linux-kernel - Re: [PATCH 23/35] media: dvb-frontends: tda10048: Use the right div

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Date: Tue, 16 Apr 2024 12:39:33 +0200
From: Ricardo Ribalda <ribalda@...omium.org>
To: Dan Carpenter <dan.carpenter@...aro.org>
Cc: Martin Tuma <martin.tuma@...iteqautomotive.com>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
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Subject: Re: [PATCH 23/35] media: dvb-frontends: tda10048: Use the right div

Hi Dan

What about going the safe way?

--- a/drivers/media/dvb-frontends/tda10048.c
+++ b/drivers/media/dvb-frontends/tda10048.c
@@ -341,7 +341,7 @@ static int tda10048_set_wref(struct dvb_frontend
*fe, u32 sample_freq_hz,
        /* t *= 2147483648 on 32bit platforms */
        t *= (2048 * 1024);
        t *= 1024;
-       z = 7 * sample_freq_hz;
+       z = (u64)7 * sample_freq_hz;
        t = div64_u64(t, z) + 5;
        do_div(t, 10);

@@ -409,6 +409,7 @@ static int tda10048_set_if(struct dvb_frontend *fe, u32 bw)
        struct tda10048_config *config = &state->config;
        int i;
        u32 if_freq_khz;
+       u64 sample_freq;

        dprintk(1, "%s(bw = %d)\n", __func__, bw);

@@ -450,9 +451,10 @@ static int tda10048_set_if(struct dvb_frontend *fe, u32 bw)
        dprintk(1, "- pll_pfactor = %d\n", state->pll_pfactor);

        /* Calculate the sample frequency */
-       state->sample_freq = state->xtal_hz * (state->pll_mfactor + 45);
-       state->sample_freq /= (state->pll_nfactor + 1);
-       state->sample_freq /= (state->pll_pfactor + 4);
+       sample_freq = (u64)state->xtal_hz * (state->pll_mfactor + 45);
+       do_div(sample_freq, state->pll_nfactor + 1);
+       do_div(sample_freq, state->pll_pfactor + 4);
+       state->sample_freq = sample_freq;
        dprintk(1, "- sample_freq = %d\n", state->sample_freq);

        /* Update the I/F */

I will add a extra patch to fix tda10048_set_if

Thanks

PS: Thanks a lot for your reviews, really appreciate! I have a v2 with
your changes, I am giving it a couple of days before re-submitting

On Tue, 16 Apr 2024 at 12:10, Dan Carpenter <dan.carpenter@...aro.org> wrote:
>
> On Mon, Apr 15, 2024 at 07:34:40PM +0000, Ricardo Ribalda wrote:
> > z does not fit in 32 bits.
> >
>
> z has to fit in 32 bits otherwise there is a different bug.
>
> > Found by cocci:
> > drivers/media/dvb-frontends/tda10048.c:345:1-7: WARNING: do_div() does a 64-by-32 division, please consider using div64_u64 instead.
> >
> > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > ---
> >  drivers/media/dvb-frontends/tda10048.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/drivers/media/dvb-frontends/tda10048.c b/drivers/media/dvb-frontends/tda10048.c
> > index 5d5e4e9e4422..b176e7803e5b 100644
> > --- a/drivers/media/dvb-frontends/tda10048.c
> > +++ b/drivers/media/dvb-frontends/tda10048.c
> > @@ -342,8 +342,7 @@ static int tda10048_set_wref(struct dvb_frontend *fe, u32 sample_freq_hz,
> >       t *= (2048 * 1024);
> >       t *= 1024;
> >       z = 7 * sample_freq_hz;
>
> sample_freq_hz is a u32 so z can't be more than U32_MAX.  Perhaps there
> is an integer overflow bug on this line.
>
> The sample frequency is set in tda10048_set_if().
>
>         state->sample_freq = state->xtal_hz * (state->pll_mfactor + 45);
>
> ->xtal_hz is set earlier in tda10048_set_if() and it goes up to
> 16,000,000.  So if ->pll_mfactor is non-zero this line will have an
> integer overflow.  16million * 46 > U32_MAX.  Maybe when .clk_freq_khz
> is TDA10048_CLK_16000 then ->pll_mfactor is zero?  Ugh...
>
> > -     do_div(t, z);
> > -     t += 5;
> > +     t = div64_u64(t, z) + 5;
> >       do_div(t, 10);
>
> regards,
> dan carpenter
>


-- 
Ricardo Ribalda