Re: [Qemu-trivial] [PATCH v2 3/5] hw/usb/dev-mtp: Fix GCC 9 build warning

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

On Tue, Apr 30, 2019 at 11:28:41PM +0000, Alistair Francis wrote:
> Fix this warning with GCC 9 on Fedora 30:
> hw/usb/dev-mtp.c:1715:36: error: taking address of packed member of ‘struct <anonymous>’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
>  1715 |                             dataset->filename);
>       |                             ~~~~~~~^~~~~~~~~~
> 
> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
> ---
>  hw/usb/dev-mtp.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/hw/usb/dev-mtp.c b/hw/usb/dev-mtp.c
> index 99548b012d..8233beacab 100644
> --- a/hw/usb/dev-mtp.c
> +++ b/hw/usb/dev-mtp.c
> @@ -1711,9 +1711,22 @@ static void usb_mtp_write_metadata(MTPState *s, uint64_t dlen)
>      assert(!s->write_pending);
>      assert(p != NULL);
>  
> +/*
> + * We are about to access a packed struct. We are confident that the pointer
> + * address won't be unaligned, so we ignore GCC warnings.
> + */

The data is mis-aligned as we're accessing an int16 array that
is immediately following an int8 field in a packed struct

This problem is fixed by the following series which Gerd has in the
USB queue:

  https://lists.gnu.org/archive/html/qemu-devel/2019-04/msg02524.html

> +#if defined(CONFIG_PRAGMA_DIAGNOSTIC_AVAILABLE) && QEMU_GNUC_PREREQ(9, 0)
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Waddress-of-packed-member"
> +#endif
> +
>      filename = utf16_to_str(MIN(dataset->length, filename_chars),
>                              dataset->filename);
>  
> +#if defined(CONFIG_PRAGMA_DIAGNOSTIC_AVAILABLE) && QEMU_GNUC_PREREQ(9, 0)
> +#pragma GCC diagnostic pop
> +#endif
> +
>      if (strchr(filename, '/')) {
>          usb_mtp_queue_result(s, RES_PARAMETER_NOT_SUPPORTED, d->trans,
>                               0, 0, 0, 0);

Regards,
Daniel
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