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

[Robin Murphy]

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)
> > > > >  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.

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.