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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Feb 25, 2021 at 4:08 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 2/25/21 2:31 PM, Andrii Nakryiko wrote:
> > On Thu, Feb 25, 2021 at 2:05 PM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> >>
> >> 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,
> >
> > ok, patch #4 confused me because of this `int *insn_idx`. You don't
> > seem to be ever updating it, so why pass it by pointer?... What did I
> > miss?
>
> We do have something later:
>
>          /* and go analyze first insn of the callee */
>          *insn_idx = target_insn;
>
> which is the old code and probably did not show up in the diff.
> The above statement changed insn_idx such that when done with
> examining the func call, the control will jump (*insn_idx)++ instruction.

So I did miss something. Thanks for explaining!

>
> >
> >>> +                            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
> >>>