Re: [PATCH qemu] vfio/spapr: Fix page size calculation

[Philippe Mathieu-Daudé]

On 3/24/20 7:39 AM, Alexey Kardashevskiy wrote:
> Coverity detected an issue (CID 1421903) with potential call of clz64(0)
> which returns 64 which make it do "<<" with a negative number.
>
> This checks the mask and avoids undefined behaviour.
>
> In practice pgsizes and memory_region_iommu_get_min_page_size() always
> have some common page sizes and even if they did not, the resulting page
> size would be 0x8000.0000.0000.0000 (gcc 9.2) and
> ioctl(VFIO_IOMMU_SPAPR_TCE_CREATE) would fail anyway.
>
> Signed-off-by: Alexey Kardashevskiy <address@hidden>
> ---
>   hw/vfio/spapr.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/hw/vfio/spapr.c b/hw/vfio/spapr.c
> index 33692fc86fd6..2900bd19417a 100644
> --- a/hw/vfio/spapr.c
> +++ b/hw/vfio/spapr.c
> @@ -147,7 +147,7 @@ int vfio_spapr_create_window(VFIOContainer *container,
>   {
>       int ret = 0;
>       IOMMUMemoryRegion *iommu_mr = IOMMU_MEMORY_REGION(section->mr);
> -    uint64_t pagesize = memory_region_iommu_get_min_page_size(iommu_mr);
> +    uint64_t pagesize = memory_region_iommu_get_min_page_size(iommu_mr), 
> pgmask;
>       unsigned entries, bits_total, bits_per_level, max_levels;
>       struct vfio_iommu_spapr_tce_create create = { .argsz = sizeof(create) };
>       long rampagesize = qemu_minrampagesize();
> @@ -159,8 +159,8 @@ int vfio_spapr_create_window(VFIOContainer *container,
>       if (pagesize > rampagesize) {
>           pagesize = rampagesize;
>       }
> -    pagesize = 1ULL << (63 - clz64(container->pgsizes &
> -                                   (pagesize | (pagesize - 1))));
> +    pgmask = container->pgsizes & (pagesize | (pagesize - 1));

Is that ROUND_UP(container->pgsizes, pagesize)?

> +    pagesize = pgmask ? (1ULL << (63 - clz64(pgmask))) : 0;
>       if (!pagesize) {
>           error_report("Host doesn't support page size 0x%"PRIx64
>                        ", the supported mask is 0x%lx",