Re: [PATCH bpf-next v3 2/2] selftests/bpf: Add ldsx selftests for ldsx and subreg compare

[Yonghong Song <yonghong.song@xxxxxxxxx>]

On 7/16/24 5:12 PM, Eduard Zingerman wrote:
> On Tue, 2024-07-16 at 15:38 -0700, Yonghong Song wrote:
>
> [...]
>
> > diff --git a/tools/testing/selftests/bpf/prog_tests/reg_bounds.c b/tools/testing/selftests/bpf/prog_tests/reg_bounds.c
> > index eb74363f9f70..c88602908cfe 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/reg_bounds.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/reg_bounds.c
> > @@ -441,6 +441,22 @@ static struct range range_refine(enum num_t x_t, struct range x, enum num_t y_t,
> >           if (t_is_32(y_t) && !t_is_32(x_t)) {
> >                   struct range x_swap;
> >    
> > +               /* If we know that
> > +                *   - *x* is in the range of signed 32bit value
> > +                *   - *y_cast* range is 32-bit sign non-negative, and
> > +                * then *x* range can be narrowed to the interaction of
> > +                * *x* and *y_cast*. Otherwise, if the new range for *x*
> > +                * allows upper 32-bit 0xffffffff then the eventual new
> > +                * range for *x* will be out of signed 32-bit range
> > +                * which violates the origin *x* range.
> > +                */
> > +               if (x_t == S64 && y_t == S32 &&
> > +                   !(y_cast.a & 0xffffffff80000000ULL) && !(y_cast.b & 0xffffffff80000000) &&
> > +                   (long long)x.a >= S32_MIN && (long long)x.b <= S32_MAX) {
> > +                               return range(S64, max_t(S64, y_cast.a, x.a),
> > +                                            min_t(S64, y_cast.b, x.b));
> > +               }
> > +
> >                   /* some combinations of upper 32 bits and sign bit can lead to
> >                    * invalid ranges, in such cases it's easier to detect them
> >                    * after cast/swap than try to enumerate all the conditions
> > @@ -2108,6 +2124,9 @@ static struct subtest_case crafted_cases[] = {
> >           {S32, U32, {(u32)S32_MIN, 0}, {0, 0}},
> >           {S32, U32, {(u32)S32_MIN, 0}, {(u32)S32_MIN, (u32)S32_MIN}},
> >           {S32, U32, {(u32)S32_MIN, S32_MAX}, {S32_MAX, S32_MAX}},
> > +       {S64, U32, {0x0, 0x1f}, {0xffffffff80000000ULL, 0x000000007fffffffULL}},
> > +       {S64, U32, {0x0, 0x1f}, {0xffffffffffff8000ULL, 0x0000000000007fffULL}},
> > +       {S64, U32, {0x0, 0x1f}, {0xffffffffffffff80ULL, 0x000000000000007fULL}},
> >    };
> >    
> >    /* Go over crafted hard-coded cases. This is fast, so we do it as part of
> >
> > The logic is very similar to kernel implementation but has a difference in generating
> > the final range. In reg_bounds implementation, the range is narrowed by intersecting
> > y_cast and x range which are necessary.
> >
> > In kernel implementation, there is no interection since we only have one register
> > and two register has been compared before.
> >
> > Eduard, could you take a look at the above code?
> I think this change is correct.
> The return clause could be simplified a bit:
>
> 	return range_improve(x_t, x, y_cast);

Indeed. This is much simpler. I will use reg_bounds testing instead of verifier_ldsx testing
in next revision.

> [...]