Re: [PATCH net-next v2 1/2] net: dsa: yt921x: Fix MIB overflow wraparound routine

[Andrew Lunn <andrew@lunn.ch>]

On Sun, Oct 26, 2025 at 01:13:10AM +0800, David Yang wrote:
> Reported by the following Smatch static checker warning:
> 
>   drivers/net/dsa/yt921x.c:702 yt921x_read_mib()
>   warn: was expecting a 64 bit value instead of '(~0)'
> 
> Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
> Closes: https://lore.kernel.org/netdev/aPsjYKQMzpY0nSXm@stanley.mountain/
> Suggested-by: David Laight <david.laight.linux@gmail.com>
> Signed-off-by: David Yang <mmyangfl@gmail.com>
> ---
>  drivers/net/dsa/yt921x.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/dsa/yt921x.c b/drivers/net/dsa/yt921x.c
> index ab762ffc4661..97a7eeb4ea15 100644
> --- a/drivers/net/dsa/yt921x.c
> +++ b/drivers/net/dsa/yt921x.c
> @@ -687,21 +687,22 @@ static int yt921x_read_mib(struct yt921x_priv *priv, int port)
>  		const struct yt921x_mib_desc *desc = &yt921x_mib_descs[i];
>  		u32 reg = YT921X_MIBn_DATA0(port) + desc->offset;
>  		u64 *valp = &((u64 *)mib)[i];
> -		u64 val = *valp;
> +		u64 val;
>  		u32 val0;
> -		u32 val1;
>  
>  		res = yt921x_reg_read(priv, reg, &val0);
>  		if (res)
>  			break;
>  
>  		if (desc->size <= 1) {
> -			if (val < (u32)val)
> -				/* overflow */
> -				val += (u64)U32_MAX + 1;
> -			val &= ~U32_MAX;
> -			val |= val0;
> +			u64 old_val = *valp;
> +
> +			val = (old_val & ~(u64)U32_MAX) | val0;
> +			if (val < old_val)
> +				val += 1ull << 32;
>  		} else {
> +			u32 val1;
> +

What David suggested, https://lore.kernel.org/all/20251024132117.43f39504@pumpkin/ was

		if (desc->size <= 1) {
			u64 old_val = *valp;
			val = upper32_bits(old_val) | val0;
			if (val < old_val)
				val += 1ull << 32;
		}

I believe there is a minor typo here, it should be upper_32_bits(),
but what you implemented is not really what David suggested.

	Andrew