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

[Marcel Apfelbaum]

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 <address@hidden>
> > ---
> >   migration/rdma.c | 3 +++
> >   1 file changed, 3 insertions(+)