On Fri, Apr 16, 2021 at 3:19 AM Dan Carpenter [off-list ref] wrote:

On Fri, Apr 09, 2021 at 03:46:47PM +0900, Masahiro Yamada wrote:

quoted

On Thu, Apr 8, 2021 at 12:25 AM Colin King [off-list ref] wrote:

quoted

From: Colin Ian King <redacted>

The for-loop iterates with a u8 loop counter i and compares this
with the loop upper limit of num_parents that is an int type.
There is a potential infinite loop if num_parents is larger than
the u8 loop counter. Fix this by making the loop counter the same
type as num_parents.

Addresses-Coverity: ("Infinite loop")
Fixes: 734d82f4a678 ("clk: uniphier: add core support code for UniPhier clock driver")
Signed-off-by: Colin Ian King <redacted>
---
 drivers/clk/uniphier/clk-uniphier-mux.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/clk/uniphier/clk-uniphier-mux.c b/drivers/clk/uniphier/clk-uniphier-mux.c
index 462c84321b2d..ce219e0d2a85 100644
--- a/drivers/clk/uniphier/clk-uniphier-mux.c
+++ b/drivers/clk/uniphier/clk-uniphier-mux.c

@@ -34,7 +34,7 @@ static u8 uniphier_clk_mux_get_parent(struct clk_hw *hw)
        int num_parents = clk_hw_get_num_parents(hw);
        int ret;
        unsigned int val;
-       u8 i;
+       int i;

        ret = regmap_read(mux->regmap, mux->reg, &val);
        if (ret)
--

2.30.2

clk_hw_get_num_parents() returns 'unsigned int', so
I think 'num_parents' should also have been 'unsigned int'.

Maybe, the loop counter 'i' also should be 'unsigned int' then?

The clk_hw_get_num_parents() function returns 0-255 so the original code
works fine.

True.  clk->core->num_parents is u8,
but it is not clear just by looking at the
prototype of clk_hw_get_num_parents().

At least, it is not clear enough for tools,
and actually Coverity raised a flag.


Personally, I prefer 'unsigned int' (or 'int')
when I count the number of something.
Historically, the clk subsystem uses u8,
(maybe to save memory??), and there exists
distortion.

For example, the return type of
uniphier_clk_mux_get_parent() is u8,
but it actually returns -EINVAL for error cases.

So, u8 is not wide enough, in my opinion.

It should basically always be "int i;"  That's the safest assumption.
There are other case where it has to be size_t but in those cases I
think people should call the list iterator something else instead of "i"
like "size_t pg_idx;".

Making everthing u32 causes more bugs than it prevents.  Signedness bugs
with comparing to zero, type promotion bugs, or subtraction bugs where
subtracting wraps to a high value.  It's rare to loop more than INT_MAX
times in the kernel.  When we do need to count about 2 million then
we're probably not going to stop counting at 4 million, we're going to
go to 10 million or higher so size_t is more appropriate than u32.

Btw, if you have a loop that does:

        for (i = 0; i < UINT_MAX; i++) {

that loop works exactly the same if "i" is an int or if it's a u32
because of type promotion.

You are right.

Perhaps, in hindsight, the following were natural:


   unsigned int num_parents = clk_hw_get_num_parents(hw);
   ...
   int i;


I am fine with this if it is not too late.
But, Stephen has already picked up this patch.

 So you have to look really hard to find a
place where changing a loop iterator from int to u32 fixes bug in real
life.

regards,
dan carpenter

-- 
Best Regards
Masahiro Yamada

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