clk: Sanitize possible_parent_show to Handle Return Value of of_clk_get_parent_name

In the possible_parent_show function, ensure proper handling of the return
value from of_clk_get_parent_name to prevent potential issues arising from
a NULL return.
The current implementation invokes seq_puts directly on the result of
of_clk_get_parent_name without verifying the return value, which can lead
to kernel panic if the function returns NULL.

This patch addresses the concern by introducing a check on the return
value of of_clk_get_parent_name. If the return value is not NULL, the
function proceeds to call seq_puts, providing the returned value as
argument.
However, if of_clk_get_parent_name returns NULL, the function provides a
static string as argument, avoiding the panic.

Fixes: 1ccc0ddf ("clk: Use seq_puts() in possible_parent_show()")
Reported-by: Philip Daly <pdaly@redhat.com>
Signed-off-by: Alessandro Carminati (Red Hat) <alessandro.carminati@gmail.com>
Link: https://lore.kernel.org/r/20230921073217.572151-1-alessandro.carminati@gmail.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/clk.c

@@ -3416,6 +3416,7 @@ static void possible_parent_show(struct seq_file *s, struct clk_core *core,
 				 unsigned int i, char terminator)
 {
 	struct clk_core *parent;
+	const char *name = NULL;
 
 	/*
 	 * Go through the following options to fetch a parent's name.
@@ -3430,18 +3431,20 @@ static void possible_parent_show(struct seq_file *s, struct clk_core *core,
 	 * registered (yet).
 	 */
 	parent = clk_core_get_parent_by_index(core, i);
-	if (parent)
+	if (parent) {
 		seq_puts(s, parent->name);
-	else if (core->parents[i].name)
+	} else if (core->parents[i].name) {
 		seq_puts(s, core->parents[i].name);
-	else if (core->parents[i].fw_name)
+	} else if (core->parents[i].fw_name) {
 		seq_printf(s, "<%s>(fw)", core->parents[i].fw_name);
-	else if (core->parents[i].index >= 0)
-		seq_puts(s,
-			 of_clk_get_parent_name(core->of_node,
-						core->parents[i].index));
-	else
-		seq_puts(s, "(missing)");
+	} else {
+		if (core->parents[i].index >= 0)
+			name = of_clk_get_parent_name(core->of_node, core->parents[i].index);
+		if (!name)
+			name = "(missing)";
+
+		seq_puts(s, name);
+	}
 
 	seq_putc(s, terminator);
 }