Re: [Xen-devel] [PATCH] tools/libxencall: enforce proper alignment of hypercall buffers

[Juergen Gross <jgross@xxxxxxxx>]

On 08/06/18 12:25, Juergen Gross wrote:
> On 08/06/18 12:09, Andrew Cooper wrote:
>> On 08/06/18 10:51, Juergen Gross wrote:
>>> xencall_alloc_buffer() is used throughout Xen tools for allocating
>>> hypercall buffers. Allocation is done at page granularity. For simple
>>> administration each allocated set of pages contains a small header
>>> holding the number of pages of that set. The hypercall buffer is
>>> located directly after the 4 byte sized header, leading to a wrong
>>> alignment for e.g. pointers.
>>>
>>> Repair that by using a 16 byte sized header enforcing the same
>>> alignment as malloc().
>>>
>>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>>> ---
>>> Not sure whether this wants to be backported. In case the answer is
>>> "yes" I think it should be part of 4.11.
>>> ---
>>>  tools/libs/call/buffer.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/tools/libs/call/buffer.c b/tools/libs/call/buffer.c
>>> index 2d8fc29ac6..0b6af2db60 100644
>>> --- a/tools/libs/call/buffer.c
>>> +++ b/tools/libs/call/buffer.c
>>> @@ -151,6 +151,7 @@ void xencall_free_buffer_pages(xencall_handle *xcall, 
>>> void *p, size_t nr_pages)
>>>  
>>>  struct allocation_header {
>>>      int nr_pages;
>>
>> This hunk reveals another bug...
>>
>> There are a rather large number of size_t => int truncations in the
>> code, which is surely going to catch people by surprise.  (Again - I was
>> certain I commented on this during the original review of this library.)
> 
> On the practical side I don't think this really matters here. Passing
> more than 2GB as a hypercall buffer is nothing we want to support...
> 
>>
>>> +    int pad[3];
>>
>> This brings it to 16 byte alignment, but is that enough?  An xsave state
>> block in x86 for example has 64 byte alignment.  I suppose we mostly
>> care about ARM here, as its memcpy() will be most impacted by this
>> misalignment.
> 
> Special purpose buffers can still be directly allocated via
> xencall_alloc_buffer_pages(). I don't think we'll need alignment better
> than malloc().
> 
>> Irrespective, might be easier to accomplish with an
>> __attribute__((__aligned__($N))) rather than explicit padding?
> 
> I don't mind which way to go here. I'll leave the decision for the
> maintainers. :-)

Ping?


Juergen


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