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

[Mark Rutland]

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@xxxxxxx>
> > > ---
> > >   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), 

I'm pretty sure that regardless of whether a type is signed, shifting more than
the type's width is undefined behaviour. That causes GCC to scream at compile
time:

|   CC      arch/arm64/kernel/setup.o
| arch/arm64/kernel/setup.c: In function 'shift_test':
| arch/arm64/kernel/setup.c:295:20: warning: left shift count >= width of type [-Wshift-count-overflow]
|   295 |         return 1UL << 64;
|       |                    ^~

... and a UBSAN splat:

| ================================================================================
| UBSAN: shift-out-of-bounds in arch/arm64/kernel/setup.c:295:13
| shift exponent 64 is too large for 64-bit type 'long unsigned int'
| CPU: 0 PID: 0 Comm: swapper Not tainted 6.7.0-rc1-00005-g06034455cb74-dirty #3
| Call trace:
|  dump_backtrace+0x90/0xe8
|  show_stack+0x18/0x24
|  dump_stack_lvl+0x48/0x60
|  dump_stack+0x18/0x24
|  __ubsan_handle_shift_out_of_bounds+0x114/0x244
|  shift_test+0x24/0x34
|  setup_arch+0x238/0x68c
|  start_kernel+0x70/0x610
|  __primary_switched+0xbc/0xc4
| ================================================================================

Mark.

> thus
> subtracting 1 results in the correct all-bits-set value for an inclusive
> 64-bit limit.
> 
> Thanks,
> Robin.
> 
> > >   	return 0;
> > >   }
> > > -static int rc_dma_get_range(struct device *dev, u64 *size)
> > > +static int rc_dma_get_range(struct device *dev, u64 *limit)
> > >   {
> > >   	struct acpi_iort_node *node;
> > >   	struct acpi_iort_root_complex *rc;
> > > @@ -1408,8 +1407,7 @@ static int rc_dma_get_range(struct device *dev, u64 *size)
> > >   		return -EINVAL;
> > >   	}
> > > -	*size = rc->memory_address_limit >= 64 ? U64_MAX :
> > > -			1ULL<<rc->memory_address_limit;
> > > +	*limit = (1ULL << rc->memory_address_limit) - 1;
> > 
> > Same thing here.
> > 
> > Will