by Marijn Suijten

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

On 2021-11-15 11:23:27, Daniel Thompson wrote:
> 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 <[email protected]>
> > > > > Reviewed-By: AngeloGioacchino Del Regno <[email protected]>
> > > > > ---
> > > > > 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.

Thanks - I presume the message we settled upon last time is helpful
enough:

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

I'll respin this, together with this warning reordered into the next
commit, and using __le16 for the cpu_to_le16 output.

- Marijn