Re: [PATCH v3 02/13] xen: harmonize return types of hypercall handlers

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 20 Dec 2021, Julien Grall wrote:
> On 18/12/2021 00:00, Stefano Stabellini wrote:
> > On Fri, 17 Dec 2021, Juergen Gross wrote:
> > > On 17.12.21 11:41, Julien Grall wrote:
> > > > Hi Juergen,
> > > > 
> > > > On 17/12/2021 08:50, Juergen Gross wrote:
> > > > > On 17.12.21 08:45, Jan Beulich wrote:
> > > > > > On 17.12.2021 06:34, Juergen Gross wrote:
> > > > > > > On 16.12.21 22:15, Stefano Stabellini wrote:
> > > > > > > > On Thu, 16 Dec 2021, Stefano Stabellini wrote:
> > > > > > > > > On Thu, 16 Dec 2021, Juergen Gross wrote:
> > > > > > > > > > On 16.12.21 03:10, Stefano Stabellini wrote:
> > > > > > > > > > > The case of XENMEM_maximum_ram_page is interesting but it
> > > > > > > > > > > is
> > > > > > > > > > > not a
> > > > > > > > > > > problem in reality because the max physical address size
> > > > > > > > > > > is
> > > > > > > > > > > only 40-bit
> > > > > > > > > > > for aarch32 guests, so 32-bit are always enough to return
> > > > > > > > > > > the
> > > > > > > > > > > highest
> > > > > > > > > > > page in memory for 32-bit guests.
> > > > > > > > > > 
> > > > > > > > > > You are aware that this isn't the guest's max page, but the
> > > > > > > > > > host's?
> > > > > > > > 
> > > > > > > > I can see now that you meant to say that, no matter what is the
> > > > > > > > max
> > > > > > > > pseudo-physical address supported by the VM,
> > > > > > > > XENMEM_maximum_ram_page
> > > > > > > > is
> > > > > > > > supposed to return the max memory page, which could go above the
> > > > > > > > addressibility limit of the VM.
> > > > > > > > 
> > > > > > > > So XENMEM_maximum_ram_page should potentially be able to return
> > > > > > > > (1<<44)
> > > > > > > > even when called by an aarch32 VM, with max IPA 40-bit.
> > > > > > > > 
> > > > > > > > I would imagine it could be useful if dom0 is 32-bit but domUs
> > > > > > > > are
> > > > > > > > 64-bit on a 64-bit hypervisor (which I think it would be a very
> > > > > > > > rare
> > > > > > > > configuration on ARM.)
> > > > > > > > 
> > > > > > > > Then it looks like XENMEM_maximum_ram_page needs to be able to
> > > > > > > > return a
> > > > > > > > value > 32-bit when called by a 32-bit guest.
> > > > > > > > 
> > > > > > > > The hypercall ABI follows the ARM C calling convention, so a
> > > > > > > > 64-bit
> > > > > > > > value should be returned using r0 and r1. But looking at
> > > > > > > > xen/arch/arm/traps.c:do_trap_hypercall, it doesn't seem it ever
> > > > > > > > sets
> > > > > > > > r1
> > > > > > > > today. Only r0 is set, so effectively we only support 32-bit
> > > > > > > > return
> > > > > > > > values on aarch32 and for aarch32 guests.
> > > > > > > > 
> > > > > > > > In other words, today all hypercalls on ARM return 64-bit to
> > > > > > > > 64-bit
> > > > > > > > guests and 32-bit to 32-bit guests. Which in the case of
> > > > > > > > memory_op
> > > > > > > > is
> > > > > > > > "technically" the correct thing to do because it matches the C
> > > > > > > > declaration in xen/include/xen/hypercall.h:
> > > > > > > > 
> > > > > > > > extern long
> > > > > > > > do_memory_op(
> > > > > > > >        unsigned long cmd,
> > > > > > > >        XEN_GUEST_HANDLE_PARAM(void) arg);
> > > > > > > > 
> > > > > > > > So...  I guess the conclusion is that on ARM do_memory_op should
> > > > > > > > return
> > > > > > > > "long" although it is not actually enough for a correct
> > > > > > > > implementation
> > > > > > > > of XENMEM_maximum_ram_page for aarch32 guests ?
> > > > > > > > 
> > > > > > > 
> > > > > > > Hence my suggestion to check the return value of _all_ hypercalls
> > > > > > > to
> > > > > > > be
> > > > > > > proper sign extended int values for 32-bit guests. This would fix
> > > > > > > all
> > > > > > > potential issues without silently returning truncated values.
> > > > > > 
> > > > > > Are we absolutely certain we have no other paths left where a
> > > > > > possibly
> > > > > > large unsigned values might be returned? In fact while
> > > > > > compat_memory_op() does the necessary saturation, I've never been
> > > > > > fully
> > > > > > convinced of this being the best way of dealing with things. The
> > > > > > range
> > > > > > of error indicators is much smaller than [-INT_MIN,-1], so almost
> > > > > > double the range of effectively unsigned values could be passed back
> > > > > > fine. (Obviously we can't change existing interfaces, so this mem-op
> > > > > > will need to remain as is.)
> > > > > 
> > > > > In fact libxenctrl tries do deal with this fact by wrapping a
> > > > > memory_op
> > > > > for a 32-bit environment into a multicall. This will work fine for a
> > > > > 32-bit Arm guest, as xen_ulong_t is a uint64 there.
> > > > > 
> > > > > So do_memory_op should return long on Arm, yes. OTOH doing so will
> > > > > continue to be a problem in case a 32-bit guest doesn't use the
> > > > > multicall technique for handling possible 64-bit return values.
> > > > > 
> > > > > So I continue to argue that on Arm the return value of a hypercall
> > > > > should be tested to fit into 32 bits.
> > > > 
> > > > It would make sense. But what would you return if the value doesn't fit?
> > > 
> > > I guess some errno value would be appropriate, like -EDOM, -ERANGE or
> > > -E2BIG.
> > 
> > This seems to be better than the alternative below as it is a lot
> > simpler.
> 
> We would still need to special case XENMEM_maximum_reservation (or rework the
> implementation of the sub-op) because the value returned is an unsigned long.
> So technically, the unsigned value for -EDOM & co could be interpreted as the
> maximum host frame number.
> 
> I also would like to see the hypercall returning 'int' when they are only
> meant to return 32-bit value. This will make easier to spot someone that
> decide to return a 64-bit value.

I am completely aligned with you on both points.

XENMEM_maximum_reservation is a bit of a distraction given that is
unused (even unsupported?) on ARM. In general, to switch to "int" as
return type we would have to (manually) check that all the sub-ops of a
given hypercall return 32-bit values, right? Otherwise, how can we be
sure that we don't start to silently truncate the top 32-bit on a sub-op
on arm64?

In theory we could use -Wconversion to automatically spot any
truncations but unfortunately -Wconversion breaks the build at the
moment.