Re: [PATCH net-next] drivers: net: sky2: Fix -Wstringop-truncation with W=1

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 3 Nov 2020 10:19:55 +0000
David Laight <David.Laight@ACULAB.COM> wrote:

> From: Jakub Kicinski
> > Sent: 03 November 2020 00:01
> > 
> > On Sat, 31 Oct 2020 18:40:28 +0100 Andrew Lunn wrote:  
> > > In function ‘strncpy’,
> > >     inlined from ‘sky2_name’ at drivers/net/ethernet/marvell/sky2.c:4903:3,
> > >     inlined from ‘sky2_probe’ at drivers/net/ethernet/marvell/sky2.c:5049:2:
> > > ./include/linux/string.h:297:30: warning: ‘__builtin_strncpy’ specified bound 16 equals destination  
> > size [-Wstringop-truncation]  
> > >
> > > None of the device names are 16 characters long, so it was never an
> > > issue, but reduce the length of the buffer size by one to avoid the
> > > warning.
> > >
> > > Signed-off-by: Andrew Lunn <andrew@lunn.ch>
> > > ---
> > >  drivers/net/ethernet/marvell/sky2.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/net/ethernet/marvell/sky2.c b/drivers/net/ethernet/marvell/sky2.c
> > > index 25981a7a43b5..35b0ec5afe13 100644
> > > --- a/drivers/net/ethernet/marvell/sky2.c
> > > +++ b/drivers/net/ethernet/marvell/sky2.c
> > > @@ -4900,7 +4900,7 @@ static const char *sky2_name(u8 chipid, char *buf, int sz)
> > >  	};
> > >
> > >  	if (chipid >= CHIP_ID_YUKON_XL && chipid <= CHIP_ID_YUKON_OP_2)
> > > -		strncpy(buf, name[chipid - CHIP_ID_YUKON_XL], sz);
> > > +		strncpy(buf, name[chipid - CHIP_ID_YUKON_XL], sz - 1);  
> > 
> > Hm. This irks the eye a little. AFAIK the idiomatic code would be:
> > 
> > 	strncpy(buf, name..., sz - 1);
> > 	buf[sz - 1] = '\0';
> > 
> > Perhaps it's easier to convert to strscpy()/strscpy_pad()?
> >   
> > >  	else
> > >  		snprintf(buf, sz, "(chip %#x)", chipid);
> > >  	return buf;  
> 
> Is the pad needed?
> It isn't present in the 'else' branch.

Since this is non-critical code and is only ther to print something useful
on boot, why not just use snprintf on both sides of statement?

diff --git a/drivers/net/ethernet/marvell/sky2.c b/drivers/net/ethernet/marvell/sky2.c
index 25981a7a43b5..96edad30006e 100644
--- a/drivers/net/ethernet/marvell/sky2.c
+++ b/drivers/net/ethernet/marvell/sky2.c
@@ -4900,7 +4900,7 @@ static const char *sky2_name(u8 chipid, char *buf, int sz)
        };
 
        if (chipid >= CHIP_ID_YUKON_XL && chipid <= CHIP_ID_YUKON_OP_2)
-               strncpy(buf, name[chipid - CHIP_ID_YUKON_XL], sz);
+               snprintf(buf, sz, name[chipid - CHIP_ID_YUKON_XL]);
        else
                snprintf(buf, sz, "(chip %#x)", chipid);
        return buf