Re: [PATCH 2/2] xen/x86: Livepatch: support patching CET-enhanced functions

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.03.2022 15:21, David Vrabel wrote:
> On 07/03/2022 14:03, Jan Beulich wrote:
>> On 07.03.2022 12:53, Bjoern Doebel wrote:
>>> @@ -104,18 +122,36 @@ void noinline arch_livepatch_revive(void)
>>>   
>>>   int arch_livepatch_verify_func(const struct livepatch_func *func)
>>>   {
>>> +    BUILD_BUG_ON(sizeof(struct x86_livepatch_meta) != 
>>> LIVEPATCH_OPAQUE_SIZE);
>>> +
>>>       /* If NOPing.. */
>>>       if ( !func->new_addr )
>>>       {
>>> +        struct x86_livepatch_meta *lp;
>>> +
>>> +        lp = (struct x86_livepatch_meta *)func->opaque;
>>>           /* Only do up to maximum amount we can put in the ->opaque. */
>>> -        if ( func->new_size > sizeof(func->opaque) )
>>> +        if ( func->new_size > sizeof(lp->instruction) )
>>>               return -EOPNOTSUPP;
>>>   
>>>           if ( func->old_size < func->new_size )
>>>               return -EINVAL;
>>>       }
>>
>> I continue to be concerned of the new local variable causing compiler
>> warnings. With the adjustment made compared to v1, the specific
>> warning would have changed, and we're now liable to see set-but-never-
>> used ones.
> 
> Linux has a sizeof_field() macro for this sort of use.
> 
>      /**
>       * sizeof_field() - Report the size of a struct field in bytes
>       *
>       * @TYPE: The structure containing the field of interest
>       * @MEMBER: The field to return the size of
>       */
>      #define sizeof_field(TYPE, MEMBER) sizeof((((TYPE *)0)->MEMBER))

Oh, I should have thought of this. Iirc it was Paul who did pull in
this one, so it should be readily available.

Thanks for pointing out,
Jan