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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

A "do_" prefix, in particular, provides no useful information to the reader.

>>> @@ -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.

> 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

which isn't a great statement about the quality of the code.

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.

~Andrew

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