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bpf: allow non-owning list-node args via __nonown_allowed

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KF_ARG_PTR_TO_LIST_NODE normally requires an owning reference
(PTR_TO_BTF_ID | MEM_ALLOC with ref_obj_id). Introduce  the
__nonown_allowed annotation on selected list-node arguments so
non-owning references with ref_obj_id==0 are accepted as well.

This patch only adds the generic verifier support and documents the
annotation. Later patches in the series will apply it to bpf_list_add
/del(), and bpf_list_is_first/last(), allowing bpf_list_front/back()
results to be used as the insertion point, deletion target, or query
target for those kfuncs.

Verifier keeps existing owning-ref checks by default; only arguments
annotated with __nonown_allowed bypass MEM_ALLOC/ref_obj_id checks
and then follow the same list-node validation path.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260521032306.97118-4-kaitao.cheng@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Kaitao Cheng 2026-05-21 11:23:01 +08:00 committed by Alexei Starovoitov
parent cfa6afa4b9
commit 7c8c71591b
2 changed files with 33 additions and 2 deletions
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22
Documentation/bpf/kfuncs.rst
View File

@ -207,8 +207,26 @@ Here, the buffer may be NULL. If the buffer is not NULL, it must be at least
buffer__szk bytes in size. The kfunc is responsible for checking if the buffer
is NULL before using it.
2.3.5 __str Annotation
----------------------------
2.3.5 __nonown_allowed Annotation
---------------------------------
This annotation is used to indicate that the parameter may be a non-owning reference.
An example is given below::
__bpf_kfunc int bpf_list_add(..., struct bpf_list_node
*prev__nonown_allowed, ...)
{
...
}
For the ``prev__nonown_allowed`` parameter (resolved as ``KF_ARG_PTR_TO_LIST_NODE``),
suffix ``__nonown_allowed`` retains the usual owning-pointer rules and also
permits a non-owning reference with no ref_obj_id (e.g. the return value of
bpf_list_front() / bpf_list_back()).
2.3.6 __str Annotation
----------------------
This annotation is used to indicate that the argument is a constant string.
An example is given below::

13
kernel/bpf/verifier.c
View File

@ -10714,6 +10714,11 @@ static bool is_kfunc_arg_nullable(const struct btf *btf, const struct btf_param
return btf_param_match_suffix(btf, arg, "__nullable");
}
static bool is_kfunc_arg_nonown_allowed(const struct btf *btf, const struct btf_param *arg)
{
return btf_param_match_suffix(btf, arg, "__nonown_allowed");
}
static bool is_kfunc_arg_const_str(const struct btf *btf, const struct btf_param *arg)
{
return btf_param_match_suffix(btf, arg, "__str");
@ -12244,6 +12249,13 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
return ret;
break;
case KF_ARG_PTR_TO_LIST_NODE:
if (is_kfunc_arg_nonown_allowed(btf, &args[i]) &&
type_is_non_owning_ref(reg->type) && !reg->ref_obj_id) {
/* Allow bpf_list_front/back return value for
* __nonown_allowed list-node arguments.
*/
goto check_ok;
}
if (reg->type != (PTR_TO_BTF_ID | MEM_ALLOC)) {
verbose(env, "%s expected pointer to allocated object\n",
reg_arg_name(env, argno));
@ -12253,6 +12265,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
verbose(env, "allocated object must be referenced\n");
return -EINVAL;
}
check_ok:
ret = process_kf_arg_ptr_to_list_node(env, reg, argno, meta);
if (ret < 0)
return ret;