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SubjectRe: [PATCH] stm class: Fix out of bound access from bitmap allocation
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On 07/04/2019 05:31, Sai Prakash Ranjan wrote:
> On 4/5/2019 6:44 PM, David Laight wrote:
>> From: Sai Prakash Ranjan
>>>
>>> diff --git a/drivers/hwtracing/stm/core.c b/drivers/hwtracing/stm/core.c
>>> index 93ce3aa740a9..21a5838f6e67 100644
>>> --- a/drivers/hwtracing/stm/core.c
>>> +++ b/drivers/hwtracing/stm/core.c
>>> @@ -168,7 +168,7 @@ static int stp_master_alloc(struct stm_device
>>> *stm, unsigned int idx)
>>>       struct stp_master *master;
>>>       size_t size;
>>>
>>> -    size = ALIGN(stm->data->sw_nchannels, 8) / 8;
>>> +    size = ALIGN(stm->data->sw_nchannels, STM_MASTER_SZ) /
>>> STM_MASTER_SZ;
>>
>> I'm not sure that using STP_MASTER_SZ improves readability at all.
>>
>
> I thought it was better to have a macro than directly specifying
> sizeof(unsigned long), anyways I can change it.
>
>> Is there something that gives the size of a bitmap for 'n' items?
>>
>
> Not sure if there is something.

If you were to ask the question "how does the bitmap code itself know
what the total size of a bitmap is?", that would quickly lead you
towards BITS_TO_LONGS() ;)

And given that stp_master::chan_map is already an appropriate type, that
suggests simplifying the entire calculation down to something neat and
tidy like:

size = offsetof(struct stp_master,
chan_map[BITS_TO_LONGS(stm->data->sw_nchannels)]);


Robin.

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