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

[Dr. David Alan Gilbert]

* Eric Blake (address@hidden) 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
> 
> > 
> > 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.

In this case it is, however, I think it's easy enough to work around;
all we need is something like:

  uint23_t tmp_rkey;
                  if (qemu_rdma_register_and_get_keys(rdma, block,
                            (uintptr_t)host_addr, NULL, &tmp_rkey,
                            chunk, chunk_start, chunk_end)) {
                    error_report("cannot get rkey");
                    ret = -EINVAL;
                    goto out;
                  }
                  reg_result->rkey = tmp_rkey

Note that I think the structure pointed to by reg_result is
visible over the wire, so we can't change it's (unfortunate) layout.

Dave

> > 
> > Signed-off-by: Marcel Apfelbaum <address@hidden>
> > ---
> >   migration/rdma.c | 3 +++
> >   1 file changed, 3 insertions(+)
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK