Re: [qemu-s390x] [PATCH 10/14] usb-mtp: avoid warning about unaligned access to filename

[Thomas Huth]

On 29/03/2019 12.11, Daniel P. Berrangé wrote:
> The 'filename' field in ObjectInfo struct is declared as a
> zero length array of uint16_t. Accessing it is equivalent
> to taking the address of the field, and taking the address
> of fields in a packed struct causes unaligned pointer
> warnings:
> 
> hw/usb/dev-mtp.c: In function ‘usb_mtp_write_metadata’:
> hw/usb/dev-mtp.c:1712:36: warning: taking address of packed member of ‘struct 
> <anonymous>’ may result in an unaligned pointer value 
> [-Waddress-of-packed-member]
>  1712 |                             dataset->filename);
>       |                             ~~~~~~~^~~~~~~~~~
> 
> The warning is in fact correct because the 'filename'
> field is preceeded by a uint8_t field which causes it
> to have bad alignment.
> 
> Using pointer arithmetic instead of accessing the zero
> length array field directly avoids the compiler warning
> but doesn't ultimately fix the bad alignment. Fixing
> that probably requires allocating a new array of
> uint16_t in the heap & then memcpy() the data before
> accessing the array elements.

If we are really using an unaligned pointer here, this code will crash
on Sparc host machines (and maybe some others). Thus I'd prefer if you
could rather fix the misalignment issue here instead of papering over
the compiler warning.

 Thomas