[PATCH 3/7] ACPI/IORT: Handle memory address size limits as limits

[Mark Rutland]

On Mon, Dec 11, 2023 at 03:30:24PM +0000, Will Deacon wrote:
> On Mon, Dec 11, 2023 at 03:01:27PM +0000, Robin Murphy wrote:
> > On 2023-12-11 1:27 pm, Will Deacon wrote:
> > > On Wed, Nov 29, 2023 at 05:43:00PM +0000, Robin Murphy wrote:
> > > > Return the Root Complex/Named Component memory address size limit as an
> > > > inclusive limit value, rather than an exclusive size.  This saves us
> > > > having to special-case 64-bit overflow, and simplifies our caller too.
> > > > 
> > > > Signed-off-by: Robin Murphy <robin.murphy at arm.com>
> > > > ---
> > > >   drivers/acpi/arm64/dma.c  |  9 +++------
> > > >   drivers/acpi/arm64/iort.c | 18 ++++++++----------
> > > >   include/linux/acpi_iort.h |  4 ++--
> > > >   3 files changed, 13 insertions(+), 18 deletions(-)
> > > 
> > > [...]
> > > 
> > > > diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c
> > > > index 6496ff5a6ba2..eb64d8e17dd1 100644
> > > > --- a/drivers/acpi/arm64/iort.c
> > > > +++ b/drivers/acpi/arm64/iort.c
> > > > @@ -1367,7 +1367,7 @@ int iort_iommu_configure_id(struct device *dev, const u32 *input_id)
> > > >   { return -ENODEV; }
> > > >   #endif
> > > > -static int nc_dma_get_range(struct device *dev, u64 *size)
> > > > +static int nc_dma_get_range(struct device *dev, u64 *limit)
> > > >   {
> > > >   	struct acpi_iort_node *node;
> > > >   	struct acpi_iort_named_component *ncomp;
> > > > @@ -1384,13 +1384,12 @@ static int nc_dma_get_range(struct device *dev, u64 *size)
> > > >   		return -EINVAL;
> > > >   	}
> > > > -	*size = ncomp->memory_address_limit >= 64 ? U64_MAX :
> > > > -			1ULL<<ncomp->memory_address_limit;
> > > > +	*limit = (1ULL << ncomp->memory_address_limit) - 1;
> > > 
> > > The old code handled 'ncomp->memory_address_limit >= 64' -- why is it safe
> > > to drop that? You mention it in the cover letter, so clearly I'm missing
> > > something!
> > 
> > Because an unsigned shift by 64 or more generates 0 (modulo 2^64), thus
> > subtracting 1 results in the correct all-bits-set value for an inclusive
> > 64-bit limit.
> 
> Oh, I'd have thought you'd have gotten one of those "left shift count >=
> width of type" warnings if you did that.

I think you'll get a UBSAN splat, but here the compiler doesn't know what
'ncomp->memory_address_limit' will be and so doesn't produce a compile-time
warning.

Regardless, it's undefined behaviour.

Mark.