Re: [PATCH] net: dsa: realtek: rtl8365mb: replace deprecated strncpy with ethtool_sprintf

[Justin Stitt]

On Tue, Oct 10, 2023 at 10:36 AM Justin Stitt <justinstitt@xxxxxxxxxx> wrote:
>
> On Tue, Oct 10, 2023 at 4:07 AM Vladimir Oltean <olteanv@xxxxxxxxx> wrote:
> >
> > Hello Justin,
> >
> > On Mon, Oct 09, 2023 at 10:43:59PM +0000, Justin Stitt wrote:
> > > `strncpy` is deprecated for use on NUL-terminated destination strings
> > > [1] and as such we should prefer more robust and less ambiguous string
> > > interfaces.
> > >
> > > ethtool_sprintf() is designed specifically for get_strings() usage.
> > > Let's replace strncpy in favor of this more robust and easier to
> > > understand interface.
> > >
> > > Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> > > Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> > > Link: https://github.com/KSPP/linux/issues/90
> > > Cc: linux-hardening@xxxxxxxxxxxxxxx
> > > Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> > > ---
> > > Note: build-tested only.
> > > ---
> > >  drivers/net/dsa/realtek/rtl8365mb.c | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/net/dsa/realtek/rtl8365mb.c b/drivers/net/dsa/realtek/rtl8365mb.c
> > > index 41ea3b5a42b1..d171c18dd354 100644
> > > --- a/drivers/net/dsa/realtek/rtl8365mb.c
> > > +++ b/drivers/net/dsa/realtek/rtl8365mb.c
> > > @@ -1303,8 +1303,7 @@ static void rtl8365mb_get_strings(struct dsa_switch *ds, int port, u32 stringset
> > >
> > >       for (i = 0; i < RTL8365MB_MIB_END; i++) {
> > >               struct rtl8365mb_mib_counter *mib = &rtl8365mb_mib_counters[i];
> > > -
> > > -             strncpy(data + i * ETH_GSTRING_LEN, mib->name, ETH_GSTRING_LEN);
> > > +             ethtool_sprintf(&data, "%s", mib->name);
> >
> > Is there any particular reason why you opted for the "%s" printf format
> > specifier when you could have simply given mib->name as the single
> > argument? This comment applies to all the ethtool_sprintf() patches
> > you've submitted.
>
> Yeah, it causes a -Wformat-security warning for me. I briefly mentioned it
> in one of my first patches like this [1].

For more context, here's some warnings in the wild:
https://lore.kernel.org/netdev/20231003183603.3887546-3-jesse.brandeburg@xxxxxxxxx/

>
> [1]: https://lore.kernel.org/all/20231005-strncpy-drivers-net-dsa-lan9303-core-c-v2-1-feb452a532db@xxxxxxxxxx/