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

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Date:   Tue, 3 Nov 2020 08:26:51 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Jakub Kicinski' <kuba@...nel.org>, Andrew Lunn <andrew@...n.ch>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] drivers: net: sky2: Fix -Wstringop-truncation
 with W=1

On Tue, 3 Nov 2020 08:25:01 -0800
Stephen Hemminger <stephen@...workplumber.org> wrote:

> On Tue, 3 Nov 2020 10:19:55 +0000
> David Laight <David.Laight@...LAB.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@...n.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?

Like this is what I meant...
diff --git a/drivers/net/ethernet/marvell/sky2.c b/drivers/net/ethernet/marvell/sky2.c
index 25981a7a43b5..ebe1406c6e64 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, "%s", name[chipid - CHIP_ID_YUKON_XL]);
        else
                snprintf(buf, sz, "(chip %#x)", chipid);
        return buf;