Re: [PATCH nft] src: osf: fix snprintf -Wformat-truncation warning

[Fernando Fernandez Mancera <ffmancera@riseup.net>]

Hi Phil,

On 7/20/19 10:21 PM, Phil Sutter wrote:
> Hi Fernando,
> 
> On Thu, Jul 18, 2019 at 01:01:46PM +0200, Fernando Fernandez Mancera wrote:
>> Fedora 30 uses very recent gcc (version 9.1.1 20190503 (Red Hat 9.1.1-1)),
>> osf produces following warnings:
>>
>> -Wformat-truncation warning have been introduced in the version 7.1 of gcc.
>> Also, remove a unneeded address check of "tmp + 1" in nf_osf_strchr().
>>
>> nfnl_osf.c: In function ‘nfnl_osf_load_fingerprints’:
>> nfnl_osf.c:292:39: warning: ‘%s’ directive output may be truncated writing
>> up to 1023 bytes into a region of size 128 [-Wformat-truncation=]
>>   292 |   cnt = snprintf(obuf, sizeof(obuf), "%s,", pbeg);
>>       |                                       ^~
>> nfnl_osf.c:292:9: note: ‘snprintf’ output between 2 and 1025 bytes into a
>> destination of size 128
>>   292 |   cnt = snprintf(obuf, sizeof(obuf), "%s,", pbeg);
>>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> nfnl_osf.c:302:46: warning: ‘%s’ directive output may be truncated writing
>> up to 1023 bytes into a region of size 32 [-Wformat-truncation=]
>>   302 |    cnt = snprintf(f.genre, sizeof(f.genre), "%s", pbeg);
>>       |                                              ^~
>> nfnl_osf.c:302:10: note: ‘snprintf’ output between 1 and 1024 bytes into a
>> destination of size 32
>>   302 |    cnt = snprintf(f.genre, sizeof(f.genre), "%s", pbeg);
>>       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> nfnl_osf.c:309:49: warning: ‘%s’ directive output may be truncated writing
>> up to 1023 bytes into a region of size 32 [-Wformat-truncation=]
>>   309 |   cnt = snprintf(f.version, sizeof(f.version), "%s", pbeg);
>>       |                                                 ^~
>> nfnl_osf.c:309:9: note: ‘snprintf’ output between 1 and 1024 bytes into a
>> destination of size 32
>>   309 |   cnt = snprintf(f.version, sizeof(f.version), "%s", pbeg);
>>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> nfnl_osf.c:317:47: warning: ‘%s’ directive output may be truncated writing
>> up to 1023 bytes into a region of size 32 [-Wformat-truncation=]
>>   317 |       snprintf(f.subtype, sizeof(f.subtype), "%s", pbeg);
>>       |                                               ^~
>> nfnl_osf.c:317:7: note: ‘snprintf’ output between 1 and 1024 bytes into a
>> destination of size 32
>>   317 |       snprintf(f.subtype, sizeof(f.subtype), "%s", pbeg);
>>       |       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> Reported-by: Florian Westphal <fw@strlen.de>
>> Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
>> ---
>>  src/nfnl_osf.c | 14 +++++++-------
>>  1 file changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/src/nfnl_osf.c b/src/nfnl_osf.c
>> index be3fd81..c99f8f3 100644
>> --- a/src/nfnl_osf.c
>> +++ b/src/nfnl_osf.c
>> @@ -81,7 +81,7 @@ static char *nf_osf_strchr(char *ptr, char c)
>>  	if (tmp)
>>  		*tmp = '\0';
>>  
>> -	while (tmp && tmp + 1 && isspace(*(tmp + 1)))
>> +	while (tmp && isspace(*(tmp + 1)))
>>  		tmp++;
>>  
>>  	return tmp;
>> @@ -212,7 +212,7 @@ static int osf_load_line(char *buffer, int len, int del,
>>  			 struct netlink_ctx *ctx)
>>  {
>>  	int i, cnt = 0;
>> -	char obuf[MAXOPTSTRLEN];
>> +	char obuf[MAXOPTSTRLEN + 1];
>>  	struct nf_osf_user_finger f;
>>  	char *pbeg, *pend;
>>  	struct nlmsghdr *nlh;
>> @@ -289,7 +289,7 @@ static int osf_load_line(char *buffer, int len, int del,
>>  	pend = nf_osf_strchr(pbeg, OSFPDEL);
>>  	if (pend) {
>>  		*pend = '\0';
>> -		cnt = snprintf(obuf, sizeof(obuf), "%s,", pbeg);
>> +		cnt = snprintf(obuf, sizeof(obuf), "%.128s", pbeg);
> 
> Not a big deal, but sizeof() and hard-coding the "precision" doesn't mix
> well in my opinion. I've solved this like so:
> 
> 		i = sizeof(obuf);
> 		cnt = snprintf(obuf, i, "%.*s,", i - 2, pbeg);
> 
> (i - 2) to leave space for the trailing comma and nul-char. Also note
> that your patch drops the trailing comma, I guess that's a bug.
> 

Oh! I am really happy that you spotted the missing trailing comma,
thanks! :-)

> Maybe you want to have a look at my patch (Message-ID
> 20190720185226.8876-2-phil@nwl.cc) and incorporate what's useful into
> yours? It's your code, so you should know better how to fix things. :)
> 
> Thanks, Phil
> 

I think your code is more readable than mine. I am going to send a v2
patch with your code but also adding the following fix.

-	while (tmp && tmp + 1 && isspace(*(tmp + 1)))
+	while (tmp && isspace(*(tmp + 1)))

I am going to send a similar patch for the iptables tree because this
file was imported from iptables.git/utils/nfnl_osf.c.

Thanks!