Re: [Qemu-devel] [PATCH v2] hw/s390/ccw.c: Don't take address of packed members

[Farhan Ali <alifm@linux.ibm.com>]

On 12/13/2018 07:02 AM, Peter Maydell wrote:
> Taking the address of a field in a packed struct is a bad idea, because
> it might not be actually aligned enough for that pointer type (and
> thus cause a crash on dereference on some host architectures). Newer
> versions of clang warn about this.
> 
> Avoid the problem by using local copies of the PMCW and SCSW
> struct fields in copy_schib_from_guest() and copy_schib_to_guest().
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> v1->v2 changes:
>   * add comment about why we're using locals
>   * name locals with underscores, as QEMU's naming conventions recommend
> 
> 
>   hw/s390x/css.c | 28 ++++++++++++++++++++++++----
>   1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/s390x/css.c b/hw/s390x/css.c
> index 04ec5cc9705..f92b046cd33 100644
> --- a/hw/s390x/css.c
> +++ b/hw/s390x/css.c
> @@ -1290,9 +1290,19 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)
>   static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)
>   {
>       int i;
> +    /*
> +     * We copy the PMCW and SCSW in and out of local variables to
> +     * avoid taking the address of members of a packed struct.
> +     */
> +    PMCW src_pmcw, dest_pmcw;
> +    SCSW src_scsw, dest_scsw;
>   
> -    copy_pmcw_to_guest(&dest->pmcw, &src->pmcw);
> -    copy_scsw_to_guest(&dest->scsw, &src->scsw);
> +    src_pmcw = src->pmcw;
> +    copy_pmcw_to_guest(&dest_pmcw, &src_pmcw);
> +    dest->pmcw = dest_pmcw;
> +    src_scsw = src->scsw;
> +    copy_scsw_to_guest(&dest_scsw, &src_scsw);
> +    dest->scsw = dest_scsw;
>       dest->mba = cpu_to_be64(src->mba);
>       for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {
>           dest->mda[i] = src->mda[i];
> @@ -1339,9 +1349,19 @@ static void copy_scsw_from_guest(SCSW *dest, const SCSW *src)
>   static void copy_schib_from_guest(SCHIB *dest, const SCHIB *src)
>   {
>       int i;
> +    /*
> +     * We copy the PMCW and SCSW in and out of local variables to
> +     * avoid taking the address of members of a packed struct.
> +     */
> +    PMCW src_pmcw, dest_pmcw;
> +    SCSW src_scsw, dest_scsw;
>   
> -    copy_pmcw_from_guest(&dest->pmcw, &src->pmcw);
> -    copy_scsw_from_guest(&dest->scsw, &src->scsw);
> +    src_pmcw = src->pmcw;
> +    copy_pmcw_from_guest(&dest_pmcw, &src_pmcw);
> +    dest->pmcw = dest_pmcw;
> +    src_scsw = src->scsw;
> +    copy_scsw_from_guest(&dest_scsw, &src_scsw);
> +    dest->scsw = dest_scsw;
>       dest->mba = be64_to_cpu(src->mba);
>       for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {
>           dest->mda[i] = src->mda[i];
> 

Reviewed-by: Farhan Ali <alifm@linux.ibm.com>