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

[Marijn Suijten]

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@xxxxxxxxxxxxxx>
> > Reviewed-By: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxxx>
> > ---
> >  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.

> This warning occurs even when there is no ambiguity.
> 
> This could be:
> 
> 	if (string_len > 0 && val != string_len)
> 
> Combined these changes allows us to give a much more helpful and assertive
> warning message:
> 
> qcom,num-strings mis-matches and will partially override
> qcom,enabled-strings (remove qcom,num-strings?)

I want to let the user know it's set regardless of whether they're
equivalent; no need to set both.

How about:

    Only one of qcom,num-strings or qcom,enabled-strings should be set

That should be more descriptive?  Otherwise, let me know if you really
want to allow users to (unnecessarily) set both - or if it can / should
be caught in DT validation instead.

- Marijn

> > +			if (val > string_len) {
> > +				dev_err(dev, "qcom,num-strings exceeds qcom,enabled-strings\n");
> > +				return -EINVAL;
> > +			}
> > +		}
> 
> 
> Daniel.