2015-12-15 14:29:39

by Jisheng Zhang

Subject: [PATCH] watchdog: dw_wdt: fix signedness bug in dw_wdt_top_in_seconds()

On 64bit platforms, "(1 << (16 + top)) / clk_get_rate(dw_wdt.clk)" is
sign-extended to 64bit then converted to unsigned 64bit, finally divide
the clk rate. If the top is the maximum TOP i.e 15, "(1 << (16 +15))"
will be sign-extended to 0xffffffff80000000, then converted to unsigned
0xffffffff80000000, which is a huge number, thus the final result is
wrong.

We fix this issue by giving usigned value(1U in this case) at first.

Let's assume clk rate is 25MHZ,
Before the patch:
dw_wdt_top_in_seconds(15) = -864612050

After the patch:
dw_wdt_top_in_seconds(15) = 85

Signed-off-by: Jisheng Zhang <[email protected]>
---
drivers/watchdog/dw_wdt.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/watchdog/dw_wdt.c b/drivers/watchdog/dw_wdt.c
index 6ea0634..8fefa4ad 100644
--- a/drivers/watchdog/dw_wdt.c
+++ b/drivers/watchdog/dw_wdt.c
@@ -81,7 +81,7 @@ static inline int dw_wdt_top_in_seconds(unsigned top)
* There are 16 possible timeout values in 0..15 where the number of
* cycles is 2 ^ (16 + i) and the watchdog counts down.
*/
- return (1 << (16 + top)) / clk_get_rate(dw_wdt.clk);
+ return (1U << (16 + top)) / clk_get_rate(dw_wdt.clk);
}

static int dw_wdt_get_top(void)
--
2.6.4

2015-12-15 16:15:44

by Guenter Roeck

Subject: Re: [PATCH] watchdog: dw_wdt: fix signedness bug in dw_wdt_top_in_seconds()

On 12/15/2015 06:25 AM, Jisheng Zhang wrote:
> On 64bit platforms, "(1 << (16 + top)) / clk_get_rate(dw_wdt.clk)" is
> sign-extended to 64bit then converted to unsigned 64bit, finally divide
> the clk rate. If the top is the maximum TOP i.e 15, "(1 << (16 +15))"
> will be sign-extended to 0xffffffff80000000, then converted to unsigned
> 0xffffffff80000000, which is a huge number, thus the final result is
> wrong.
>
> We fix this issue by giving usigned value(1U in this case) at first.
>
> Let's assume clk rate is 25MHZ,
> Before the patch:
> dw_wdt_top_in_seconds(15) = -864612050
>
> After the patch:
> dw_wdt_top_in_seconds(15) = 85
>
> Signed-off-by: Jisheng Zhang <[email protected]>

Nice catch.

Reviewed-by: Guenter Roeck <[email protected]>

> ---
> drivers/watchdog/dw_wdt.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/watchdog/dw_wdt.c b/drivers/watchdog/dw_wdt.c
> index 6ea0634..8fefa4ad 100644
> --- a/drivers/watchdog/dw_wdt.c
> +++ b/drivers/watchdog/dw_wdt.c
> @@ -81,7 +81,7 @@ static inline int dw_wdt_top_in_seconds(unsigned top)
> * There are 16 possible timeout values in 0..15 where the number of
> * cycles is 2 ^ (16 + i) and the watchdog counts down.
> */
> - return (1 << (16 + top)) / clk_get_rate(dw_wdt.clk);
> + return (1U << (16 + top)) / clk_get_rate(dw_wdt.clk);
> }
>
> static int dw_wdt_get_top(void)
>

2015-12-27 20:06:11

by Wim Van Sebroeck

Subject: Re: [PATCH] watchdog: dw_wdt: fix signedness bug in dw_wdt_top_in_seconds()

Hi Jisheng,

> On 64bit platforms, "(1 << (16 + top)) / clk_get_rate(dw_wdt.clk)" is
> sign-extended to 64bit then converted to unsigned 64bit, finally divide
> the clk rate. If the top is the maximum TOP i.e 15, "(1 << (16 +15))"
> will be sign-extended to 0xffffffff80000000, then converted to unsigned
> 0xffffffff80000000, which is a huge number, thus the final result is
> wrong.
>
> We fix this issue by giving usigned value(1U in this case) at first.
>
> Let's assume clk rate is 25MHZ,
> Before the patch:
> dw_wdt_top_in_seconds(15) = -864612050
>
> After the patch:
> dw_wdt_top_in_seconds(15) = 85
>
> Signed-off-by: Jisheng Zhang <[email protected]>
> ---
> drivers/watchdog/dw_wdt.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/watchdog/dw_wdt.c b/drivers/watchdog/dw_wdt.c
> index 6ea0634..8fefa4ad 100644
> --- a/drivers/watchdog/dw_wdt.c
> +++ b/drivers/watchdog/dw_wdt.c
> @@ -81,7 +81,7 @@ static inline int dw_wdt_top_in_seconds(unsigned top)
> * There are 16 possible timeout values in 0..15 where the number of
> * cycles is 2 ^ (16 + i) and the watchdog counts down.
> */
> - return (1 << (16 + top)) / clk_get_rate(dw_wdt.clk);
> + return (1U << (16 + top)) / clk_get_rate(dw_wdt.clk);
> }
>
> static int dw_wdt_get_top(void)
> --
> 2.6.4
>

Patch is added to linux-watchdog-next.

Kind regards,
Wim.