Messages in this thread | | | Subject | Re: [PATCH] stm class: Fix out of bound access from bitmap allocation | From | Robin Murphy <> | Date | Mon, 8 Apr 2019 11:52:23 +0100 |
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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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