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Commit fb8d251e authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by Daniel Borkmann
Browse files

bpf: extend is_branch_taken to registers 



This patch extends is_branch_taken() logic from JMP+K instructions
to JMP+X instructions.
Conditional branches are often done when src and dst registers
contain known scalars. In such case the verifier can follow
the branch that is going to be taken when program executes.
That speeds up the verification and is essential feature to support
bounded loops.

Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarAndrii Nakryiko <andriin@fb.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent fc559a70

kernel/bpf/verifier.c

+19 −15
Original line number Diff line number Diff line
@@ -5266,9 +5266,10 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 
	struct bpf_verifier_state *this_branch = env->cur_state;

	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;

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

	u8 opcode = BPF_OP(insn->code);

	bool is_jmp32;

	int pred = -1;

	int err;



	/* Only conditional jumps are expected to reach here. */
@@ -5293,6 +5294,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 
				insn->src_reg);

			return -EACCES;

		}

		src_reg = &regs[insn->src_reg];

	} else {

		if (insn->src_reg != BPF_REG_0) {

			verbose(env, "BPF_JMP/JMP32 uses reserved fields\n");
@@ -5308,10 +5310,13 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 
	dst_reg = &regs[insn->dst_reg];

	is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;



	if (BPF_SRC(insn->code) == BPF_K) {

		int pred = is_branch_taken(dst_reg, insn->imm, opcode,

					   is_jmp32);



	if (BPF_SRC(insn->code) == BPF_K)

		pred = is_branch_taken(dst_reg, insn->imm,

				       opcode, is_jmp32);

	else if (src_reg->type == SCALAR_VALUE &&

		 tnum_is_const(src_reg->var_off))

		pred = is_branch_taken(dst_reg, src_reg->var_off.value,

				       opcode, is_jmp32);

	if (pred == 1) {

		/* only follow the goto, ignore fall-through */

		*insn_idx += insn->off;
@@ -5322,7 +5327,6 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 
		 */

		return 0;

	}

	}



	other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx,

				  false);