[PATCH v3 1/2] clk:Fix divide-by-zero in divider_ro_round_rate_parent

[PATCH v3 1/2] clk:Fix divide-by-zero in divider_ro_round_rate_parent

[nixiaoming]

In the function divider_recalc_rate() The judgment of the return value of
_get_div() indicates that the return value of _get_div() can be 0.
In order to avoid the divide-by-zero error, add check for return value
of _get_div() in the divider_ro_round_rate_parent()

Signed-off-by: nixiaoming <nixiaoming@huawei.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
---
 drivers/clk/clk-divider.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
index e5a1726..f4bf7a4 100644
--- a/drivers/clk/clk-divider.c
+++ b/drivers/clk/clk-divider.c
@@ -347,6 +347,9 @@ long divider_ro_round_rate_parent(struct clk_hw *hw, struct clk_hw *parent,
 	int div;
 
 	div = _get_div(table, val, flags, width);
+	/* avoid divide-by-zero */
+	if (!div)
+		return -EINVAL;
 
 	/* Even a read-only clock can propagate a rate change */
 	if (clk_hw_get_flags(hw) & CLK_SET_RATE_PARENT) {
-- 
1.8.5.6

Re: [PATCH v3 1/2] clk:Fix divide-by-zero in divider_ro_round_rate_parent

[Stephen Boyd]

Quoting nixiaoming (2019-03-30 06:54:50)
> In the function divider_recalc_rate() The judgment of the return value of
> _get_div() indicates that the return value of _get_div() can be 0.

When does _get_div() return 0? It can't be CLK_DIVIDER_MAX_AT_ZERO or
CLK_DIVIDER_POWER_OF_TWO. I suppose it could be CLK_DIVIDER_ONE_BASED if
CLK_DIVIDER_ALLOW_ZERO is set? Or just CLK_DIVIDER_ALLOW_ZERO is set? Or
a table that has 0 in it for some odd reason.

> In order to avoid the divide-by-zero error, add check for return value
> of _get_div() in the divider_ro_round_rate_parent()
> 
> Signed-off-by: nixiaoming <nixiaoming@huawei.com>
> Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
> ---
>  drivers/clk/clk-divider.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> index e5a1726..f4bf7a4 100644
> --- a/drivers/clk/clk-divider.c
> +++ b/drivers/clk/clk-divider.c
> @@ -347,6 +347,9 @@ long divider_ro_round_rate_parent(struct clk_hw *hw, struct clk_hw *parent,
>         int div;
>  
>         div = _get_div(table, val, flags, width);
> +       /* avoid divide-by-zero */
> +       if (!div)
> +               return -EINVAL;

Can you please give more details on what's happening here? Who's the
caller? What are the arguments being passed in? Shouldn't we check for
CLK_DIVIDER_ALLOW_ZERO and then return prate as it comes in instead of
returning an error?

>  
>         /* Even a read-only clock can propagate a rate change */
>         if (clk_hw_get_flags(hw) & CLK_SET_RATE_PARENT) {

RE: [PATCH v3 1/2] clk:Fix divide-by-zero in divider_ro_round_rate_parent

[Nixiaoming]

On Wed, Apr 24, 2019 at 6:52 AM Stephen Boyd <sboyd@kernel.org> wrote:
>Quoting nixiaoming (2019-03-30 06:54:50)
>> In the function divider_recalc_rate() The judgment of the return value of
>> _get_div() indicates that the return value of _get_div() can be 0.
>
>When does _get_div() return 0? It can't be CLK_DIVIDER_MAX_AT_ZERO or
>CLK_DIVIDER_POWER_OF_TWO. I suppose it could be CLK_DIVIDER_ONE_BASED if
>CLK_DIVIDER_ALLOW_ZERO is set? Or just CLK_DIVIDER_ALLOW_ZERO is set? Or
>a table that has 0 in it for some odd reason.
>
divider_ro_round_rate_parent() is an exported function.
There is no parameter check or return value check before 
and after calling _get_div(), which may result in a divide by zero error.

Case1: The "flags" contains CLK_DIVIDER_ONE_BASED, and "val" is 0.
Case2: The "flags" does not contain CLK_DIVIDER_ONE_BASED, 
    CLK_DIVIDER_POWER_OF_TWO, CLK_DIVIDER_MAX_AT_ZERO,  
    "table" is NULL.  "val" is 0xffffffff
In both cases _get_div() returns 0

>> In order to avoid the divide-by-zero error, add check for return value
>> of _get_div() in the divider_ro_round_rate_parent()
>> 
>> Signed-off-by: nixiaoming <nixiaoming@huawei.com>
>> Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
>> ---
>>  drivers/clk/clk-divider.c | 3 +++
>>  1 file changed, 3 insertions(+)
>> 
>> diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
>> index e5a1726..f4bf7a4 100644
>> --- a/drivers/clk/clk-divider.c
>> +++ b/drivers/clk/clk-divider.c
>> @@ -347,6 +347,9 @@ long divider_ro_round_rate_parent(struct clk_hw *hw, struct clk_hw *parent,
>>         int div;
>>  
>>         div = _get_div(table, val, flags, width);
>> +       /* avoid divide-by-zero */
>> +       if (!div)
>> +               return -EINVAL;
>
>Can you please give more details on what's happening here? Who's the
>caller? What are the arguments being passed in? Shouldn't we check for
>CLK_DIVIDER_ALLOW_ZERO and then return prate as it comes in instead of
>returning an error?
>
I found that there may be a divide-by-zero error by code review,
 for example: "flags" is CLK_DIVIDER_ONE_BASED and "val" is 0.
So simply add a return value check to avoid divide-by-zero

thanks for your suggestion, 
I will resend the patch later
refer to your advice and divider_recalc_rate() to add a check for CLK_DIVIDER_ALLOW_ZERO

thanks

RE: [PATCH v3 1/2] clk:Fix divide-by-zero in divider_ro_round_rate_parent

[Stephen Boyd]

Quoting Nixiaoming (2019-04-24 08:31:23)
> On Wed, Apr 24, 2019 at 6:52 AM Stephen Boyd <sboyd@kernel.org> wrote:
> >Quoting nixiaoming (2019-03-30 06:54:50)
> >> In the function divider_recalc_rate() The judgment of the return value of
> >> _get_div() indicates that the return value of _get_div() can be 0.
> >
> >When does _get_div() return 0? It can't be CLK_DIVIDER_MAX_AT_ZERO or
> >CLK_DIVIDER_POWER_OF_TWO. I suppose it could be CLK_DIVIDER_ONE_BASED if
> >CLK_DIVIDER_ALLOW_ZERO is set? Or just CLK_DIVIDER_ALLOW_ZERO is set? Or
> >a table that has 0 in it for some odd reason.
> >
> divider_ro_round_rate_parent() is an exported function.
> There is no parameter check or return value check before 
> and after calling _get_div(), which may result in a divide by zero error.
> 
> Case1: The "flags" contains CLK_DIVIDER_ONE_BASED, and "val" is 0.
> Case2: The "flags" does not contain CLK_DIVIDER_ONE_BASED, 
>     CLK_DIVIDER_POWER_OF_TWO, CLK_DIVIDER_MAX_AT_ZERO,  
>     "table" is NULL.  "val" is 0xffffffff
> In both cases _get_div() returns 0

Alright, so this is all theoretical and not happening in practice? Best
to do nothing then I think.

> 
> >> In order to avoid the divide-by-zero error, add check for return value
> >> of _get_div() in the divider_ro_round_rate_parent()
> >> 
> >> Signed-off-by: nixiaoming <nixiaoming@huawei.com>
> >> Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
> >> ---
> >>  drivers/clk/clk-divider.c | 3 +++
> >>  1 file changed, 3 insertions(+)
> >> 
> >> diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> >> index e5a1726..f4bf7a4 100644
> >> --- a/drivers/clk/clk-divider.c
> >> +++ b/drivers/clk/clk-divider.c
> >> @@ -347,6 +347,9 @@ long divider_ro_round_rate_parent(struct clk_hw *hw, struct clk_hw *parent,
> >>         int div;
> >>  
> >>         div = _get_div(table, val, flags, width);
> >> +       /* avoid divide-by-zero */
> >> +       if (!div)
> >> +               return -EINVAL;
> >
> >Can you please give more details on what's happening here? Who's the
> >caller? What are the arguments being passed in? Shouldn't we check for
> >CLK_DIVIDER_ALLOW_ZERO and then return prate as it comes in instead of
> >returning an error?
> >
> I found that there may be a divide-by-zero error by code review,
>  for example: "flags" is CLK_DIVIDER_ONE_BASED and "val" is 0.
> So simply add a return value check to avoid divide-by-zero
> 
> thanks for your suggestion, 
> I will resend the patch later
> refer to your advice and divider_recalc_rate() to add a check for CLK_DIVIDER_ALLOW_ZERO
> 

Ok. I'm actually wondering why divider_ro_round_rate_parent() doesn't
use clk_divider_bestdiv() instead of _get_div(). I think we did this to
make things simpler.

We could make clk_divider_bestdiv() take a 'val' argument to start the
divider loop (and make that 0 for the "normal" case) and then look at
the 'flags' for CLK_DIVIDER_READ_ONLY to restrict the 'maxdiv' and 'i'
arguments there to be whatever the val is passed in to be. It would
require passing the val into _get_maxdiv() too, and only caring about
the val when the read only flag is set.

Then I think we would naturally get div-by-zero avoidance code and we
could collapse the divider code paths substantially into
clk_divider_bestdiv(). It would be even better if that patch could come
after moving the clk-divider ops to use .determine_rate() instead of
.round_rate() clk_ops. If we did that then we could enhance the divider
code to handle rate request clamping.