Re: [Xen-devel] [PATCH v4 10/12] livepatch: Handle arbitrary size names with the list operation

["Wieczorkiewicz, Pawel" <wipawel@xxxxxxxxx>]

> On 30. Sep 2019, at 10:50, Jan Beulich <jbeulich@xxxxxxxx> wrote:
> 
> On 28.09.2019 17:13, Pawel Wieczorkiewicz wrote:
>> --- a/xen/include/public/sysctl.h
>> +++ b/xen/include/public/sysctl.h
>> @@ -925,10 +925,11 @@ struct xen_sysctl_livepatch_get {
>>  *
>> 
snip
>>     uint32_t pad;                           /* IN: Must be zero. */
>> +    uint32_t name_total_size;               /* OUT: Total size of all 
>> transfer names */
>>     XEN_GUEST_HANDLE_64(xen_livepatch_status_t) status;  /* OUT. Must have 
>> enough
>>                                                space allocate for nr of 
>> them. */
>>     XEN_GUEST_HANDLE_64(char) name;         /* OUT: Array of names. Each 
>> member
>> -                                               MUST XEN_LIVEPATCH_NAME_SIZE 
>> in size.
>> -                                               Must have nr of them. */
>> +                                               may have an arbitrary length 
>> up to
>> +                                               XEN_LIVEPATCH_NAME_SIZE 
>> bytes. Must have
>> +                                               nr of them. */
>>     XEN_GUEST_HANDLE_64(uint32) len;        /* OUT: Array of lengths of 
>> name's.
>>                                                Must have nr of them. */
>> };
> 
> Non-binary-compatible changes require an interface version bump.

The bump happens with this patch of the patchset:
https://patchwork.kernel.org/patch/11165427/

> I wonder though why you don't use the "pad" field; in fact the
> way you make the change you'd have to introduce a 2nd padding
> field, to make padding explicit _and_ check it's zero on input
> (for future extensibility _without_ having to bump the interface
> version).
> 

I do not use the pad field because I introduce another field with the
next patch of the patchset: https://patchwork.kernel.org/patch/11165433/
Then I would have to re-add the pad field again I suppose.
Also, I was (false?) impression that the pad field is dedicated to
the future input parameters, so I should not touch it.

> Jan

Best Regards,
Pawel Wieczorkiewicz






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