[PATCH bpf-next v3 6/7] bpf: Let bpf_iter_task_new accept null task ptr

Chuyi Zhou
2023-09-25T10:56:50Z

When using task_iter to iterate all threads of a specific task, we enforce
that the user must pass a valid task pointer to ensure safety. However,
when iterating all threads/process in the system, BPF verifier still
require a valid ptr instead of "nullable" pointer, even though it's
pointless, which is a kind of surprising from usability standpoint. It
would be nice if we could let that kfunc accept a explicit null pointer
when we are using BPF_TASK_ITER_ALL/BPF_TASK_ITER_PROC and a valid pointer
when using BPF_TASK_ITER_THREAD.

Given a trival kfunc:
__bpf_kfunc void FN(struct TYPE_A *obj)

BPF Prog would reject a nullptr for obj. The error info is:
"arg#x pointer type xx xx must point to scalar, or struct with scalar"
reported by get_kfunc_ptr_arg_type(). The reg->type is SCALAR_VALUE and
the btf type of ref_t is not scalar or scalar_struct which leads to the
rejection of get_kfunc_ptr_arg_type.

This patch reuse the __opt annotation which was used to indicate that
the buffer associated with an __sz or __szk argument may be null:
__bpf_kfunc void FN(struct TYPE_A *obj__opt)
Here __opt indicates obj can be optional, user can pass a explicit nullptr
or a normal TYPE_A pointer. In get_kfunc_ptr_arg_type(), we will detect
whether the current arg is optional and register is null, If so, return
a new kfunc_ptr_arg_type KF_ARG_PTR_TO_NULL and skip to the next arg in
check_kfunc_args().

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
---
kernel/bpf/task_iter.c | 7 +++++--
kernel/bpf/verifier.c | 13 ++++++++++++-
2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index 9bcd3f9922b1..7ac007f161cc 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -867,7 +867,7 @@ struct bpf_iter_task_kern {
unsigned int type;
} __attribute__((aligned(8)));

-__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task, unsigned int type)
+__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task__opt, unsigned int type)
{
struct bpf_iter_task_kern *kit = (void *)it;
BUILD_BUG_ON(sizeof(struct bpf_iter_task_kern) != sizeof(struct bpf_iter_task));
@@ -877,14 +877,17 @@ __bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *
switch (type) {
case BPF_TASK_ITER_ALL:
case BPF_TASK_ITER_PROC:
+ break;
case BPF_TASK_ITER_THREAD:
+ if (!task__opt)
+ return -EINVAL;
break;
default:
return -EINVAL;
}

if (type == BPF_TASK_ITER_THREAD)
- kit->task = task;
+ kit->task = task__opt;
else
kit->task = &init_task;
kit->pos = kit->task;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a065e18a0b3a..a79204c75a90 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10331,6 +10331,7 @@ enum kfunc_ptr_arg_type {
KF_ARG_PTR_TO_CALLBACK,
KF_ARG_PTR_TO_RB_ROOT,
KF_ARG_PTR_TO_RB_NODE,
+ KF_ARG_PTR_TO_NULL,
};

enum special_kfunc_type {
@@ -10425,6 +10426,12 @@ static bool is_kfunc_bpf_rcu_read_unlock(struct bpf_kfunc_call_arg_meta *meta)
return meta->func_id == special_kfunc_list[KF_bpf_rcu_read_unlock];
}

+static inline bool is_kfunc_arg_optional_null(struct bpf_reg_state *reg,
+ const struct btf *btf, const struct btf_param *arg)
+{
+ return register_is_null(reg) && is_kfunc_arg_optional(btf, arg);
+}
+
static enum kfunc_ptr_arg_type
get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
struct bpf_kfunc_call_arg_meta *meta,
@@ -10497,6 +10504,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
*/
if (!btf_type_is_scalar(ref_t) && !__btf_type_is_scalar_struct(env, meta->btf, ref_t, 0) &&
(arg_mem_size ? !btf_type_is_void(ref_t) : 1)) {
+ if (is_kfunc_arg_optional_null(reg, meta->btf, &args[argno]))
+ return KF_ARG_PTR_TO_NULL;
verbose(env, "arg#%d pointer type %s %s must point to %sscalar, or struct with scalar\n",
argno, btf_type_str(ref_t), ref_tname, arg_mem_size ? "void, " : "");
return -EINVAL;
@@ -11028,7 +11037,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
}

if ((is_kfunc_trusted_args(meta) || is_kfunc_rcu(meta)) &&
- (register_is_null(reg) || type_may_be_null(reg->type))) {
+ (register_is_null(reg) || type_may_be_null(reg->type)) && !is_kfunc_arg_optional(meta->btf, &args[i])) {
verbose(env, "Possibly NULL pointer passed to trusted arg%d\n", i);
return -EACCES;
}
@@ -11053,6 +11062,8 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
return kf_arg_type;

switch (kf_arg_type) {
+ case KF_ARG_PTR_TO_NULL:
+ continue;
case KF_ARG_PTR_TO_ALLOC_BTF_ID:
case KF_ARG_PTR_TO_BTF_ID:
if (!is_kfunc_trusted_args(meta) && !is_kfunc_rcu(meta))
--
2.20.1