Subject: Re: [oss-drivers] Re: [RFC bpf-next 2/8] bpf: extend list based insn patching infra to verification layer

From: Jiong Wang <[email-masked]>
Date: Mon, 15 Jul 2019 11:02:28 +0100

Andrii Nakryiko writes:

> On Thu, Jul 11, 2019 at 5:20 AM Jiong Wang <[email-masked]> wrote:
>>
>>
>> Jiong Wang writes:
>>
>> > Andrii Nakryiko writes:
>> >
>> >> On Thu, Jul 4, 2019 at 2:32 PM Jiong Wang <[email-masked]> wrote:
>> >>>
>> >>> Verification layer also needs to handle auxiliar info as well as adjusting
>> >>> subprog start.
>> >>>
>> >>> At this layer, insns inside patch buffer could be jump, but they should
>> >>> have been resolved, meaning they shouldn't jump to insn outside of the
>> >>> patch buffer. Lineration function for this layer won't touch insns inside
>> >>> patch buffer.
>> >>>
>> >>> Adjusting subprog is finished along with adjusting jump target when the
>> >>> input will cover bpf to bpf call insn, re-register subprog start is cheap.
>> >>> But adjustment when there is insn deleteion is not considered yet.
>> >>>
>> >>> Signed-off-by: Jiong Wang <[email-masked]>
>> >>> ---
>> >>>  kernel/bpf/verifier.c | 150 ++++++++++++++++++++++++++++++++++++++++++++++++++
>> >>>  1 file changed, 150 insertions(+)
>> >>>
>> >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> >>> index a2e7637..2026d64 100644
>> >>> --- a/kernel/bpf/verifier.c
>> >>> +++ b/kernel/bpf/verifier.c
>> >>> @@ -8350,6 +8350,156 @@ static void opt_hard_wire_dead_code_branches(struct bpf_verifier_env *env)
>> >>>         }
>> >>>  }
>> >>>
>> >>> +/* Linearize bpf list insn to array (verifier layer). */
>> >>> +static struct bpf_verifier_env *
>> >>> +verifier_linearize_list_insn(struct bpf_verifier_env *env,
>> >>> +                            struct bpf_list_insn *list)
>> >>
>> >> It's unclear why this returns env back? It's not allocating a new env,
>> >> so it's weird and unnecessary. Just return error code.
>> >
>> > The reason is I was thinking we have two layers in BPF, the core and the
>> > verifier.
>> >
>> > For core layer (the relevant file is core.c), when doing patching, the
>> > input is insn list and bpf_prog, the linearization should linearize the
>> > insn list into insn array, and also whatever others affect inside bpf_prog
>> > due to changing on insns, for example line info inside prog->aux. So the
>> > return value is bpf_prog for core layer linearization hook.
>> >
>> > For verifier layer, it is similar, but the context if bpf_verifier_env, the
>> > linearization hook should linearize the insn list, and also those affected
>> > inside env, for example bpf_insn_aux_data, so the return value is
>> > bpf_verifier_env, meaning returning an updated verifier context
>> > (bpf_verifier_env) after insn list linearization.
>>
>> Realized your point is no new env is allocated, so just return error
>> code. Yes, the env pointer is not changed, just internal data is
>> updated. Return bpf_verifier_env mostly is trying to make the hook more
>> clear that it returns an updated "context" where the linearization happens,
>> for verifier layer, it is bpf_verifier_env, and for core layer, it is
>> bpf_prog, so return value was designed to return these two types.
>
> Oh, I missed that core layer returns bpf_prog*. I think this is
> confusing as hell and is very contrary to what one would expect. If
> the function doesn't allocate those objects, it shouldn't return them,
> except for rare cases of some accessor functions. Me reading this,
> I'll always be suprised and will have to go skim code just to check
> whether those functions really return new bpf_prog or
> bpf_verifier_env, respectively.

bpf_prog_realloc do return new bpf_prog, so we will need to return bpf_prog
* for core layer.

>
> Please change them both to just return error code.
>
>>
>> >
>> > Make sense?
>> >
>> > Regards,
>> > Jiong
>> >
>> >>
>> >>> +{
>> >>> +       u32 *idx_map, idx, orig_cnt, fini_cnt = 0;
>> >>> +       struct bpf_subprog_info *new_subinfo;
>> >>> +       struct bpf_insn_aux_data *new_data;
>> >>> +       struct bpf_prog *prog = env->prog;
>> >>> +       struct bpf_verifier_env *ret_env;
>> >>> +       struct bpf_insn *insns, *insn;
>> >>> +       struct bpf_list_insn *elem;
>> >>> +       int ret;
>> >>> +
>> >>> +       /* Calculate final size. */
>> >>> +       for (elem = list; elem; elem = elem->next)
>> >>> +               if (!(elem->flag & LIST_INSN_FLAG_REMOVED))
>> >>> +                       fini_cnt++;
>> >>> +
>> >>> +       orig_cnt = prog->len;
>> >>> +       insns = prog->insnsi;
>> >>> +       /* If prog length remains same, nothing else to do. */
>> >>> +       if (fini_cnt == orig_cnt) {
>> >>> +               for (insn = insns, elem = list; elem; elem = elem->next, insn++)
>> >>> +                       *insn = elem->insn;
>> >>> +               return env;
>> >>> +       }
>> >>> +       /* Realloc insn buffer when necessary. */
>> >>> +       if (fini_cnt > orig_cnt)
>> >>> +               prog = bpf_prog_realloc(prog, bpf_prog_size(fini_cnt),
>> >>> +                                       GFP_USER);
>> >>> +       if (!prog)
>> >>> +               return ERR_PTR(-ENOMEM);
>> >>> +       insns = prog->insnsi;
>> >>> +       prog->len = fini_cnt;
>> >>> +       ret_env = env;
>> >>> +
>> >>> +       /* idx_map[OLD_IDX] = NEW_IDX */
>> >>> +       idx_map = kvmalloc(orig_cnt * sizeof(u32), GFP_KERNEL);
>> >>> +       if (!idx_map)
>> >>> +               return ERR_PTR(-ENOMEM);
>> >>> +       memset(idx_map, 0xff, orig_cnt * sizeof(u32));
>> >>> +
>> >>> +       /* Use the same alloc method used when allocating env->insn_aux_data. */
>> >>> +       new_data = vzalloc(array_size(sizeof(*new_data), fini_cnt));
>> >>> +       if (!new_data) {
>> >>> +               kvfree(idx_map);
>> >>> +               return ERR_PTR(-ENOMEM);
>> >>> +       }
>> >>> +
>> >>> +       /* Copy over insn + calculate idx_map. */
>> >>> +       for (idx = 0, elem = list; elem; elem = elem->next) {
>> >>> +               int orig_idx = elem->orig_idx - 1;
>> >>> +
>> >>> +               if (orig_idx >= 0) {
>> >>> +                       idx_map[orig_idx] = idx;
>> >>> +
>> >>> +                       if (elem->flag & LIST_INSN_FLAG_REMOVED)
>> >>> +                               continue;
>> >>> +
>> >>> +                       new_data[idx] = env->insn_aux_data[orig_idx];
>> >>> +
>> >>> +                       if (elem->flag & LIST_INSN_FLAG_PATCHED)
>> >>> +                               new_data[idx].zext_dst =
>> >>> +                                       insn_has_def32(env, &elem->insn);
>> >>> +               } else {
>> >>> +                       new_data[idx].seen = true;
>> >>> +                       new_data[idx].zext_dst = insn_has_def32(env,
>> >>> +                                                               &elem->insn);
>> >>> +               }
>> >>> +               insns[idx++] = elem->insn;
>> >>> +       }
>> >>> +
>> >>> +       new_subinfo = kvzalloc(sizeof(env->subprog_info), GFP_KERNEL);
>> >>> +       if (!new_subinfo) {
>> >>> +               kvfree(idx_map);
>> >>> +               vfree(new_data);
>> >>> +               return ERR_PTR(-ENOMEM);
>> >>> +       }
>> >>> +       memcpy(new_subinfo, env->subprog_info, sizeof(env->subprog_info));
>> >>> +       memset(env->subprog_info, 0, sizeof(env->subprog_info));
>> >>> +       env->subprog_cnt = 0;
>> >>> +       env->prog = prog;
>> >>> +       ret = add_subprog(env, 0);
>> >>> +       if (ret < 0) {
>> >>> +               ret_env = ERR_PTR(ret);
>> >>> +               goto free_all_ret;
>> >>> +       }
>> >>> +       /* Relocate jumps using idx_map.
>> >>> +        *   old_dst = jmp_insn.old_target + old_pc + 1;
>> >>> +        *   new_dst = idx_map[old_dst] = jmp_insn.new_target + new_pc + 1;
>> >>> +        *   jmp_insn.new_target = new_dst - new_pc - 1;
>> >>> +        */
>> >>> +       for (idx = 0, elem = list; elem; elem = elem->next) {
>> >>> +               int orig_idx = elem->orig_idx;
>> >>> +
>> >>> +               if (elem->flag & LIST_INSN_FLAG_REMOVED)
>> >>> +                       continue;
>> >>> +               if ((elem->flag & LIST_INSN_FLAG_PATCHED) || !orig_idx) {
>> >>> +                       idx++;
>> >>> +                       continue;
>> >>> +               }
>> >>> +
>> >>> +               ret = bpf_jit_adj_imm_off(&insns[idx], orig_idx - 1, idx,
>> >>> +                                         idx_map);
>> >>> +               if (ret < 0) {
>> >>> +                       ret_env = ERR_PTR(ret);
>> >>> +                       goto free_all_ret;
>> >>> +               }
>> >>> +               /* Recalculate subprog start as we are at bpf2bpf call insn. */
>> >>> +               if (ret > 0) {
>> >>> +                       ret = add_subprog(env, idx + insns[idx].imm + 1);
>> >>> +                       if (ret < 0) {
>> >>> +                               ret_env = ERR_PTR(ret);
>> >>> +                               goto free_all_ret;
>> >>> +                       }
>> >>> +               }
>> >>> +               idx++;
>> >>> +       }
>> >>> +       if (ret < 0) {
>> >>> +               ret_env = ERR_PTR(ret);
>> >>> +               goto free_all_ret;
>> >>> +       }
>> >>> +
>> >>> +       env->subprog_info[env->subprog_cnt].start = fini_cnt;
>> >>> +       for (idx = 0; idx <= env->subprog_cnt; idx++)
>> >>> +               new_subinfo[idx].start = env->subprog_info[idx].start;
>> >>> +       memcpy(env->subprog_info, new_subinfo, sizeof(env->subprog_info));
>> >>> +
>> >>> +       /* Adjust linfo.
>> >>> +        * FIXME: no support for insn removal at the moment.
>> >>> +        */
>> >>> +       if (prog->aux->nr_linfo) {
>> >>> +               struct bpf_line_info *linfo = prog->aux->linfo;
>> >>> +               u32 nr_linfo = prog->aux->nr_linfo;
>> >>> +
>> >>> +               for (idx = 0; idx < nr_linfo; idx++)
>> >>> +                       linfo[idx].insn_off = idx_map[linfo[idx].insn_off];
>> >>> +       }
>> >>> +       vfree(env->insn_aux_data);
>> >>> +       env->insn_aux_data = new_data;
>> >>> +       goto free_mem_list_ret;
>> >>> +free_all_ret:
>> >>> +       vfree(new_data);
>> >>> +free_mem_list_ret:
>> >>> +       kvfree(new_subinfo);
>> >>> +       kvfree(idx_map);
>> >>> +       return ret_env;
>> >>> +}
>> >>> +
>> >>>  static int opt_remove_dead_code(struct bpf_verifier_env *env)
>> >>>  {
>> >>>         struct bpf_insn_aux_data *aux_data = env->insn_aux_data;
>> >>> --
>> >>> 2.7.4
>> >>>
>>