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


From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] migration/rdma: clang compilation fix
Date: Mon, 4 Mar 2019 12:11:10 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1

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.


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



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