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[80.7.220.175]) by smtp.gmail.com with ESMTPSA id h7sm13665207wrt.64.2021.11.15.03.23.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 15 Nov 2021 03:23:29 -0800 (PST) Date: Mon, 15 Nov 2021 11:23:27 +0000 From: Daniel Thompson To: Marijn Suijten , phone-devel@vger.kernel.org, Andy Gross , Bjorn Andersson , Rob Herring , Lee Jones , Jingoo Han , ~postmarketos/upstreaming@lists.sr.ht, AngeloGioacchino Del Regno , Konrad Dybcio , Martin Botka , Jami Kettunen , Pavel Dubrova , Kiran Gunda , Bryan Wu , linux-arm-msm@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, Courtney Cavin Subject: Re: [RESEND PATCH v2 04/13] backlight: qcom-wled: Fix off-by-one maximum with default num_strings Message-ID: <20211115112327.tklic3fggrv5mzjt@maple.lan> References: <20211112002706.453289-1-marijn.suijten@somainline.org> <20211112002706.453289-5-marijn.suijten@somainline.org> <20211112120839.i6g747vewg6bkyk7@maple.lan> <20211112123501.pz5e6g7gavlinung@SoMainline.org> <20211112214337.r5xrpeyjgdygzc3n@SoMainline.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211112214337.r5xrpeyjgdygzc3n@SoMainline.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > > > Reviewed-By: AngeloGioacchino Del Regno > > > > --- > > > > 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.