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

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Date:   Mon, 2 Nov 2020 16:01:06 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] drivers: net: sky2: Fix -Wstringop-truncation
 with W=1

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;