On Mon, Oct 30, 2023 at 11:12 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Mon, Oct 30, 2023 at 7:12 PM Alexei Starovoitov
> <alexei.starovoitov@xxxxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > > 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.
Not just 'saves us'. The tnum <-> bounds sync is mandatory.
I think we have a test where a function returns [-errno, 0]
and then we do if (ret < 0) check. At this point the reg has
to be tnum_is_const and zero.
So if smin1 == smax1 == umin1 == umax1 it should be tnum_is_const.
Otherwise it's a bug in sync logic.
I think instead of doing redundant and confusing check may be
add WARN either here or in sync logic to make sure it's all good ?
