Re: [Xen-devel] [PATCH v3 09/11] fuzz/x86emul: update fuzzer

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Feb 02, 2017 at 10:01:46AM -0700, Jan Beulich wrote:
> >>> On 02.02.17 at 17:50, <wei.liu2@xxxxxxxxxx> wrote:
> > On Thu, Feb 02, 2017 at 05:20:56AM -0700, Jan Beulich wrote:
> >> >>> On 01.02.17 at 13:02, <wei.liu2@xxxxxxxxxx> wrote:
> >> > +static int fuzz_read_segment(
> >> > +    enum x86_segment seg,
> >> > +    struct segment_register *reg,
> >> > +    struct x86_emulate_ctxt *ctxt)
> >> > +{
> >> > +    int rc;
> >> > +
> >> > +    if ( seg >= SEG_NUM )
> >> > +        return X86EMUL_UNHANDLEABLE;
> >> > +
> >> > +    rc = maybe_fail("read_segment", true);
> >> > +
> >> > +    if ( rc == X86EMUL_OKAY )
> >> > +        *reg = input.segments[seg];
> >> > +
> >> > +    return rc;
> >> > +}
> >> 
> >> Just like with ->read_cr(), this must not vary in returned state
> >> between multiple invocations.
> > 
> > Fixed for both read_segment and write_segment.
> 
> Why for write_segment? That one may fail at any time (and wouldn't
> normally be invoked more than once for a given segment anyway).

OK, that makes sense. I will revert it back.

> 
> >> > +static int _fuzz_read_msr(
> >> > +    unsigned int reg,
> >> > +    uint64_t *val,
> >> > +    struct x86_emulate_ctxt *ctxt)
> >> > +{
> >> > +    unsigned int idx;
> >> > +
> >> > +    switch ( reg )
> >> > +    {
> >> > +    case MSR_TSC_AUX:
> >> > +    case MSR_IA32_TSC:
> >> > +        return data_read("read_msr", val, sizeof(*val));
> >> 
> >> Strictly speaking the above applies to TSC_AUX too. And TSC should
> >> return monotonically increasing values. I don't think though that
> >> producing random output here matters right now. A comment may
> >> be worthwhile.
> >> 
> > 
> > Right, I will add the following:
> > 
> >         /*
> >          * TSC should return monotonically increasing values, but
> >          * returning random values is fine in fuzzer.
> >          */
> 
> What about TSC_AUX then?
> 

What model would you like it to follow? I suppose returning random value
is also fine? I.e. I should just add TSC_AUX to the comment as well.

Wei.

> Jan
> 

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