linux-kernel - Re: [PATCH v1 04/10] powerpc/kernel/iommu: Add new iommu_table_in

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Date:   Thu, 27 Aug 2020 15:34:55 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Alexey Kardashevskiy <aik@...abs.ru>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Joel Stanley <joel@....id.au>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Ram Pai <linuxram@...ibm.com>,
        Brian King <brking@...ux.vnet.ibm.com>,
        Murilo Fossa Vicentini <muvic@...ux.ibm.com>,
        David Dai <zdai@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 04/10] powerpc/kernel/iommu: Add new
 iommu_table_in_use() helper

On Sat, 2020-08-22 at 20:34 +1000, Alexey Kardashevskiy wrote:
> > +
> > +	/*ignore reserved bit0*/
> 
> s/ignore reserved bit0/ ignore reserved bit0 /  (add spaces)

Fixed

> > +	if (tbl->it_offset == 0)
> > +		p1_start = 1;
> > +
> > +	/* Check if reserved memory is valid*/
> 
> A missing space here.

Fixed

> 
> > +	if (tbl->it_reserved_start >= tbl->it_offset &&
> > +	    tbl->it_reserved_start <= (tbl->it_offset + tbl->it_size) &&
> > +	    tbl->it_reserved_end   >= tbl->it_offset &&
> > +	    tbl->it_reserved_end   <= (tbl->it_offset + tbl->it_size)) {
> 
> Uff. What if tbl->it_reserved_end is bigger than tbl->it_offset +
> tbl->it_size?
> 
> The reserved area is to preserve MMIO32 so it is for it_offset==0 only
> and the boundaries are checked in the only callsite, and it is unlikely
> to change soon or ever.
> 
> Rather that bothering with fixing that, may be just add (did not test):
> 
> if (WARN_ON((
> (tbl->it_reserved_start || tbl->it_reserved_end) && (it_offset != 0))
> (tbl->it_reserved_start > it_offset && tbl->it_reserved_end < it_offset
> + it_size) && (it_offset == 0)) )
>  return true;
> 
> Or simply always look for it_offset..it_reserved_start and
> it_reserved_end..it_offset+it_size and if there is no reserved area,
> initialize it_reserved_start=it_reserved_end=it_offset so the first
> it_offset..it_reserved_start becomes a no-op.

The problem here is that the values of it_reserved_{start,end} are not
necessarily valid. I mean, on iommu_table_reserve_pages() the values
are stored however they are given (bit reserving is done only if they
are valid). 

Having a it_reserved_{start,end} value outside the valid ranges would
cause find_next_bit() to run over memory outside the bitmap.
Even if the those values are < tbl->it_offset, the resulting
subtraction on unsigned would cause it to become a big value and run
over memory outside the bitmap.

But I think you are right. That is not the place to check if the
reserved values are valid. It should just trust them here.
I intent to change iommu_table_reserve_pages() to only store the
parameters in it_reserved_{start,end} if they are in the range, and or
it_offset in both of them if they are not.

What do you think?

Thanks for the feedback!
Leonardo Bras