Re: [Xen-devel] [PATCH v7 1/3] AMD/IOMMU: allocate one device table per PCI segment

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.10.2019 19:28, Andrew Cooper wrote:
> On 04/10/2019 14:30, Jan Beulich wrote:
>> On 04.10.2019 15:18, Andrew Cooper wrote:
>>> On 26/09/2019 15:28, Jan Beulich wrote:
>>>> @@ -1068,8 +1067,29 @@ static void * __init allocate_ppr_log(st
>>>>                                  IOMMU_PPR_LOG_DEFAULT_ENTRIES, "PPR Log");
>>>>  }
>>>>  
>>>> +/*
>>>> + * Within ivrs_mappings[] we allocate an extra array element to store
>>>> + * - segment number,
>>>> + * - device table.
>>>> + */
>>>> +#define IVRS_MAPPINGS_SEG(m) (m)[ivrs_bdf_entries].dte_requestor_id
>>>> +#define IVRS_MAPPINGS_DEVTAB(m) (m)[ivrs_bdf_entries].intremap_table
>>>> +
>>>> +static void __init free_ivrs_mapping(void *ptr)
>>>> +{
>>>> +    const struct ivrs_mappings *ivrs_mappings = ptr;
>>> How absolutely certain are we that ptr will never be NULL?
>> As certain as we can be by never installing a NULL pointer into the
>> radix tree, and by observing that neither radix_tree_destroy() nor
>> radix_tree_node_destroy() would ever call the callback for a NULL
>> node.
>>
>>> It might be better to rename this to radix_tree_free_ivrs_mappings() to
>>> make it clear who calls it, and also provide a hint as to why the
>>> parameter is void.
>> I'm not happy to add a radix_tree_ prefix; I'd be fine with adding
>> e.g. do_ instead, in case this provides enough of a hint for your
>> taste that this is actually a callback function.
> 
> How about a _callback() suffix?  I'm looking to make it obvious that you
> code shouldn't simply call it directly.

As indicated I've done this.

>>>> @@ -1082,13 +1102,15 @@ static int __init amd_iommu_init_one(str
>>>>      if ( intr && !set_iommu_interrupt_handler(iommu) )
>>>>          goto error_out;
>>>>  
>>>> -    /* To make sure that device_table.buffer has been successfully 
>>>> allocated */
>>>> -    if ( device_table.buffer == NULL )
>>>> +    /* Make sure that the device table has been successfully allocated. */
>>>> +    ivrs_mappings = get_ivrs_mappings(iommu->seg);
>>>> +    if ( !IVRS_MAPPINGS_DEVTAB(ivrs_mappings) )
>>> This is still going to crash with a NULL pointer deference in the case
>>> described by the comment.  (Then again, it may not crash, and hit
>>> userspace at the 64M mark.)
>>>
>>> You absolutely need to check ivrs_mappings being non NULL before using
>>> IVRS_MAPPINGS_DEVTAB(), or perhaps roll the check into the macro.
>> I can only repeat what I've said in reply to your respective v6 remark:
>> We won't come here for an IOMMU which didn't have its ivrs_mappings
>> successfully allocated.
> 
> Right, but to a first approximation, I don't care.  I can picture
> exactly what Coverity will say about this, in that radix_tree_lookup()
> may return NULL, and it is used here unconditionally where in most other
> contexts, the pointer gets checked before use.

Just one more word on top of the prior discussion: Would you also
insist on an explicit check here (when ...

>> You also seem to be mixing up this and the
>> device table allocation - the comment refers to the latter, while your
>> NULL deref concern is about the former. (If you go through the code
>> you'll find that we have numerous other places utilizing the fact that
>> get_ivrs_mappings() can't fail in cases like the one above.)
> 
> The existing code being terrible isn't a reasonable justification for
> adding to the mess.
> 
> It appears we have:
> 
> 1x assert not null
> 14x blind use
> 3x check

... none exists on basically all similar paths elsewhere) if the
IVRS mappings array hung off of struct amd_iommu as a plain pointer,
rather than being taken from a guaranteed populated (by this point
in time) radix tree slot?

> Seeing as we are pushed to the deadline for 4.13, begrudgingly A-by
> (preferably with the _callback() suffix), but I'm still not happy with
> the overall quality of the code.  At least it isn't getting
> substantially worse as a consequence here.

Juergen, since I didn't hear back from Andrew, would you be willing
to give a release ack on this series, as at this point I don't see
any good alternative to using the "begrudgingly A-by" give above?

Jan

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