backlight: qcom-wled: Fix off-by-one maximum with default num_strings

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 enables more stringent checks on the maximum value when
qcom,enabled-strings is provided in the DT, by parsing num-strings after
enabled-strings to allow it to check against (and in a subsequent patch
override) the length of enabled-strings: it is invalid to set
num-strings higher than that.
The DT currently utilizes it to get around an incorrect fixed read of
four elements from that array (has been addressed in a prior patch) by
setting a lower num-strings where desired.

Fixes: 93c64f1ea1 ("leds: add Qualcomm PM8941 WLED driver")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-By: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211115203459.1634079-5-marijn.suijten@somainline.org

drivers/video/backlight/qcom-wled.c

@@ -1255,21 +1255,6 @@ static const struct wled_var_cfg wled5_ovp_cfg = {
 	.size = 16,
 };
 
-static u32 wled3_num_strings_values_fn(u32 idx)
-{
-	return idx + 1;
-}
-
-static const struct wled_var_cfg wled3_num_strings_cfg = {
-	.fn = wled3_num_strings_values_fn,
-	.size = 3,
-};
-
-static const struct wled_var_cfg wled4_num_strings_cfg = {
-	.fn = wled3_num_strings_values_fn,
-	.size = 4,
-};
-
 static u32 wled3_switch_freq_values_fn(u32 idx)
 {
 	return 19200 / (2 * (1 + idx));
@@ -1343,11 +1328,6 @@ static int wled_configure(struct wled *wled)
 			.val_ptr = &cfg->switch_freq,
 			.cfg = &wled3_switch_freq_cfg,
 		},
-		{
-			.name = "qcom,num-strings",
-			.val_ptr = &cfg->num_strings,
-			.cfg = &wled3_num_strings_cfg,
-		},
 	};
 
 	const struct wled_u32_opts wled4_opts[] = {
@@ -1371,11 +1351,6 @@ static int wled_configure(struct wled *wled)
 			.val_ptr = &cfg->switch_freq,
 			.cfg = &wled3_switch_freq_cfg,
 		},
-		{
-			.name = "qcom,num-strings",
-			.val_ptr = &cfg->num_strings,
-			.cfg = &wled4_num_strings_cfg,
-		},
 	};
 
 	const struct wled_u32_opts wled5_opts[] = {
@@ -1399,11 +1374,6 @@ static int wled_configure(struct wled *wled)
 			.val_ptr = &cfg->switch_freq,
 			.cfg = &wled3_switch_freq_cfg,
 		},
-		{
-			.name = "qcom,num-strings",
-			.val_ptr = &cfg->num_strings,
-			.cfg = &wled4_num_strings_cfg,
-		},
 		{
 			.name = "qcom,modulator-sel",
 			.val_ptr = &cfg->mod_sel,
@@ -1522,8 +1492,6 @@ static int wled_configure(struct wled *wled)
 			*bool_opts[i].val_ptr = true;
 	}
 
-	cfg->num_strings = cfg->num_strings + 1;
-
 	string_len = of_property_count_elems_of_size(dev->of_node,
 						     "qcom,enabled-strings",
 						     sizeof(u32));
@@ -1554,6 +1522,22 @@ 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 && val > string_len) {
+			dev_err(dev, "qcom,num-strings exceeds qcom,enabled-strings\n");
+			return -EINVAL;
+		}
+
+		cfg->num_strings = val;
+	}
+
 	return 0;
 }