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

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Oct 12, 2020 at 02:04:26PM +0100, Ferruh Yigit wrote:
> 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"?
>

Nope, no issue with checking that too.

/Bruce