Re: [dpdk-dev] [PATCH] net/af_xdp: use snprintf instead of strncpy

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

On Fri, Oct 09, 2020 at 12:36:30PM +0200, Gaëtan Rivet wrote:
> On 07/10/20 12:45 +0100, Ferruh Yigit wrote:
> > On 10/7/2020 11:28 AM, Bruce Richardson wrote:
> > > On Wed, Oct 07, 2020 at 11:26:38AM +0100, Bruce Richardson wrote:
> > > > On Wed, Oct 07, 2020 at 11:51:31AM +0200, Olivier Matz wrote:
> > > > > On Wed, Oct 07, 2020 at 10:40:32AM +0100, Ferruh Yigit wrote:
> > > > > > On 10/7/2020 10:01 AM, Ciara Loftus wrote:
> > > > > > > strncpy may leave the destination buffer not NULL terminated so use
> > > > > > > snprintf instead.
> > > > > > 
> > > > > > What do you think using 'strlcpy'?
> > > > > 
> > > > > Or even better, rte_strscpy()
> > > > > https://git.dpdk.org/dpdk/commit/?id=b0236c7cf761
> > > > > 
> > > > I think this is largely a matter of preference, and unless there is a good
> > > > reason not to, I tend towards strlcpy as the older and more common (till
> > > > now) interface. The main thing is just to use a function that will
> > > > guarantee dest is null-terminated here, and both strlcpy and strscpy meet
> > > > that criteria.
> > > > 
> > > I'd also add that strlcpy is more likely to be recognised by tools like
> > > coverity, compared to rte_strscpy which is DPDK-specific.
> > > 
> > 
> > +1 to 'strlcpy'
> 
> Using strlcpy will be more recognized by static analyzer indeed.
> 
> But strscpy API is better:
> 
> * It helps checking string truncation by making it easier:
> 
>   if (strlcpy(dst, src, dstsize) >= dstsize)
>         /* Dev + reviewer needs to think about using >= and not >, dstsize is
>          * repeated so either dst is an array or it needs a dedicated variable.
>          * Deal with truncation.
>          */
> 
>   if (rte_strscpy(dst, src, dstsize) < 0)
>         /* deal with truncation. */
> 
> * It is safer when dealing with unknown data source. strlcpy will always
>   read all of src, because the API (uselessly) defines the return value
>   to strlen(src).
> 
> Having yet another string copy function is contentious, but we can avoid
> using worse API to please tools.
> 
> And detecting string truncation *is* helpful. String are used as IDs in
> DPDK for some objects. Using strlcpy / snprintf at least protects from
> buffer overflow, which is a bare minimum. A good implementation would
> also warn the user about a config error / memory corruption happening
> sooner.
> 
> In any case, sure to fix a sanity check strlcpy / snprintf will work.
> 

Yes.

My main issue with strscpy right now is that it's got to be a DPDK-specific
function, since AFAIK it's defined in no standard C library, just in the
Linux kernel. If we get strscpy added to e.g. glibc, then we can see about
starting to use it - letting meson do the work of detect if it's present
and allowing us to define a fallback only in case it's not (i.e. as is done
with strlcpy).