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SubjectRe: [PATCH] stm class: Fix out of bound access from bitmap allocation
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On 08/04/2019 11:33, David Laight wrote:
> From: Robin Murphy
>> Sent: 08 April 2019 11:24
>> 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)]);
>
> Except that is invalid.
> You can't use offsetof() with something that isn't a compile time constant.

Oh, I see the standard does actually say that, although there seem to be
enough non-constant uses in the kernel to suggest that it still works in
practice.

However, while writing the above I was still trying to remember the
other thing I'd seen for handling precisely this variable-sized-struct
situation, which I've now found again, namely struct_size(). I guess now
I understand why that isn't implemented in terms of offsetof(), thanks
for the nudge :)

Robin.

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