Commit befae758 authored by Eduard Zingerman's avatar Eduard Zingerman Committed by Alexei Starovoitov
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bpf: propagate nullness information for reg to reg comparisons 



Propagate nullness information for branches of register to register
equality compare instructions. The following rules are used:
- suppose register A maybe null
- suppose register B is not null
- for JNE A, B, ... - A is not null in the false branch
- for JEQ A, B, ... - A is not null in the true branch

E.g. for program like below:

  r6 = skb->sk;
  r7 = sk_fullsock(r6);
  r0 = sk_fullsock(r6);
  if (r0 == 0) return 0;    (a)
  if (r0 != r7) return 0;   (b)
  *r7->type;                (c)
  return 0;

It is safe to dereference r7 at point (c), because of (a) and (b).

Signed-off-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Acked-by: default avatarYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221115224859.2452988-2-eddyz87@gmail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 32637e33

kernel/bpf/verifier.c

+33 −2
Original line number Diff line number Diff line
@@ -10267,6 +10267,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 
	struct bpf_verifier_state *other_branch;

	struct bpf_reg_state *regs = this_branch->frame[this_branch->curframe]->regs;

	struct bpf_reg_state *dst_reg, *other_branch_regs, *src_reg = NULL;

	struct bpf_reg_state *eq_branch_regs;

	u8 opcode = BPF_OP(insn->code);

	bool is_jmp32;

	int pred = -1;
@@ -10376,8 +10377,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 
	/* detect if we are comparing against a constant value so we can adjust

	 * our min/max values for our dst register.

	 * this is only legit if both are scalars (or pointers to the same

	 * object, I suppose, but we don't support that right now), because

	 * otherwise the different base pointers mean the offsets aren't

	 * object, I suppose, see the PTR_MAYBE_NULL related if block below),

	 * because otherwise the different base pointers mean the offsets aren't

	 * comparable.

	 */

	if (BPF_SRC(insn->code) == BPF_X) {
@@ -10426,6 +10427,36 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 
		find_equal_scalars(other_branch, &other_branch_regs[insn->dst_reg]);

	}



	/* if one pointer register is compared to another pointer

	 * register check if PTR_MAYBE_NULL could be lifted.

	 * E.g. register A - maybe null

	 *      register B - not null

	 * for JNE A, B, ... - A is not null in the false branch;

	 * for JEQ A, B, ... - A is not null in the true branch.

	 */

	if (!is_jmp32 && BPF_SRC(insn->code) == BPF_X &&

	    __is_pointer_value(false, src_reg) && __is_pointer_value(false, dst_reg) &&

	    type_may_be_null(src_reg->type) != type_may_be_null(dst_reg->type)) {

		eq_branch_regs = NULL;

		switch (opcode) {

		case BPF_JEQ:

			eq_branch_regs = other_branch_regs;

			break;

		case BPF_JNE:

			eq_branch_regs = regs;

			break;

		default:

			/* do nothing */

			break;

		}

		if (eq_branch_regs) {

			if (type_may_be_null(src_reg->type))

				mark_ptr_not_null_reg(&eq_branch_regs[insn->src_reg]);

			else

				mark_ptr_not_null_reg(&eq_branch_regs[insn->dst_reg]);

		}

	}



	/* detect if R == 0 where R is returned from bpf_map_lookup_elem().

	 * NOTE: these optimizations below are related with pointer comparison

	 *       which will never be JMP32.