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

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Message-ID: <CAAXyoMOa1Ngze9VwwUJy0E7U52=w=fQE8cxwAviGm53MSQXVEA@mail.gmail.com>
Date: Mon, 27 Oct 2025 10:51:45 +0800
From: Yangfl <mmyangfl@...il.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: netdev@...r.kernel.org, Vladimir Oltean <olteanv@...il.com>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org, 
	Dan Carpenter <dan.carpenter@...aro.org>, David Laight <david.laight.linux@...il.com>
Subject: Re: [PATCH net-next v2 1/2] net: dsa: yt921x: Fix MIB overflow
 wraparound routine

On Mon, Oct 27, 2025 at 10:30 AM Andrew Lunn <andrew@...n.ch> wrote:
>
> 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@...aro.org>
> > Closes: https://lore.kernel.org/netdev/aPsjYKQMzpY0nSXm@stanley.mountain/
> > Suggested-by: David Laight <david.laight.linux@...il.com>
> > Signed-off-by: David Yang <mmyangfl@...il.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

I didn't find the definition for upper32_bits, so...