| Message ID | 20231011120857.251943-7-zhouchuyi@bytedance.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | Add Open-coded task, css_task and css iters | expand |
On Wed, Oct 11, 2023 at 5:09 AM Chuyi Zhou <zhouchuyi@bytedance.com> wrote: > > 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@bytedance.com> > --- > kernel/bpf/task_iter.c | 7 +++++-- > kernel/bpf/verifier.c | 13 ++++++++++++- > 2 files changed, 17 insertions(+), 3 deletions(-) > Looks good to me, but someone better versed in kfunc internals should double-check. Acked-by: Andrii Nakryiko <andrii@kernel.org> > 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 { > }; > [...]
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], ®s[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))
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@bytedance.com> --- kernel/bpf/task_iter.c | 7 +++++-- kernel/bpf/verifier.c | 13 ++++++++++++- 2 files changed, 17 insertions(+), 3 deletions(-)