Re: [PATCH bpf-next 02/13] bpf: generalize is_scalar_branch_taken() logic

Eduard Zingerman
2023-11-03T16:47:37Z

On Thu, 2023-11-02 at 17:08 -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>

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

(With the same nitpick that '<' cases could be converted to '>' cases).

> ---
> kernel/bpf/verifier.c | 103 ++++++++++++++++++++++++++----------------
> 1 file changed, 63 insertions(+), 40 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 52934080042c..2627461164ed 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -14187,82 +14187,104 @@ 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 uval = is_jmp32 ? (u32)tnum_subreg(reg2->var_off).value : reg2->var_off.value;
> - s64 sval = is_jmp32 ? (s32)uval : (s64)uval;
> + 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, uval);
> - else if (uval < umin1 || uval > umax1)
> + /* constants, umin/umax and smin/smax checks would be
> + * redundant in this case because they all should match
> + */
> + if (tnum_is_const(t1) && tnum_is_const(t2))
> + return t1.value == t2.value;
> + /* const ranges */
> + if (umin1 == umax1 && umin2 == umax2)
> + return umin1 == umin2;
> + if (smin1 == smax1 && smin2 == smax2)
> + return smin1 == smin2;
> + /* non-overlapping ranges */
> + if (umin1 > umax2 || umax1 < umin2)
> return 0;
> - else if (sval < smin1 || sval > smax1)
> + if (smin1 > smax2 || smax1 < smin2)
> return 0;
> break;
> case BPF_JNE:
> - if (tnum_is_const(t1))
> - return !tnum_equals_const(t1, uval);
> - else if (uval < umin1 || uval > umax1)
> + /* constants, umin/umax and smin/smax checks would be
> + * redundant in this case because they all should match
> + */
> + if (tnum_is_const(t1) && tnum_is_const(t2))
> + return t1.value != t2.value;
> + /* non-overlapping ranges */
> + if (umin1 > umax2 || umax1 < umin2)
> return 1;
> - else if (sval < smin1 || sval > smax1)
> + if (smin1 > smax2 || smax1 < smin2)
> return 1;
> break;
> case BPF_JSET:
> - if ((~t1.mask & t1.value) & uval)
> + if (!is_reg_const(reg2, is_jmp32)) {
> + swap(reg1, reg2);
> + swap(t1, t2);
> + }
> + if (!is_reg_const(reg2, is_jmp32))
> + return -1;
> + if ((~t1.mask & t1.value) & t2.value)
> return 1;
> - if (!((t1.mask | t1.value) & uval))
> + if (!((t1.mask | t1.value) & t2.value))
> return 0;
> break;
> case BPF_JGT:
> - if (umin1 > uval )
> + if (umin1 > umax2)
> return 1;
> - else if (umax1 <= uval)
> + else if (umax1 <= umin2)
> return 0;
> break;
> case BPF_JSGT:
> - if (smin1 > sval)
> + if (smin1 > smax2)
> return 1;
> - else if (smax1 <= sval)
> + else if (smax1 <= smin2)
> return 0;
> break;
> case BPF_JLT:
> - if (umax1 < uval)
> + if (umax1 < umin2)
> return 1;
> - else if (umin1 >= uval)
> + else if (umin1 >= umax2)
> return 0;
> break;
> case BPF_JSLT:
> - if (smax1 < sval)
> + if (smax1 < smin2)
> return 1;
> - else if (smin1 >= sval)
> + else if (smin1 >= smax2)
> return 0;
> break;
> case BPF_JGE:
> - if (umin1 >= uval)
> + if (umin1 >= umax2)
> return 1;
> - else if (umax1 < uval)
> + else if (umax1 < umin2)
> return 0;
> break;
> case BPF_JSGE:
> - if (smin1 >= sval)
> + if (smin1 >= smax2)
> return 1;
> - else if (smax1 < sval)
> + else if (smax1 < smin2)
> return 0;
> break;
> case BPF_JLE:
> - if (umax1 <= uval)
> + if (umax1 <= umin2)
> return 1;
> - else if (umin1 > uval)
> + else if (umin1 > umax2)
> return 0;
> break;
> case BPF_JSLE:
> - if (smax1 <= sval)
> + if (smax1 <= smin2)
> return 1;
> - else if (smin1 > sval)
> + else if (smin1 > smax2)
> return 0;
> break;
> }
> @@ -14341,28 +14363,28 @@ static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
> static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
> u8 opcode, bool is_jmp32)
> {
> - u64 val;
> -
> if (reg_is_pkt_pointer_any(reg1) && reg_is_pkt_pointer_any(reg2) && !is_jmp32)
> return is_pkt_ptr_branch_taken(reg1, reg2, opcode);
>
> - /* try to make sure reg2 is a constant SCALAR_VALUE */
> - if (!is_reg_const(reg2, is_jmp32)) {
> - opcode = flip_opcode(opcode);
> - swap(reg1, reg2);
> - }
> - /* for now we expect reg2 to be a constant to make any useful decisions */
> - if (!is_reg_const(reg2, is_jmp32))
> - return -1;
> - val = reg_const_value(reg2, is_jmp32);
> + if (__is_pointer_value(false, reg1) || __is_pointer_value(false, reg2)) {
> + u64 val;
> +
> + /* arrange that reg2 is a scalar, and reg1 is a pointer */
> + if (!is_reg_const(reg2, is_jmp32)) {
> + opcode = flip_opcode(opcode);
> + swap(reg1, reg2);
> + }
> + /* and ensure that reg2 is a constant */
> + if (!is_reg_const(reg2, is_jmp32))
> + return -1;
>
> - if (__is_pointer_value(false, reg1)) {
> if (!reg_not_null(reg1))
> return -1;
>
> /* If pointer is valid tests against zero will fail so we can
> * use this to direct branch taken.
> */
> + val = reg_const_value(reg2, is_jmp32);
> if (val != 0)
> return -1;
>
> @@ -14376,6 +14398,7 @@ static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg
> }
> }
>
> + /* now deal with two scalars, but not necessarily constants */
> return is_scalar_branch_taken(reg1, reg2, opcode, is_jmp32);
> }
>