[PATCH] clk: clk-loongson2: Fix memory corruption bug in struct loongson2_clk_provider

[Gustavo A. R. Silva]

Some heap space is allocated for the flexible structure `struct
clk_hw_onecell_data` and its flexible-array member `hws` through
the composite structure `struct loongson2_clk_provider` in function
`loongson2_clk_probe()`, as shown below:

289         struct loongson2_clk_provider *clp;
        ...
296         for (p = data; p->name; p++)
297                 clks_num++;
298
299         clp = devm_kzalloc(dev, struct_size(clp, clk_data.hws, clks_num),
300                            GFP_KERNEL);

Then some data is written into the flexible array:

350                 clp->clk_data.hws[p->id] = hw;

This corrupts `clk_lock`, which is the spinlock variable immediately
following the `clk_data` member in `struct loongson2_clk_provider`:

struct loongson2_clk_provider {
        void __iomem *base;
        struct device *dev;
        struct clk_hw_onecell_data clk_data;
        spinlock_t clk_lock;    /* protect access to DIV registers */
};

The problem is that the flexible structure is currently placed in the
middle of `struct loongson2_clk_provider` instead of at the end.

Fix this by moving `struct clk_hw_onecell_data clk_data;` to the end of
`struct loongson2_clk_provider`. Also, add a code comment to help
prevent this from happening again in case new members are added to the
structure in the future.

This change also fixes the following -Wflex-array-member-not-at-end
warning:

drivers/clk/clk-loongson2.c:32:36: warning: structure containing a flexible 
array member is not at the end of another structure 
[-Wflex-array-member-not-at-end]

Fixes: 9796ec0bd04b ("clk: clk-loongson2: Refactor driver for adding new 
platforms")
Cc: sta...@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo...@kernel.org>
---
 drivers/clk/clk-loongson2.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/clk/clk-loongson2.c b/drivers/clk/clk-loongson2.c
index 820bb1e9e3b7..e99ba79feec6 100644
--- a/drivers/clk/clk-loongson2.c
+++ b/drivers/clk/clk-loongson2.c
@@ -29,8 +29,10 @@ enum loongson2_clk_type {
 struct loongson2_clk_provider {
        void __iomem *base;
        struct device *dev;
-       struct clk_hw_onecell_data clk_data;
        spinlock_t clk_lock;    /* protect access to DIV registers */
+
+       /* Must be last --ends in a flexible-array member. */
+       struct clk_hw_onecell_data clk_data;
 };
 
 struct loongson2_clk_data {
-- 
2.43.0