Re: [Qemu-devel] [PATCH 2/3 V3] s390: implement pci instructions

[Markus Armbruster]

Peter Maydell <address@hidden> writes:

> On 21 January 2015 at 11:54, Markus Armbruster <address@hidden> wrote:
>> Markus Armbruster <address@hidden> writes:
>>
>>> Frank Blaschka <address@hidden> writes:
>>>
>>>> On Tue, Jan 20, 2015 at 01:56:09PM +0100, Markus Armbruster wrote:
>>>>> Markus Armbruster <address@hidden> writes:
>>>>> > 1. pbdev->isc gets promoted from uint8_t to int as operand of binary <<
>>>>> >    (usual arithmetic conversions ISO/IEC 9899:1999 6.3.1.8)
>>>>> >
>>>>> > 2. The int result is shifted left 28 bits.  This can set the MSB.
>>>>> >
>>>>> > 3. Likewise: pbdev->noi gets promoted from uint64_t to int, and shifted
>>>>> >    left 16 bits.
>>>> uint16_t to int
>>>
>>> Yes, that's what I meant :)
>>>
>>>>> >
>>>>> > 4. The two shift results stay int and get ored.
>>>>> >
>>>>> > 5. pbdev->routes.adapter.ind_offset stays uint64_t, and is shifted left
>>>>> >    8 bits.
>>>>> >
>>>>> > 6. The next or's left operand is the int result of 4 and the right
>>>>> >    operant is the uint64_t result of 5.  Therefore, the left operand is
>>>>> >    *sign-extended* from int to uint64_t.  This copies bit#7 of
>>>>> >    pbdev->isc to bits#31..63.  Whoops.
>>>>>
>>>>> I neglected to say: we don't currently use the upper 32 bits, and as
>>>>> long as we do that, the sign extension is harmless.  I'd recommend to
>>>>> avoid it all the same, for robustness, and to hush up Coverity.
>>>>>
>
>> diff --git a/hw/s390x/s390-pci-inst.c b/hw/s390x/s390-pci-inst.c
>> index 5ea13e5..2bed3f5 100644
>> --- a/hw/s390x/s390-pci-inst.c
>> +++ b/hw/s390x/s390-pci-inst.c
>> @@ -785,8 +785,8 @@ int stpcifc_service_call(S390CPU *cpu, uint8_t
>> r1, uint64_t fiba)
>>      stq_p(&fib.fmb_addr, pbdev->fmb_addr);
>>
>>      data = (pbdev->isc << 28) | (pbdev->noi << 16) |
>> -           (pbdev->routes.adapter.ind_offset << 8) | (pbdev->sum << 7) |
>> -           pbdev->routes.adapter.summary_offset;
>> +           ((uint32_t)pbdev->routes.adapter.ind_offset << 8) |
>> +           (pbdev->sum << 7) | pbdev->routes.adapter.summary_offset;
>>      stw_p(&fib.data, data);
>>
>>      if (pbdev->fh >> ENABLE_BIT_OFFSET) {
>>
>
> This doesn't make sense to me as a fix for the problem you describe
> above. Either
>  (1) pbdev->isc may have bit 3 set: in this case shifting it left
>      by 28 is undefined behaviour in C,

Correct.

>                                         and we must not do it

I suspect we shift signed values all over the place, without regard for
signed overflow.  Machines are fine with that, but some day some
compiler wiseguy may find a way to save a femtosecond or two for some
program that never does that, breaking programs that do it, and then
we'll be in trouble.

We should follow the kernel's lead and compile with
-fno-strict-overflow.

>      (and adding a cast to ind_offset doesn't help us at all)

Correct, it doesn't help with the signed left shift of pbdev->isc.

>  (2) pbdev->isc is guaranteed never to have bit 3 set: in this
>      case the sign extension to uint64_t in step 6 above will
>      have no effect, because the sign bit in the int result will
>      be clear
>
> So you can either:
>  (1) cast pbdev->isc to uint32_t before shifting, thus ensuring that
>      we do all our | operations on unsigned types and that we won't
>      shift into the sign bit regardless of pbdev->isc's value
>  (2) state that we know pbdev->isc is always less than 8 and so this
>      is a coverity false positive to be suppressed via the web UI
>
> But the patch you have doesn't seem like the right thing to me.

Frank's code, Frank's choice :)