Re: [v2 PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation for 32-bit sid size

From: Yang Shi
Date: Thu Oct 03 2024 - 11:31:42 EST




On 10/3/24 4:16 AM, Jason Gunthorpe wrote:
On Wed, Oct 02, 2024 at 01:05:08PM -0700, Yang Shi wrote:
It would make some sense to have something like:

u64 size = arm_smmu_strtab_max_sid()

/* Would require too much memory */
if (size > SZ_512M)
return -EINVAL;
Why not just check smmu->sid_bits?

For example,

if (smmu->sid_bits > 28)
    return -EINVAL;

The check can happen before the shift.
Sure, but IMHO it reads a bit better to check the size computed from
the helper

MAX_PAGE_ORDER is often 10, so kmalloc will always fail before we
reach 28 bits of sid space.

I'm wondering we may just use 31 instead of using some magic number:

if (smmu->sid_bits > 31)
    return -EINVAL;

If I understand correctly, the check is mainly used to avoid the u64 -> u32 overflow. This check guarantee no overflow. If some crazy hardware really requests that large memory, the allocation will fail.


Jason