Re: [Xen-devel] [PATCH 2/4] hvm/dmop: Implement copy_{to, from}_guest_buf() in terms of raw accessors

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/04/2017 08:27, Jan Beulich wrote:
>>>> On 20.04.17 at 19:59, <jennifer.herbert@xxxxxxxxxx> wrote:
>> From: Jennifer Herbert <Jennifer.Herbert@xxxxxxxxxx>
> Is this correct, considering that iirc the patch was new in v5 and ...
>
>> This also allows the usual cases to be simplified, by omitting an 
>> unnecessary
>> buf parameters, and because the macros can appropriately size the object.
>>
>> This makes copying to or from a buf that isn't big enough an error.
>> If the buffer isnt big enough, trying to carry on regardless
>> can only cause trouble later on.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Signed-off-by: Jennifer Herbert <Jennifer.Herbert@xxxxxxxxxx>
> ... this sequence of S-o-b-s?
>
>> --- a/xen/arch/x86/hvm/dm.c
>> +++ b/xen/arch/x86/hvm/dm.c
>> @@ -32,36 +32,47 @@ struct dmop_args {
>>      struct xen_dm_op_buf buf[2];
>>  };
>>  
>> -static bool copy_buf_from_guest(const xen_dm_op_buf_t bufs[],
>> -                                unsigned int nr_bufs, void *dst,
>> -                                unsigned int idx, size_t dst_size)
>> +static bool _raw_copy_from_guest_buf(void *dst,
>> +                                     const struct dmop_args *args,
>> +                                     unsigned int buf_idx,
>> +                                     size_t dst_bytes)
>>  {
>> -    size_t size;
>> +    size_t buf_bytes;
>>  
>> -    if ( idx >= nr_bufs )
>> +    if ( buf_idx >= args->nr_bufs )
>>          return false;
>>  
>> -    memset(dst, 0, dst_size);
>> +    buf_bytes =  args->buf[buf_idx].size;
>>  
>> -    size = min_t(size_t, dst_size, bufs[idx].size);
>> +    if ( dst_bytes > buf_bytes )
>> +        return false;
> While this behavioral change is now being mentioned in the
> description, I'm not sure I buy the argument of basically being
> guaranteed to cause trouble down the road. Did you consider the
> forward compatibility aspect here, allowing us to extend interface
> structures by adding fields to their ends without breaking old
> callers? Paul, what are your thoughts here?

DMOP is a stable ABI.  There is no legal extending of any objects.

The previous semantics are guaranteed to break the ABI with future
subops, which is why I removed it.  In the case that the guest
originally passed an overly long buffer, and someone tried be "clever"
here passing the same object here expecting _raw_copy_from_guest_buf()
to DTRT, the function will end up copying too much data from the guest,
and you will end up with something the guest wasn't intending to be part
of the structure replacing the zero extension.

New subops which take a longer object should use a brand new object.

$MAGIC compatibility logic like you want has no business living in the
copy helper.  Had I spotted the intention during the original dmop
series, I would have rejected it during review.

~Andrew

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