Commit 58aa2afb authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by Andrii Nakryiko
Browse files

bpf: Allow ld_imm64 instruction to point to kfunc. 



Allow ld_imm64 insn with BPF_PSEUDO_BTF_ID to hold the address of kfunc. The
ld_imm64 pointing to a valid kfunc will be seen as non-null PTR_TO_MEM by
is_branch_taken() logic of the verifier, while libbpf will resolve address to
unknown kfunc as ld_imm64 reg, 0 which will also be recognized by
is_branch_taken() and the verifier will proceed dead code elimination. BPF
programs can use this logic to detect at load time whether kfunc is present in
the kernel with bpf_ksym_exists() macro that is introduced in the next patches.

Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Reviewed-by: default avatarMartin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: default avatarToke Høiland-Jørgensen <toke@redhat.com>
Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20230317201920.62030-2-alexei.starovoitov@gmail.com
parent 0f10f647

kernel/bpf/verifier.c

+11 −6
Original line number Diff line number Diff line
@@ -15952,8 +15952,8 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env, 
		goto err_put;

	}









	if (!btf_type_is_var(t)) {









		verbose(env, "pseudo btf_id %d in ldimm64 isn't KIND_VAR.\n", id);









	if (!btf_type_is_var(t) && !btf_type_is_func(t)) {









		verbose(env, "pseudo btf_id %d in ldimm64 isn't KIND_VAR or KIND_FUNC\n", id);









		err = -EINVAL;









		goto err_put;









	}








@@ -15966,6 +15966,14 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,







		err = -ENOENT;









		goto err_put;









	}









	insn[0].imm = (u32)addr;









	insn[1].imm = addr >> 32;

















	if (btf_type_is_func(t)) {









		aux->btf_var.reg_type = PTR_TO_MEM | MEM_RDONLY;









		aux->btf_var.mem_size = 0;









		goto check_btf;









	}

















	datasec_id = find_btf_percpu_datasec(btf);









	if (datasec_id > 0) {








@@ -15978,9 +15986,6 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,







		}









	}

















	insn[0].imm = (u32)addr;









	insn[1].imm = addr >> 32;

















	type = t->type;









	t = btf_type_skip_modifiers(btf, type, NULL);









	if (percpu) {








@@ -16008,7 +16013,7 @@ static int check_pseudo_btf_id(struct bpf_verifier_env *env,







		aux->btf_var.btf = btf;









		aux->btf_var.btf_id = type;









	}

















check_btf:









	/* check whether we recorded this BTF (and maybe module) already */









	for (i = 0; i < env->used_btf_cnt; i++) {









		if (env->used_btfs[i].btf == btf) {