Re: [PATCH bpf-next v3 03/11] bpf: refactor check_func_call() to allow callback function

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Yonghong Song <yhs@fb.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Cong Wang <xiyou.wangcong@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next v3 03/11] bpf: refactor check_func_call() to allow callback function
Date: Thu, 25 Feb 2021 14:05:59 -0800	[thread overview]
Message-ID: <CAEf4BzZn125xN0p=mUvAfFzq+Pbequm9Yp0rSN0B=ru4X8X8Jg@mail.gmail.com> (raw)
In-Reply-To: <20210225073312.4120415-1-yhs@fb.com>

On Thu, Feb 25, 2021 at 1:35 AM Yonghong Song <yhs@fb.com> wrote:
>
> Later proposed bpf_for_each_map_elem() helper has callback
> function as one of its arguments. This patch refactored
> check_func_call() to permit callback function which sets
> callee state. Different callback functions may have
> different callee states.
>
> There is no functionality change for this patch except
> it added a case to handle where subprog number is known
> and there is no need to do find_subprog(). This case
> is used later by implementing bpf_for_each_map() helper.
>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  kernel/bpf/verifier.c | 54 ++++++++++++++++++++++++++++++++-----------
>  1 file changed, 41 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index a657860ecba5..092d2c734dd8 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5250,13 +5250,19 @@ static void clear_caller_saved_regs(struct bpf_verifier_env *env,
>         }
>  }
>
> -static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> -                          int *insn_idx)
> +typedef int (*set_callee_state_fn)(struct bpf_verifier_env *env,
> +                                  struct bpf_func_state *caller,
> +                                  struct bpf_func_state *callee,
> +                                  int insn_idx);
> +
> +static int __check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> +                            int *insn_idx, int subprog,
> +                            set_callee_state_fn set_callee_st)

nit: s/set_callee_st/set_callee_state_cb|set_calle_state_fn/

_st is quite an unusual suffix

>  {
>         struct bpf_verifier_state *state = env->cur_state;
>         struct bpf_func_info_aux *func_info_aux;
>         struct bpf_func_state *caller, *callee;
> -       int i, err, subprog, target_insn;
> +       int err, target_insn;
>         bool is_global = false;
>
>         if (state->curframe + 1 >= MAX_CALL_FRAMES) {
> @@ -5265,12 +5271,16 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>                 return -E2BIG;
>         }
>
> -       target_insn = *insn_idx + insn->imm;
> -       subprog = find_subprog(env, target_insn + 1);
>         if (subprog < 0) {
> -               verbose(env, "verifier bug. No program starts at insn %d\n",
> -                       target_insn + 1);
> -               return -EFAULT;
> +               target_insn = *insn_idx + insn->imm;
> +               subprog = find_subprog(env, target_insn + 1);
> +               if (subprog < 0) {
> +                       verbose(env, "verifier bug. No program starts at insn %d\n",
> +                               target_insn + 1);
> +                       return -EFAULT;
> +               }
> +       } else {
> +               target_insn = env->subprog_info[subprog].start - 1;
>         }
>
>         caller = state->frame[state->curframe];
> @@ -5327,11 +5337,9 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>         if (err)
>                 return err;
>
> -       /* copy r1 - r5 args that callee can access.  The copy includes parent
> -        * pointers, which connects us up to the liveness chain
> -        */
> -       for (i = BPF_REG_1; i <= BPF_REG_5; i++)
> -               callee->regs[i] = caller->regs[i];
> +       err = set_callee_st(env, caller, callee, *insn_idx);
> +       if (err)
> +               return err;
>
>         clear_caller_saved_regs(env, caller->regs);
>
> @@ -5350,6 +5358,26 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>         return 0;
>  }
>
> +static int set_callee_state(struct bpf_verifier_env *env,
> +                           struct bpf_func_state *caller,
> +                           struct bpf_func_state *callee, int insn_idx)
> +{
> +       int i;
> +
> +       /* copy r1 - r5 args that callee can access.  The copy includes parent
> +        * pointers, which connects us up to the liveness chain
> +        */
> +       for (i = BPF_REG_1; i <= BPF_REG_5; i++)
> +               callee->regs[i] = caller->regs[i];
> +       return 0;
> +}
> +
> +static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> +                          int *insn_idx)
> +{
> +       return __check_func_call(env, insn, insn_idx, -1, set_callee_state);

I think it would be much cleaner to not have this -1 special case in
__check_func_call and instead search for the right subprog right here
in check_func_call(). Related question, is meta.subprogno (in patch
#4) expected to sometimes be < 0? If not, then I think
__check_func_call() definitely shouldn't support -1 case at all.

> +}
> +
>  static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
>  {
>         struct bpf_verifier_state *state = env->cur_state;
> --
> 2.24.1
>