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

Alexei Starovoitov
2023-10-31T02:12:08Z

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.

> + if (smin1 == smax1 && smin2 == smax2)
> + return umin1 == umin2;

here it's even more confusing. smin == smax -> singel const,
but then compare umin1 with umin2 ?!

> + /* 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.