linux-kernel - Re: [RESEND PATCH v2 04/13] backlight: qcom-wled: Fix off-by-one maximum with default num

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Message-ID: <20211115112327.tklic3fggrv5mzjt@maple.lan>
Date:   Mon, 15 Nov 2021 11:23:27 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Marijn Suijten <marijn.suijten@...ainline.org>,
        phone-devel@...r.kernel.org, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        ~postmarketos/upstreaming@...ts.sr.ht,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Martin Botka <martin.botka@...ainline.org>,
        Jami Kettunen <jami.kettunen@...ainline.org>,
        Pavel Dubrova <pashadubrova@...il.com>,
        Kiran Gunda <kgunda@...eaurora.org>,
        Bryan Wu <cooloney@...il.com>, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        Courtney Cavin <courtney.cavin@...ymobile.com>
Subject: Re: [RESEND PATCH v2 04/13] backlight: qcom-wled: Fix off-by-one
 maximum with default num_strings

On Fri, Nov 12, 2021 at 10:43:37PM +0100, Marijn Suijten wrote:
> On 2021-11-12 13:35:03, Marijn Suijten wrote:
> > On 2021-11-12 12:08:39, Daniel Thompson wrote:
> > > On Fri, Nov 12, 2021 at 01:26:57AM +0100, Marijn Suijten wrote:
> > > > When not specifying num-strings in the DT the default is used, but +1 is
> > > > added to it which turns WLED3 into 4 and WLED4/5 into 5 strings instead
> > > > of 3 and 4 respectively, causing out-of-bounds reads and register
> > > > read/writes.  This +1 exists for a deficiency in the DT parsing code,
> > > > and is simply omitted entirely - solving this oob issue - by parsing the
> > > > property separately much like qcom,enabled-strings.
> > > > 
> > > > This also allows more stringent checks on the maximum value when
> > > > qcom,enabled-strings is provided in the DT.  Note that num-strings is
> > > > parsed after enabled-strings to give it final sign-off over the length,
> > > > which DT currently utilizes to get around an incorrect fixed read of
> > > > four elements from that array (has been addressed in a prior patch).
> > > > 
> > > > Fixes: 93c64f1ea1e8 ("leds: add Qualcomm PM8941 WLED driver")
> > > > Signed-off-by: Marijn Suijten <marijn.suijten@...ainline.org>
> > > > Reviewed-By: AngeloGioacchino Del Regno <angelogioacchino.delregno@...ainline.org>
> > > > ---
> > > >  drivers/video/backlight/qcom-wled.c | 51 +++++++++++------------------
> > > >  1 file changed, 19 insertions(+), 32 deletions(-)
> > > > 
> > > > diff --git a/drivers/video/backlight/qcom-wled.c b/drivers/video/backlight/qcom-wled.c
> > > > index 977cd75827d7..c5232478a343 100644
> > > > --- a/drivers/video/backlight/qcom-wled.c
> > > > +++ b/drivers/video/backlight/qcom-wled.c
> > > > @@ -1552,6 +1520,25 @@ static int wled_configure(struct wled *wled)
> > > >  		}
> > > >  	}
> > > > 
> > > > +	rc = of_property_read_u32(dev->of_node, "qcom,num-strings", &val);
> > > > +	if (!rc) {
> > > > +		if (val < 1 || val > wled->max_string_count) {
> > > > +			dev_err(dev, "qcom,num-strings must be between 1 and %d\n",
> > > > +				wled->max_string_count);
> > > > +			return -EINVAL;
> > > > +		}
> > > > +
> > > > +		if (string_len > 0) {
> > > > +			dev_warn(dev, "qcom,num-strings and qcom,enabled-strings are ambiguous\n");
> > > 
> > > The warning should also be below the error message on the next if statement.
> > 
> > Agreed.
> 
> Thinking about this again while reworking the patches, I initially put
> this above the error to make DT writers aware.  There's no point telling
> them that their values are out of sync (num-strings >
> len(enabled-strings)), when they "shouldn't even" (don't need to) set
> both in the first place.  They might needlessly fix the discrepancy, see
> the driver finally probe (working backlight) and carry on without
> noticing this warning that now appears.
> 
> Sorry for bringing this back up, but I'm curious about your opinion.

With a more helpful warning about how to fix then I think it is OK to
have both the warning and the error.


Daniel.