i2c: mux: demux-pinctrl: Annotate struct i2c_demux_pinctrl_priv with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct i2c_demux_pinctrl_priv.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
[wsa: improved blank lines]
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Author: kees

drivers/i2c/muxes/i2c-demux-pinctrl.c

@@ -32,7 +32,7 @@ struct i2c_demux_pinctrl_priv {
 	const char *bus_name;
 	struct i2c_adapter cur_adap;
 	struct i2c_algorithm algo;
-	struct i2c_demux_pinctrl_chan chan[];
+	struct i2c_demux_pinctrl_chan chan[] __counted_by(num_chan);
 };
 
 static int i2c_demux_master_xfer(struct i2c_adapter *adap, struct i2c_msg msgs[], int num)
@@ -226,6 +226,8 @@ static int i2c_demux_pinctrl_probe(struct platform_device *pdev)
 	if (!priv || !props)
 		return -ENOMEM;
 
+	priv->num_chan = num_chan;
+
 	err = of_property_read_string(np, "i2c-bus-name", &priv->bus_name);
 	if (err)
 		return err;
@@ -249,9 +251,7 @@ static int i2c_demux_pinctrl_probe(struct platform_device *pdev)
 		of_changeset_update_property(&priv->chan[i].chgset, adap_np, &props[i]);
 	}
 
-	priv->num_chan = num_chan;
 	priv->dev = &pdev->dev;
-
 	platform_set_drvdata(pdev, priv);
 
 	pm_runtime_no_callbacks(&pdev->dev);