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

[Peter Maydell]

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, and we must not do it
     (and adding a cast to ind_offset doesn't help us at all)
 (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.

thanks
-- PMM