Re: [PATCH v5 bpf-next 17/23] bpf: generalize reg_set_min_max() to handle two sets of two registers

Eduard Zingerman
2023-10-31T18:14:05Z

On Fri, 2023-10-27 at 11:13 -0700, Andrii Nakryiko wrote:
> Change reg_set_min_max() to take FALSE/TRUE sets of two registers each,
> instead of assuming that we are always comparing to a constant. For now
> we still assume that right-hand side registers are constants (and make
> sure that's the case by swapping src/dst regs, if necessary), but
> subsequent patches will remove this limitation.
>
> Taking two by two registers allows to further unify and simplify
> check_cond_jmp_op() logic. We utilize fake register for BPF_K
> conditional jump case, just like with is_branch_taken() part.
>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

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

> ---
> kernel/bpf/verifier.c | 112 ++++++++++++++++++------------------------
> 1 file changed, 49 insertions(+), 63 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index dde04b17c3a3..522566699fbe 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -14387,26 +14387,43 @@ static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg
> * In JEQ/JNE cases we also adjust the var_off values.
> */
> static void reg_set_min_max(struct bpf_reg_state *true_reg1,
> + struct bpf_reg_state *true_reg2,
> struct bpf_reg_state *false_reg1,
> - u64 val, u32 val32,
> + struct bpf_reg_state *false_reg2,
> u8 opcode, bool is_jmp32)
> {
> - struct tnum false_32off = tnum_subreg(false_reg1->var_off);
> - struct tnum false_64off = false_reg1->var_off;
> - struct tnum true_32off = tnum_subreg(true_reg1->var_off);
> - struct tnum true_64off = true_reg1->var_off;
> - s64 sval = (s64)val;
> - s32 sval32 = (s32)val32;
> -
> - /* If the dst_reg is a pointer, we can't learn anything about its
> - * variable offset from the compare (unless src_reg were a pointer into
> - * the same object, but we don't bother with that.
> - * Since false_reg1 and true_reg1 have the same type by construction, we
> - * only need to check one of them for pointerness.
> + struct tnum false_32off, false_64off;
> + struct tnum true_32off, true_64off;
> + u64 val;
> + u32 val32;
> + s64 sval;
> + s32 sval32;
> +
> + /* If either register is a pointer, we can't learn anything about its
> + * variable offset from the compare (unless they were a pointer into
> + * the same object, but we don't bother with that).
> */
> - if (__is_pointer_value(false, false_reg1))
> + if (false_reg1->type != SCALAR_VALUE || false_reg2->type != SCALAR_VALUE)
> + return;
> +
> + /* we expect right-hand registers (src ones) to be constants, for now */
> + if (!is_reg_const(false_reg2, is_jmp32)) {
> + opcode = flip_opcode(opcode);
> + swap(true_reg1, true_reg2);
> + swap(false_reg1, false_reg2);
> + }
> + if (!is_reg_const(false_reg2, is_jmp32))
> return;
>
> + false_32off = tnum_subreg(false_reg1->var_off);
> + false_64off = false_reg1->var_off;
> + true_32off = tnum_subreg(true_reg1->var_off);
> + true_64off = true_reg1->var_off;
> + val = false_reg2->var_off.value;
> + val32 = (u32)tnum_subreg(false_reg2->var_off).value;
> + sval = (s64)val;
> + sval32 = (s32)val32;
> +
> switch (opcode) {
> /* JEQ/JNE comparison doesn't change the register equivalence.
> *
> @@ -14543,22 +14560,6 @@ static void reg_set_min_max(struct bpf_reg_state *true_reg1,
> }
> }
>
> -/* Same as above, but for the case that dst_reg holds a constant and src_reg is
> - * the variable reg.
> - */
> -static void reg_set_min_max_inv(struct bpf_reg_state *true_reg,
> - struct bpf_reg_state *false_reg,
> - u64 val, u32 val32,
> - u8 opcode, bool is_jmp32)
> -{
> - opcode = flip_opcode(opcode);
> - /* This uses zero as "not present in table"; luckily the zero opcode,
> - * BPF_JA, can't get here.
> - */
> - if (opcode)
> - reg_set_min_max(true_reg, false_reg, val, val32, opcode, is_jmp32);
> -}
> -
> /* Regs are known to be equal, so intersect their min/max/var_off */
> static void __reg_combine_min_max(struct bpf_reg_state *src_reg,
> struct bpf_reg_state *dst_reg)
> @@ -14891,45 +14892,30 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
> * comparable.
> */
> if (BPF_SRC(insn->code) == BPF_X) {
> - struct bpf_reg_state *src_reg = &regs[insn->src_reg];
> + reg_set_min_max(&other_branch_regs[insn->dst_reg],
> + &other_branch_regs[insn->src_reg],
> + dst_reg, src_reg, opcode, is_jmp32);
>
> if (dst_reg->type == SCALAR_VALUE &&
> - src_reg->type == SCALAR_VALUE) {
> - if (tnum_is_const(src_reg->var_off) ||
> - (is_jmp32 &&
> - tnum_is_const(tnum_subreg(src_reg->var_off))))
> - reg_set_min_max(&other_branch_regs[insn->dst_reg],
> - dst_reg,
> - src_reg->var_off.value,
> - tnum_subreg(src_reg->var_off).value,
> - opcode, is_jmp32);
> - else if (tnum_is_const(dst_reg->var_off) ||
> - (is_jmp32 &&
> - tnum_is_const(tnum_subreg(dst_reg->var_off))))
> - reg_set_min_max_inv(&other_branch_regs[insn->src_reg],
> - src_reg,
> - dst_reg->var_off.value,
> - tnum_subreg(dst_reg->var_off).value,
> - opcode, is_jmp32);
> - else if (!is_jmp32 &&
> - (opcode == BPF_JEQ || opcode == BPF_JNE))
> - /* Comparing for equality, we can combine knowledge */
> - reg_combine_min_max(&other_branch_regs[insn->src_reg],
> - &other_branch_regs[insn->dst_reg],
> - src_reg, dst_reg, opcode);
> - if (src_reg->id &&
> - !WARN_ON_ONCE(src_reg->id != other_branch_regs[insn->src_reg].id)) {
> - find_equal_scalars(this_branch, src_reg);
> - find_equal_scalars(other_branch, &other_branch_regs[insn->src_reg]);
> - }
> -
> + src_reg->type == SCALAR_VALUE &&
> + !is_jmp32 && (opcode == BPF_JEQ || opcode == BPF_JNE)) {
> + /* Comparing for equality, we can combine knowledge */
> + reg_combine_min_max(&other_branch_regs[insn->src_reg],
> + &other_branch_regs[insn->dst_reg],
> + src_reg, dst_reg, opcode);
> }
> } else if (dst_reg->type == SCALAR_VALUE) {
> - reg_set_min_max(&other_branch_regs[insn->dst_reg],
> - dst_reg, insn->imm, (u32)insn->imm,
> - opcode, is_jmp32);
> + reg_set_min_max(&other_branch_regs[insn->dst_reg], src_reg, /* fake one */
> + dst_reg, src_reg /* same fake one */,
> + opcode, is_jmp32);
> }
>
> + if (BPF_SRC(insn->code) == BPF_X &&
> + src_reg->type == SCALAR_VALUE && src_reg->id &&
> + !WARN_ON_ONCE(src_reg->id != other_branch_regs[insn->src_reg].id)) {
> + find_equal_scalars(this_branch, src_reg);
> + find_equal_scalars(other_branch, &other_branch_regs[insn->src_reg]);
> + }
> if (dst_reg->type == SCALAR_VALUE && dst_reg->id &&
> !WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id)) {
> find_equal_scalars(this_branch, dst_reg);