RE: [PATCH] stm class: Fix out of bound access from bitmap allocation

[David Laight]

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)
> >>> ÂÂÂÂÂ struct stp_master *master;
> >>> ÂÂÂÂÂ size_t size;
> >>>
> >>> -ÂÂÂ size = ALIGN(stm->data->sw_nchannels, 8) / 8;
> >>> +ÂÂÂ 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.

	David

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