netdev - Re: [PATCH net-next 4/4] s390/qeth: Convert sprintf/snprintf to scnprintf

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Date:   Wed, 8 Feb 2023 19:19:29 +0100
From:   Alexandra Winter <wintera@...ux.ibm.com>
To:     Simon Horman <simon.horman@...igine.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Thorsten Winkler <twinkler@...ux.ibm.com>,
        Jules Irenge <jbi.octave@...il.com>
Subject: Re: [PATCH net-next 4/4] s390/qeth: Convert sprintf/snprintf to
 scnprintf



On 07.02.23 16:42, Simon Horman wrote:
> On Mon, Feb 06, 2023 at 06:27:54PM +0100, Alexandra Winter wrote:
>> From: Thorsten Winkler <twinkler@...ux.ibm.com>
>>
>> This LWN article explains the rationale for this change
>> https: //lwn.net/Articles/69419/
> 
> https://lwn.net/Articles/69419/
> 
>> Ie. snprintf() returns what *would* be the resulting length,
>> while scnprintf() returns the actual length.
> 
> Ok, but in most cases in this patch the return value is not checked.
> Is there any value in this change in those cases?
> 

Jules Irenge reported a coccinnelle warning to use scnprintf in 
show() functions [1]. (Thorsten Winkler changed these instances to
sysfs_emit in patch 3 of this series.)
We read the article as a call to implement the plan to upgrade the kernel
to the newer *scnprintf functions. Is that not intended?

I totally agree, that in these cases no real problem was fixed, it is
more of a style improvement.

[1] https://lore.kernel.org/netdev/YzHyniCyf+G%2F2xI8@fedora/T/

>> Reported-by: Jules Irenge <jbi.octave@...il.com>
>> Reviewed-by: Alexandra Winkler <wintera@...ux.ibm.com>
> 
> s/Winkler/Winter/ ?

Of course. Wow, you have good eyes!

> 
>> Signed-off-by: Thorsten Winkler <twinkler@...ux.ibm.com>
>> Signed-off-by: Alexandra Winter <wintera@...ux.ibm.com>
> 
> ...
> 
>> diff --git a/drivers/s390/net/qeth_l3_main.c b/drivers/s390/net/qeth_l3_main.c
>> index 1cf4e354693f..af4e60d2917e 100644
>> --- a/drivers/s390/net/qeth_l3_main.c
>> +++ b/drivers/s390/net/qeth_l3_main.c
>> @@ -47,9 +47,9 @@ int qeth_l3_ipaddr_to_string(enum qeth_prot_versions proto, const u8 *addr,
>>  			     char *buf)
>>  {
>>  	if (proto == QETH_PROT_IPV4)
>> -		return sprintf(buf, "%pI4", addr);
>> +		return scnprintf(buf, INET_ADDRSTRLEN, "%pI4", addr);
>>  	else
>> -		return sprintf(buf, "%pI6", addr);
>> +		return scnprintf(buf, INET6_ADDRSTRLEN, "%pI6", addr);
>>  }
> 
> 
> This seems to be the once case where the return value is not ignored.
> 
> Of the 4 callers of qeth_l3_ipaddr_to_string, two don't ignore the return
> value. And I agree in those cases this change seems correct.
> 
> However, amongst other usages of the return value,
> those callers also check for a return < 0 from this function.
> Can that occur, in the sprintf or scnprintf case?

I was under the impression this was a safeguard against a bad address format,
but I tried it out and it never resulted in a negative return.
Thanks a lot for pointing this out, we can further simplify patch 3 with that.