[PATCH 12/16] clk: avoid circular clock topology

From: Masahiro Yamada
Date: Mon Nov 30 2015 - 02:37:37 EST


Currently, clk_register() never checks a circular parent looping,
but clock providers could register such an insane clock topology.
For example, "clk_a" could have "clk_b" as a parent, and vice versa.
In this case, clk_core_reparent() creates a circular parent list
and __clk_recalc_accuracies() calls itself recursively forever.

The core infrastructure should be kind enough to bail out, showing
an appropriate error message in such a case. This helps to easily
find a bug in clock providers. (uh, I made such a silly mistake
when I was implementing my clock providers first. I was upset
because the kernel did not respond, without any error message.)

This commit adds a new helper function, __clk_is_ancestor(). It
returns true if the second argument is a possible ancestor of the
first one. If a clock core is a possible ancestor of itself, it
would make a loop when it were registered. That should be detected
as an error.

Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
---

drivers/clk/clk.c | 40 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 40 insertions(+)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index ef6fedb..a1d046c 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -2235,6 +2235,38 @@ static inline void clk_debug_unregister(struct clk_core *core)
#endif

/**
+ * __clk_is_ancestor - check if a clk_core is a possible ancestor of another
+ * @core: clock core
+ * @ancestor: ancestor clock core
+ *
+ * Returns true if there is a possibility that @ancestor can be an ancestor
+ * of @core, false otherwise.
+ *
+ * This function can be used against @core or @ancestor that has not been
+ * registered yet.
+ */
+static bool __clk_is_ancestor(struct clk_core *core, struct clk_core *ancestor)
+{
+ struct clk_core *parent;
+ int i;
+
+ for (i = 0; i < core->num_parents; i++) {
+ parent = clk_core_get_parent_by_index(core, i);
+ /*
+ * If ancestor has not been added to clk_{root,orphan}_list
+ * yet, clk_core_lookup() cannot find it. If parent is NULL,
+ * compare the name strings, too.
+ */
+ if ((parent && (parent == ancestor ||
+ __clk_is_ancestor(parent, ancestor))) ||
+ (!parent && !strcmp(core->parent_names[i], ancestor->name)))
+ return true;
+ }
+
+ return false;
+}
+
+/**
* __clk_core_init - initialize the data structures in a struct clk_core
* @core: clk_core being initialized
*
@@ -2297,6 +2329,14 @@ static int __clk_core_init(struct clk_core *core)
"%s: invalid NULL in %s's .parent_names\n",
__func__, core->name);

+ /* If core is an ancestor of itself, it would make a loop. */
+ if (__clk_is_ancestor(core, core)) {
+ pr_err("%s: %s would create circular parent\n", __func__,
+ core->name);
+ ret = -EINVAL;
+ goto out;
+ }
+
core->parent = __clk_init_parent(core);

/*
--
1.9.1

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/