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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.)

Jan