Re: [Qemu-devel] [PATCH] migration/rdma: clang compilation fix

[Marcel Apfelbaum <marcel.apfelbaum@gmail.com>]

On 3/4/19 8:11 PM, Eric Blake wrote:
> On 3/4/19 2:42 AM, Marcel Apfelbaum wrote:
>> Configuring QEMU with:
>>          ../configure --cc=clang --enable-rdma
>>
>> Leads to compilation error:
>>
>>    CC      migration/rdma.o
>>    CC      migration/block.o
>>    qemu/migration/rdma.c:3615:58: error: taking address of packed 
>> member 'rkey' of class or structure
>>        'RDMARegisterResult' may result in an unaligned pointer value 
>> [-Werror,-Waddress-of-packed-member]
>>                              (uintptr_t)host_addr, NULL, 
>> &reg_result->rkey,
>> ^~~~~~~~~~~~~~~~
>>
>> This is a false warning; even if RDMARegisterResult is "packed", rkey
>> is the first field so is guaranteed to be aligned.
>
> Not so. If you packed struct, gcc is free to abut that struct next to 
> some other unaligned field in a larger struct:
>
> https://lists.gnu.org/archive/html/qemu-devel/2019-02/msg06743.html
>

I wasn't referring to general case, but to this specific case. Actually 
gcc does not complain, only clang.

Anyway, Dave's suggestion is clean and works for me.

Thanks,
Marcel

>>
>> Fix it by disabling the warning only for this instance.
>
> Ignoring the bug is not the same as fixing the bug.  You need to 
> rework this, as the compiler warning is not a false negative.
>
>>
>> Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
>> ---
>>   migration/rdma.c | 3 +++
>>   1 file changed, 3 insertions(+)