Messages in this thread | | | Date | Mon, 11 Dec 2023 16:13:38 +0000 | Subject | Re: [PATCH 3/7] ACPI/IORT: Handle memory address size limits as limits | From | Robin Murphy <> |
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On 2023-12-11 3:39 pm, Mark Rutland wrote: > 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@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.
Urgh, you're right... I double-checked 6.5.7.4 in the standard but managed to miss 6.5.7.3. So yeah, even though "4 << 62" or "2 << 63" are well-defined here, "1 << 64" isn't, dang. Thanks, funky old ISAs which did weird things for crazy large shifts and have no relevance to this code :(
Cheers, Robin.
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