Re: [dpdk-dev] [PATCH v2] net/ring: fix unchecked return value

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 10/12/2020 1:45 PM, Bruce Richardson wrote:
> On Mon, Oct 12, 2020 at 12:57:11PM +0100, Ferruh Yigit wrote:
>> On 10/1/2020 6:09 PM, Kevin Laatz wrote:
>>> Add a check for the return value of the sscanf call in
>>> parse_internal_args(), returning an error if we don't get the expected
>>> result.
>>>
>>> Coverity issue: 362049
>>> Fixes: 96cb19521147 ("net/ring: use EAL APIs in PMD specific API")
>>> Cc: stable@dpdk.org
>>>
>>> Signed-off-by: Kevin Laatz <kevin.laatz@intel.com>
>>>
>>> ---
>>> v2: added consumed characters count check
>>> ---
>>>    drivers/net/ring/rte_eth_ring.c | 7 ++++++-
>>>    1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/net/ring/rte_eth_ring.c b/drivers/net/ring/rte_eth_ring.c
>>> index 40fe1ca4ba..66367465fd 100644
>>> --- a/drivers/net/ring/rte_eth_ring.c
>>> +++ b/drivers/net/ring/rte_eth_ring.c
>>> @@ -538,8 +538,13 @@ parse_internal_args(const char *key __rte_unused, const char *value,
>>>    {
>>>    	struct ring_internal_args **internal_args = data;
>>>    	void *args;
>>> +	int n;
>>> -	sscanf(value, "%p", &args);
>>> +	if (sscanf(value, "%p%n", &args, &n) != 1 || (size_t)n != strlen(value)) {
>>
>> two small details,
>>
>> 1- I see following note in the sscanf manual: https://linux.die.net/man/3/sscanf
>> "
>> The C standard says: "Execution of a %n directive does not increment the
>> assignment count returned at the completion of execution" but the
>> Corrigendum seems to contradict this. Probably it is wise not to make any
>> assumptions on the effect of %n conversions on the return value.
>> "
>>
>> So what do you think checking return value as " == 0" ?
>>
> 
> Maybe in that copy of the man page but on my Ubuntu system there is no such
> disclaimer, and I don't see it either on the kernel.org man page reference:
> 
> https://man7.org/linux/man-pages/man3/sscanf.3.html
> 
> That official man page reference clearly states that the behaviour is that
> %n does not increase the reference count.
> 

My Linux box also doesn't have that note, but just to prevent the PMD fails for 
something like this.

Do you see any downside of checking as "sscanf() == 0"?