[PATCH bpf-next v2 09/23] bpf: Set OBJ_RELEASE when generating kfunc argument types

[Amery Hung <ameryhung@gmail.com>]

check_kfunc_args() ORs OBJ_RELEASE into arg_type on every verification
of every argument of every call, from a comparison against
meta->release_regno. Neither side of that comparison depends on the
call site. bpf_fetch_kfunc_arg_meta() sets release_regno to BPF_REG_1
when the kfunc is KF_RELEASE and leaves it zero otherwise, and regno
is derived from the argument index, so the test is true exactly for
argument 0 of a KF_RELEASE kfunc.

Set the flag in get_kfunc_arg_type() instead, where the rest
of the classification is built, and drop the comparison. The release
handling in check_func_proto() is helper-only, so an OBJ_RELEASE in a
generated kfunc prototype does not feed back into meta->release_regno.

meta->release_regno itself stays: it is still what tells the referenced
PTR_TO_BTF_ID check and release_reg() which register to act on.

No functional change.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
---
 kernel/bpf/verifier.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c7822addfc64..20e7da6674a1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12311,6 +12311,13 @@ get_kfunc_arg_type(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
 	if (is_kfunc_arg_nullable(meta->btf, &args[arg]))
 		arg_type |= PTR_MAYBE_NULL;
 
+	/*
+	 * Only the first argument of a KF_RELEASE kfunc releases anything, and
+	 * bpf_fetch_kfunc_arg_meta() only ever records BPF_REG_1 for it.
+	 */
+	if (is_kfunc_release(meta) && arg == 0)
+		arg_type |= OBJ_RELEASE;
+
 	return arg_type;
 }
 
@@ -12997,8 +13004,6 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_call_arg_me
 			ref_tname = btf_name_by_offset(btf, ref_t->name_off);
 		}
 
-		if (regno == meta->release_regno)
-			arg_type |= OBJ_RELEASE;
 		ret = check_func_arg_reg_off(env, reg, argno, arg_type);
 		if (ret < 0)
 			return ret;
-- 
2.52.0