Re: [PATCH] xen/x86: Annotate deliberate fallthrough cases from XSA-154

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/02/16 13:38, Jan Beulich wrote:
>>>> On 18.02.16 at 13:26, <andrew.cooper3@citrix.com> wrote:
>> Coverity objects otherwise.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>> ---
>> CC: Jan Beulich <JBeulich@suse.com>
>> ---
>>  xen/arch/x86/mm.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
>> index a05edc3..0bff7dd 100644
>> --- a/xen/arch/x86/mm.c
>> +++ b/xen/arch/x86/mm.c
>> @@ -924,10 +924,15 @@ get_page_from_l1e(
>>              {
>>              case 0:
>>                  break;
>> +
>>              case 1:
>>                  if ( is_hardware_domain(l1e_owner) )
>> +                {
>> +                    /* Fallthrough. */
>>              case -1:
>>                      return 0;
>> +                }
>> +                /* Fallthrough. */
>>              default:
> This second fall-through is actually a bug (luckily noticable only
> on debug builds).
>
> I'll commit the patch suitably adjusted, albeit I have a hard time
> seeing how
>
>             case 1:
>                 if ( is_hardware_domain(l1e_owner) )
>             case -1:
>
> cannot be seen as obviously deliberate.

Many C programmers are not aware that it is even valid syntax.

The point of the MISSING_BREAK check is to second guess what the
programmer has actually written.

>  Or did Coverity perhaps
> only complain about the second, indeed buggy one?

It only complained about the first.  The second is a misaligned
unconditional return when considered in isolation.

~Andrew