linux-kernel - Re: [PATCH 1/2] iommu: Fix iommu_unmap and iommu_unmap

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Message-ID: <19766001-07c9-7b67-ae10-8b5467fff748@arm.com>
Date:   Thu, 1 Feb 2018 12:17:22 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        iommu@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     jroedel@...e.de
Subject: Re: [PATCH 1/2] iommu: Fix iommu_unmap and iommu_unmap_fast return
 type

On 01/02/18 05:03, Suravee Suthikulpanit wrote:
> Hi Robin,
> 
> On 2/1/18 1:02 AM, Robin Murphy wrote:
>> Hi Suravee,
>>
>> On 31/01/18 01:48, Suravee Suthikulpanit wrote:
>>> Currently, iommu_unmap and iommu_unmap_fast return unmapped
>>> pages with size_t.  However, the actual value returned could
>>> be error codes (< 0), which can be misinterpreted as large
>>> number of unmapped pages. Therefore, change the return type to ssize_t.
>>>
>>> Cc: Joerg Roedel <joro@...tes.org>
>>> Cc: Alex Williamson <alex.williamson@...hat.com>
>>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
>>> ---
>>>   drivers/iommu/amd_iommu.c   |  6 +++---
>>>   drivers/iommu/intel-iommu.c |  4 ++--
>>
>> Er, there are a few more drivers than that implementing iommu_ops ;)
> 
> Ahh right.
>>
>> It seems like it might be more sensible to fix the single instance of 
>> a driver returning -EINVAL (which appears to be a "should never happen 
>> if used correctly" kinda thing anyway) and leave the API-internal 
>> callback prototype as-is. I do agree the inconsistency of 
>> iommu_unmap() itself wants sorting, though (particularly the 
>> !IOMMU_API stubs which are wrong either way).
>>
>> Robin.
> 
> Make sense. I'll leave the API alone, and change the code to not 
> returning error then.

Actually, on a second look I think that check in amd_iommu is 99% 
redundant anyway, since PAGE_MODE_NONE is only normally set for 
IOMMU_DOMAIN_IDENTITY domains, thus iommu_unmap() would have bailed out 
from the __IOMMU_DOMAIN_PAGING check before ops->unmap could be called. 
AFAICS the only way to hit it at all is if a caller somehow managed to 
get hold of the dev_state->domain set up in amd_iommu_init_device(), 
then tried to unmap something from that, which seems such a very wrong 
thing to do it should probably just crash and burn with extreme 
prejudice anyway.

Cheers,
Robin.