Re: [PULL 35/38] spapr: nested: Introduce H_GUEST_[GET|SET]_STATE hcalls.

[Peter Maydell]

On Wed, 27 Mar 2024 at 05:41, Harsh Prateek Bora <harshpb@linux.ibm.com> wrote:
>
>
>
> On 3/26/24 21:32, Peter Maydell wrote:
> > On Tue, 12 Mar 2024 at 17:11, Nicholas Piggin <npiggin@gmail.com> wrote:
> >>
> >> From: Harsh Prateek Bora <harshpb@linux.ibm.com>
> >>
> >> Introduce the nested PAPR hcalls:
> >>      - H_GUEST_GET_STATE which is used to get state of a nested guest or
> >>        a guest VCPU. The value field for each element in the request is
> >>        destination to be updated to reflect current state on success.
> >>      - H_GUEST_SET_STATE which is used to modify the state of a guest or
> >>        a guest VCPU. On success, guest (or its VCPU) state shall be
> >>        updated as per the value field for the requested element(s).
> >>
> >> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
> >> Signed-off-by: Michael Neuling <mikey@neuling.org>
> >> Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
> >> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> >
> > Hi; Coverity points out a problem with this code (CID 1540008, 1540009):
> >
> >
> >
> >> +static target_ulong h_guest_getset_state(PowerPCCPU *cpu,
> >> +                                         SpaprMachineState *spapr,
> >> +                                         target_ulong *args,
> >> +                                         bool set)
> >> +{
> >> +    target_ulong flags = args[0];
> >> +    target_ulong lpid = args[1];
> >> +    target_ulong vcpuid = args[2];
> >> +    target_ulong buf = args[3];
> >> +    target_ulong buflen = args[4];
> >> +    struct guest_state_request gsr;
> >> +    SpaprMachineStateNestedGuest *guest;
> >> +
> >> +    guest = spapr_get_nested_guest(spapr, lpid);
> >> +    if (!guest) {
> >> +        return H_P2;
> >> +    }
> >> +    gsr.buf = buf;
> >> +    assert(buflen <= GSB_MAX_BUF_SIZE);
> >> +    gsr.len = buflen;
> >> +    gsr.flags = 0;
> >> +    if (flags & H_GUEST_GETSET_STATE_FLAG_GUEST_WIDE) {
> >
> > flags is a target_ulong, which means it might only be 32 bits.
> > But H_GUEST_GETSET_STATE_FLAG_GUEST_WIDE has a bit set in the
> > upper 32 bits only. So Coverity complains about this condition
> > being always-zero and the body of the if being dead code.
> >
> > What was the intention here?
>
> Hi Peter,
> Ideally this is intended to be running on a ppc64 where target_ulong
> should be uint64_t. I guess same holds true for existing nested-hv code
> as well.

Sorry, I'm afraid I misread the Coverity report here;
sorry for the confusion. The 32-vs-64 bits question is a red
herring.

What Coverity is actually pointing out is in this next bit:

> >> +        gsr.flags |= GUEST_STATE_REQUEST_GUEST_WIDE;
> >> +    }
> >> +    if (flags & !H_GUEST_GETSET_STATE_FLAG_GUEST_WIDE) {

The C operator ! is the logical-NOT operator; since
H_GUEST_GETSET_STATE_FLAG_GUEST_WIDE is a non-zero value
that means that !H_GUEST_GETSET_STATE_FLAG_GUEST_WIDE is 0;
so we're testing (flags & 0), which is always false, and this
is the if() body which is dead-code as a result.

Should this be the bitwise-NOT ~  (ie "if any flag other
than this one is set"), or should this be an else clause
to the previous if() (ie "if this flag is not set") ?

> >> +        return H_PARAMETER; /* flag not supported yet */
> >> +    }
> >> +
> >> +    if (set) {
> >> +        gsr.flags |= GUEST_STATE_REQUEST_SET;
> >> +    }
> >> +    return map_and_getset_state(cpu, guest, vcpuid, &gsr);
> >> +}
> >

thanks
-- PMM