From: Alexei Starovoitov <ast@xxxxxxxxxx>
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: Alexei Starovoitov <ast@xxxxxxxxxx>
Reviewed-by: Martin KaFai Lau <martin.lau@xxxxxxxxxx>
Acked-by: John Fastabend <john.fastabend@xxxxxxxxx>
Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
---
kernel/bpf/verifier.c | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d62b7127ff2a..1bc8a6d6fdd2 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -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) {
--
2.34.1
