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

[Robin Murphy]

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)]);


Robin.