Re: [PATCH 07/12] win32: use compiler option instead of attribute gcc_struct

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Oct 30, 2024 at 09:04:21PM -0700, Pierrick Bouvier wrote:
> This attribute is not recognized by clang, but the associated option is.
> 
> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
> ---
>  meson.build                               | 8 ++++----
>  include/qemu/compiler.h                   | 7 +------
>  subprojects/libvhost-user/libvhost-user.h | 6 +-----
>  3 files changed, 6 insertions(+), 15 deletions(-)
> 
> diff --git a/meson.build b/meson.build
> index d8af08299e0..d0d5dbe1479 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -330,10 +330,10 @@ elif host_os == 'sunos'
>  elif host_os == 'haiku'
>    qemu_common_flags += ['-DB_USE_POSITIVE_POSIX_ERRORS', '-D_BSD_SOURCE', '-fPIC']
>  elif host_os == 'windows'
> -  if not compiler.compiles('struct x { int y; } __attribute__((gcc_struct));',
> -                           args: '-Werror')
> -    error('Your compiler does not support __attribute__((gcc_struct)) - please use GCC instead of Clang')
> -  endif
> +  # https://gcc.gnu.org/onlinedocs/gcc/x86-Type-Attributes.html
> +  # We use this compilation option instead of relying on gcc_struct attribute
> +  # because clang does not support it (but supports the option).
> +  qemu_common_flags += ['-mno-ms-bitfields']
>  endif

Is this really safe for us to use ?   The current gcc_struct
attribute affects only structs marked as QEMU_PACKED. This
flag will affect all code.

If we call from QEMU code into Windows native APIs, and pass
or receive structs, then those structs' layouts would be
affected by this flag. I don't have a specific example, but
this feels unsafe to me, otherwise we would have done this
originally rather than only targetting internal packed structs
with the gcc_struct attribute.


With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|