Re: [PATCH v2 2/2] selftests/bpf: Add testcases for BPF_ADD and BPF_SUB

[Harishankar Vishwanathan]

On Wed, Jun 18, 2025 at 5:22 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
> On Tue, 2025-06-17 at 19:17 -0400, Harishankar Vishwanathan wrote:
> > The previous commit improves the precision in scalar(32)_min_max_add,
> > and scalar(32)_min_max_sub. The improvement in precision occurs in
> > cases when all outcomes overflow or underflow, respectively. This
> > commit adds selftests that exercise those cases.
> >
> > Co-developed-by: Matan Shachnai <m.shachnai@xxxxxxxxxxx>
> > Signed-off-by: Matan Shachnai <m.shachnai@xxxxxxxxxxx>
> > Signed-off-by: Harishankar Vishwanathan <harishankar.vishwanathan@xxxxxxxxx>
> > ---
>
> Could you please also add test cases when one bound overflows while
> another does not? Or these are covered by some other tests?

Yes this is possible and I can add such test cases. These are not covered by
other tests as far as I can see.

[...]

> > +SEC("socket")
> > +__description("64-bit addition overflow, all outcomes overflow")
> > +__success __log_level(2)
> > +__msg("7: (0f) r5 += r3 {{.*}} R5_w=scalar(smin=0x800003d67e960f7d,umin=0x551ee3d67e960f7d,umax=0xc0149fffffffffff,smin32=0xfe960f7d,umin32=0x7e960f7d,var_off=(0x3d67e960f7d; 0xfffffc298169f082))")
>
> Would it be possible to pick some more "human readable" constants here?
> As-is it is hard to make sense what verifier actually computes.
>
> > +__retval(0)
> > +__naked void add64_ovf(void)
> > +{
> > +     asm volatile (
> > +     "call %[bpf_get_prandom_u32];"
> > +     "r3 = r0;"
> > +     "r4 = 0x950a43d67e960f7d ll;"
> > +     "r3 |= r4;"
> > +     "r5 = 0xc014a00000000000 ll;"
> > +     "r5 += r3;"
> > +     "r0 = 0;"
> > +     "exit"
> > +     :
> > +     : __imm(bpf_get_prandom_u32)
> > +     : __clobber_all);
> > +}

It is possible to pick more human readable constants, but the precision gains
might not be as apparent. For instance, with the above (current) test case,
the old scalar_min_max_add() produced
[umin_value=0x3d67e960f7d, umax_value=U64_MAX],
while the updated scalar_min_max_add() produces a much more
precise [0x551ee3d67e960f7d, 0xc0149fffffffffff], a bound that has close to
2**63 fewer inhabitants.

For the purposes of a test case, if human readability is more important
than the demonstration of a large precision gain, I can prefer one that is more
readable, similar to the one shown in the commit message of v1 of the
patch [1]:

With the old scalar_min_max_add(), we get r3's bounds set to unbounded, i.e.,
[0, U64_MAX] after instruction 6: (0f) r3 += r3

0: R1=ctx() R10=fp0
0: (18) r3 = 0x8000000000000000       ; R3_w=0x8000000000000000
2: (18) r4 = 0x0                      ; R4_w=0
4: (87) r4 = -r4                      ; R4_w=scalar()
5: (4f) r3 |= r4                      ;
R3_w=scalar(smax=-1,umin=0x8000000000000000,var_off=(0x8000000000000000;
0x7fffffffffffffff)) R4_w=scalar()
6: (0f) r3 += r3                      ; R3_w=scalar()
7: (b7) r0 = 1                        ; R0_w=1
8: (95) exit

With the new scalar_min_max_add(), we get r3's bounds set to
[0, 0xfffffffffffffffe], a bound that is more precise by having only 1 less
inhabitant.

...
6: (0f) r3 += r3                      ; R3_w=scalar(umax=0xfffffffffffffffe)
7: (b7) r0 = 1                        ; R0_w=1
8: (95) exit

Please advise which test cases to prefer. I will follow up with a v3.

[1]: https://lore.kernel.org/bpf/20250610221356.2663491-1-harishankar.vishwanathan@xxxxxxxxx/

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