        Karsten Graul <kgraul@...ux.ibm.com>
Subject: Re: [PATCH net-next 3/9] s390/qeth: clean up string ops in
 qeth_l3_parse_ipatoe()

On 23.09.20 12:55, David Laight wrote:
> From: Julian Wiedmann
>> Sent: 23 September 2020 09:37
>>
>> Indicate the max number of to-be-parsed characters, and avoid copying
>> the address sub-string.
>>
>> Signed-off-by: Julian Wiedmann <jwi@...ux.ibm.com>
>> ---
>>  drivers/s390/net/qeth_l3_sys.c | 27 ++++++++++++++-------------
>>  1 file changed, 14 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/s390/net/qeth_l3_sys.c b/drivers/s390/net/qeth_l3_sys.c
>> index ca9c95b6bf35..05fa986e30fc 100644
>> --- a/drivers/s390/net/qeth_l3_sys.c
>> +++ b/drivers/s390/net/qeth_l3_sys.c
>> @@ -409,21 +409,22 @@ static ssize_t qeth_l3_dev_ipato_add4_show(struct device *dev,
>>  static int qeth_l3_parse_ipatoe(const char *buf, enum qeth_prot_versions proto,
>>  		  u8 *addr, int *mask_bits)
>>  {
>> -	const char *start, *end;
>> -	char *tmp;
>> -	char buffer[40] = {0, };
>> +	const char *start;
>> +	char *sep, *tmp;
>> +	int rc;
>>
>> -	start = buf;
>> -	/* get address string */
>> -	end = strchr(start, '/');
>> -	if (!end || (end - start >= 40)) {
>> +	/* Expected input pattern: %addr/%mask */
>> +	sep = strnchr(buf, 40, '/');
>> +	if (!sep)
>>  		return -EINVAL;
>> -	}
>> -	strncpy(buffer, start, end - start);
>> -	if (qeth_l3_string_to_ipaddr(buffer, proto, addr)) {
>> -		return -EINVAL;
>> -	}
>> -	start = end + 1;
>> +
>> +	/* Terminate the %addr sub-string, and parse it: */
>> +	*sep = '\0';
> 
> Is it valid to write into the input buffer here?
> 

It's a private buffer that was handed to us by the kernfs write code.

>> +	rc = qeth_l3_string_to_ipaddr(buf, proto, addr);
>> +	if (rc)
>> +		return rc;
>> +
>> +	start = sep + 1;
>>  	*mask_bits = simple_strtoul(start, &tmp, 10);
> 
> The use of strnchr() rather implies that the input
> buffer may not be '\0' terminated.

It's a kernfs write buffer, so guaranteed to be terminated.

> If that is true then you've just run off the end of the
> input buffer.
> 
>>  	if (!strlen(start) ||
>>  	    (tmp == start) ||
> 
> Hmmm... delete the strlen() clause.
> It ought to test start[0], but the 'tmp == start' test
> covers that case.
> 

See the next patch in this series, all this goes away.

> I don't understand why simple_strtoul() is deprecated.
> I don't recall any of the replacements returning the
> address of the terminating character.
> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

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