Re: [Qemu-devel] [PATCH v3 3/3] hw/s390x/ioinst: Fix alignment problem in struct SubchDev

[Peter Maydell]

On Mon, 10 Dec 2018 at 13:32, Dr. David Alan Gilbert
<address@hidden> wrote:
> Is the problem here that the field could actually be misaligned (on
> any conceivable build) or is it just a matter of convincing clang it's
> safe?

This is mostly a "clang doesn't know that the struct field
will actually always be 4 aligned and the function being
called only requires 4 alignment" thing. But there is an
actual-in-theory problem too: because the SCHIB struct is
marked QEMU_PACKED it has no alignment requirements at all.
So in for instance css_do_msch() the compiler is entitled
to align the local "SCHIB schib" at any alignment it likes,
which may not be the 4-alignment that copy_pmcw_from_guest()
assumes.

An option (3) which I hadn't previously thought of:
copy the pmcw and scsw fields into and out of locals
which are guaranteed to be correctly aligned:

--- a/hw/s390x/css.c
+++ b/hw/s390x/css.c
@@ -1290,9 +1290,15 @@ void copy_scsw_to_guest(SCSW *dest, const SCSW *src)
 static void copy_schib_to_guest(SCHIB *dest, const SCHIB *src)
 {
     int i;
+    PMCW srcpmcw, destpmcw;
+    SCSW srcscsw, destscsw;

-    copy_pmcw_to_guest(&dest->pmcw, &src->pmcw);
-    copy_scsw_to_guest(&dest->scsw, &src->scsw);
+    srcpmcw = src->pmcw;
+    copy_pmcw_to_guest(&destpmcw, &srcpmcw);
+    dest->pmcw = destpmcw;
+    srcscsw = src->scsw;
+    copy_scsw_to_guest(&destscsw, &srcscsw);
+    dest->scsw = destscsw;
     dest->mba = cpu_to_be64(src->mba);
     for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {
         dest->mda[i] = src->mda[i];
@@ -1339,9 +1345,15 @@ static void copy_scsw_from_guest(SCSW *dest,
const SCSW *src)
 static void copy_schib_from_guest(SCHIB *dest, const SCHIB *src)
 {
     int i;
+    PMCW srcpmcw, destpmcw;
+    SCSW srcscsw, destscsw;

-    copy_pmcw_from_guest(&dest->pmcw, &src->pmcw);
-    copy_scsw_from_guest(&dest->scsw, &src->scsw);
+    srcpmcw = src->pmcw;
+    copy_pmcw_from_guest(&destpmcw, &srcpmcw);
+    dest->pmcw = destpmcw;
+    srcscsw = src->scsw;
+    copy_scsw_from_guest(&destscsw, &srcscsw);
+    dest->scsw = destscsw;
     dest->mba = be64_to_cpu(src->mba);
     for (i = 0; i < ARRAY_SIZE(dest->mda); i++) {
         dest->mda[i] = src->mda[i];

thanks
-- PMM