Multicall result missing sign extension in Xen or Linux

[Daniel De Graaf <dgdegra@tycho.nsa.gov>]

While trying to figure out why a failing component of a multicall did not
properly return its result, I discovered that multicall results are not
sign-extended when placed in the unsigned long result field. For hypercalls
such as do_mmu_update which return a (signed) int, this results in Linux
incorrectly thinking the hypercall succeeded when it has actually failed
since arch/x86/xen/multicalls.c uses a signed long for "result" and checks
(b->entries[i].result < 0).

Is this a bug in Xen (using the wrong return type for do_mmu_op and other
hypercalls) or in Linux (assuming all returns are signed longs)? One or the
other needs to be changed, because the current setup is silently hiding
failed memory mapping operations.

-- 
Daniel De Graaf
National Security Agency