Re: [PATCH bpf-next 3/7] bpf: Add support for kprobe multi session cookie

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Apr 22, 2024 at 5:13 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding support for cookie within the session of kprobe multi
> entry and return program.
>
> The session cookie is u64 value and can be retrieved be new
> kfunc bpf_session_cookie, which returns pointer to the cookie
> value. The bpf program can use the pointer to store (on entry)
> and load (on return) the value.
>
> The cookie value is implemented via fprobe feature that allows
> to share values between entry and return ftrace fprobe callbacks.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/bpf/verifier.c    |  7 +++++++
>  kernel/trace/bpf_trace.c | 19 ++++++++++++++++---
>  2 files changed, 23 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 68cfd6fc6ad4..baaca451aebc 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -10987,6 +10987,7 @@ enum special_kfunc_type {
>         KF_bpf_percpu_obj_drop_impl,
>         KF_bpf_throw,
>         KF_bpf_iter_css_task_new,
> +       KF_bpf_session_cookie,
>  };
>
>  BTF_SET_START(special_kfunc_set)
> @@ -11013,6 +11014,7 @@ BTF_ID(func, bpf_throw)
>  #ifdef CONFIG_CGROUPS
>  BTF_ID(func, bpf_iter_css_task_new)
>  #endif
> +BTF_ID(func, bpf_session_cookie)
>  BTF_SET_END(special_kfunc_set)
>
>  BTF_ID_LIST(special_kfunc_list)
> @@ -11043,6 +11045,7 @@ BTF_ID(func, bpf_iter_css_task_new)
>  #else
>  BTF_ID_UNUSED
>  #endif
> +BTF_ID(func, bpf_session_cookie)
>
>  static bool is_kfunc_ret_null(struct bpf_kfunc_call_arg_meta *meta)
>  {
> @@ -12409,6 +12412,10 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>                                  * because packet slices are not refcounted (see
>                                  * dynptr_type_refcounted)
>                                  */
> +                       } else if (meta.func_id == special_kfunc_list[KF_bpf_session_cookie]) {
> +                               mark_reg_known_zero(env, regs, BPF_REG_0);
> +                               regs[BPF_REG_0].type = PTR_TO_MEM;
> +                               regs[BPF_REG_0].mem_size = sizeof(u64);

Are you sure you need this?

} else if (!__btf_type_is_struct(ptr_type)) {

block should have handled it automatically.

> +__bpf_kfunc __u64 *bpf_session_cookie(void)
> +{

...