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: Thu, 11 Jul 2019 13:20:19 +0100

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.

>
> 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
>>>