Re: [PATCH v5 bpf-next 19/23] bpf: generalize is_scalar_branch_taken() logic

Andrii Nakryiko
2023-10-31T06:18:50Z

On Mon, Oct 30, 2023 at 7:12 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Fri, Oct 27, 2023 at 11:13:42AM -0700, Andrii Nakryiko wrote:
> > Generalize is_branch_taken logic for SCALAR_VALUE register to handle
> > cases when both registers are not constants. Previously supported
> > <range> vs <scalar> cases are a natural subset of more generic <range>
> > vs <range> set of cases.
> >
> > Generalized logic relies on straightforward segment intersection checks.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > ---
> > kernel/bpf/verifier.c | 104 ++++++++++++++++++++++++++----------------
> > 1 file changed, 64 insertions(+), 40 deletions(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 4c974296127b..f18a8247e5e2 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -14189,82 +14189,105 @@ static int is_scalar_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_sta
> > u8 opcode, bool is_jmp32)
> > {
> > struct tnum t1 = is_jmp32 ? tnum_subreg(reg1->var_off) : reg1->var_off;
> > + struct tnum t2 = is_jmp32 ? tnum_subreg(reg2->var_off) : reg2->var_off;
> > u64 umin1 = is_jmp32 ? (u64)reg1->u32_min_value : reg1->umin_value;
> > u64 umax1 = is_jmp32 ? (u64)reg1->u32_max_value : reg1->umax_value;
> > s64 smin1 = is_jmp32 ? (s64)reg1->s32_min_value : reg1->smin_value;
> > s64 smax1 = is_jmp32 ? (s64)reg1->s32_max_value : reg1->smax_value;
> > - u64 val = is_jmp32 ? (u32)tnum_subreg(reg2->var_off).value : reg2->var_off.value;
> > - s64 sval = is_jmp32 ? (s32)val : (s64)val;
> > + u64 umin2 = is_jmp32 ? (u64)reg2->u32_min_value : reg2->umin_value;
> > + u64 umax2 = is_jmp32 ? (u64)reg2->u32_max_value : reg2->umax_value;
> > + s64 smin2 = is_jmp32 ? (s64)reg2->s32_min_value : reg2->smin_value;
> > + s64 smax2 = is_jmp32 ? (s64)reg2->s32_max_value : reg2->smax_value;
> >
> > switch (opcode) {
> > case BPF_JEQ:
> > - if (tnum_is_const(t1))
> > - return !!tnum_equals_const(t1, val);
> > - else if (val < umin1 || val > umax1)
> > + /* const tnums */
> > + if (tnum_is_const(t1) && tnum_is_const(t2))
> > + return t1.value == t2.value;
> > + /* const ranges */
> > + if (umin1 == umax1 && umin2 == umax2)
> > + return umin1 == umin2;
>
> I don't follow this logic.
> umin1 == umax1 means that it's a single constant and
> it should have been handled by earlier tnum_is_const check.

I think you follow the logic, you just think it's redundant. Yes, it's
basically the same as

if (tnum_is_const(t1) && tnum_is_const(t2))
return t1.value == t2.value;

but based on ranges. I didn't feel comfortable to assume that if umin1
== umax1 then tnum_is_const(t1) will always be true. At worst we'll
perform one redundant check.

In short, I don't trust tnum to be as precise as umin/umax and other ranges.

>
> > + if (smin1 == smax1 && smin2 == smax2)
> > + return umin1 == umin2;
>
> here it's even more confusing. smin == smax -> singel const,
> but then compare umin1 with umin2 ?!

Eagle eyes! Typo, sorry :( it should be `smin1 == smin2`, of course.

What saves us is reg_bounds_sync(), and if we have umin1 == umax1 then
we'll have also smin1 == smax1 == umin1 == umax1 (and corresponding
relation for second register). But I fixed these typos in both BPF_JEQ
and BPF_JNE branches.

>
> > + /* non-overlapping ranges */
> > + if (umin1 > umax2 || umax1 < umin2)
> > return 0;
> > - else if (sval < smin1 || sval > smax1)
> > + if (smin1 > smax2 || smax1 < smin2)
> > return 0;
>
> this part makes sense.