[PATCH bpf-next v5 6/8] bpf: Let bpf_iter_task_new accept null task ptr

From: Chuyi Zhou
Date: Wed Oct 11 2023 - 08:11:48 EST


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_{PROCS, THREADS} 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 add "__nullable" annotation:
__bpf_kfunc void FN(struct TYPE_A *obj__nullable);
Here __nullable 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@xxxxxxxxxxxxx>
---
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 caeddad3d2f1..0772545568f1 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -873,7 +873,7 @@ enum {
};

__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it,
- struct task_struct *task, unsigned int flags)
+ struct task_struct *task__nullable, unsigned int flags)
{
struct bpf_iter_task_kern *kit = (void *)it;

@@ -885,14 +885,17 @@ __bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it,
switch (flags) {
case BPF_TASK_ITER_ALL_THREADS:
case BPF_TASK_ITER_ALL_PROCS:
+ break;
case BPF_TASK_ITER_PROC_THREADS:
+ if (!task__nullable)
+ return -EINVAL;
break;
default:
return -EINVAL;
}

if (flags == BPF_TASK_ITER_PROC_THREADS)
- kit->task = task;
+ kit->task = task__nullable;
else
kit->task = &init_task;
kit->pos = kit->task;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3a60cc87520e..d09697dbfd9c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10310,6 +10310,11 @@ static bool is_kfunc_arg_refcounted_kptr(const struct btf *btf, const struct btf
return __kfunc_param_match_suffix(btf, arg, "__refcounted_kptr");
}

+static bool is_kfunc_arg_nullable(const struct btf *btf, const struct btf_param *arg)
+{
+ return __kfunc_param_match_suffix(btf, arg, "__nullable");
+}
+
static bool is_kfunc_arg_scalar_with_name(const struct btf *btf,
const struct btf_param *arg,
const char *name)
@@ -10452,6 +10457,7 @@ 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 {
@@ -10608,6 +10614,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
if (is_kfunc_arg_callback(env, meta->btf, &args[argno]))
return KF_ARG_PTR_TO_CALLBACK;

+ if (is_kfunc_arg_nullable(meta->btf, &args[argno]) && register_is_null(reg))
+ return KF_ARG_PTR_TO_NULL;

if (argno + 1 < nargs &&
(is_kfunc_arg_mem_size(meta->btf, &args[argno + 1], &regs[regno + 1]) ||
@@ -11158,7 +11166,8 @@ 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_nullable(meta->btf, &args[i])) {
verbose(env, "Possibly NULL pointer passed to trusted arg%d\n", i);
return -EACCES;
}
@@ -11183,6 +11192,8 @@ 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