Re: [PATCH bpf-next v1 1/3] bpf: Add skb dynptrs

[Martin KaFai Lau <kafai@fb.com>]

On Fri, Jul 29, 2022 at 01:26:31PM -0700, Joanne Koong wrote:
> On Thu, Jul 28, 2022 at 4:39 PM Martin KaFai Lau <kafai@fb.com> wrote:
> >
> > On Tue, Jul 26, 2022 at 11:47:04AM -0700, Joanne Koong wrote:
> > > @@ -1567,6 +1607,18 @@ BPF_CALL_3(bpf_dynptr_data, struct bpf_dynptr_kern *, ptr, u32, offset, u32, len
> > >       if (bpf_dynptr_is_rdonly(ptr))
> > Is it possible to allow data slice for rdonly dynptr-skb?
> > and depends on the may_access_direct_pkt_data() check in the verifier.
> 
> Ooh great idea. This should be very simple to do, since the data slice
> that gets returned is assigned as PTR_TO_PACKET. So any stx operations
> on it will by default go through the may_access_direct_pkt_data()
> check. I'll add this for v2.
It will be great.  Out of all three helpers (bpf_dynptr_read/write/data),
bpf_dynptr_data will be the useful one to parse the header data (e.g. tcp-hdr-opt)
that has runtime variable length because bpf_dynptr_data() can take a non-cost
'offset' argument.  It is useful to get a consistent usage across all bpf
prog types that are either read-only or read-write of the skb.

> 
> >
> > >               return 0;
> > >
> > > +     type = bpf_dynptr_get_type(ptr);
> > > +
> > > +     if (type == BPF_DYNPTR_TYPE_SKB) {
> > > +             struct sk_buff *skb = ptr->data;
> > > +
> > > +             /* if the data is paged, the caller needs to pull it first */
> > > +             if (ptr->offset + offset + len > skb->len - skb->data_len)
> > > +                     return 0;
> > > +
> > > +             return (unsigned long)(skb->data + ptr->offset + offset);
> > > +     }
> > > +
> > >       return (unsigned long)(ptr->data + ptr->offset + offset);
> > >  }
> >
> > [ ... ]
> >
> > > -static u32 stack_slot_get_id(struct bpf_verifier_env *env, struct bpf_reg_state *reg)
> > > +static void stack_slot_get_dynptr_info(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
> > > +                                    struct bpf_call_arg_meta *meta)
> > >  {
> > >       struct bpf_func_state *state = func(env, reg);
> > >       int spi = get_spi(reg->off);
> > >
> > > -     return state->stack[spi].spilled_ptr.id;
> > > +     meta->ref_obj_id = state->stack[spi].spilled_ptr.id;
> > > +     meta->type = state->stack[spi].spilled_ptr.dynptr.type;
> > >  }
> > >
> > >  static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> > > @@ -6052,6 +6057,9 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> > >                               case DYNPTR_TYPE_RINGBUF:
> > >                                       err_extra = "ringbuf ";
> > >                                       break;
> > > +                             case DYNPTR_TYPE_SKB:
> > > +                                     err_extra = "skb ";
> > > +                                     break;
> > >                               default:
> > >                                       break;
> > >                               }
> > > @@ -6065,8 +6073,10 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
> > >                                       verbose(env, "verifier internal error: multiple refcounted args in BPF_FUNC_dynptr_data");
> > >                                       return -EFAULT;
> > >                               }
> > > -                             /* Find the id of the dynptr we're tracking the reference of */
> > > -                             meta->ref_obj_id = stack_slot_get_id(env, reg);
> > > +                             /* Find the id and the type of the dynptr we're tracking
> > > +                              * the reference of.
> > > +                              */
> > > +                             stack_slot_get_dynptr_info(env, reg, meta);
> > >                       }
> > >               }
> > >               break;
> > > @@ -7406,7 +7416,11 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> > >               regs[BPF_REG_0].type = PTR_TO_TCP_SOCK | ret_flag;
> > >       } else if (base_type(ret_type) == RET_PTR_TO_ALLOC_MEM) {
> > >               mark_reg_known_zero(env, regs, BPF_REG_0);
> > > -             regs[BPF_REG_0].type = PTR_TO_MEM | ret_flag;
> > > +             if (func_id == BPF_FUNC_dynptr_data &&
> > > +                 meta.type == BPF_DYNPTR_TYPE_SKB)
> > > +                     regs[BPF_REG_0].type = PTR_TO_PACKET | ret_flag;
> > > +             else
> > > +                     regs[BPF_REG_0].type = PTR_TO_MEM | ret_flag;
> > >               regs[BPF_REG_0].mem_size = meta.mem_size;
> > check_packet_access() uses range.
> > It took me a while to figure range and mem_size is in union.
> > Mentioning here in case someone has similar question.
> For v2, I'll add this as a comment in the code or I'll include
> "regs[BPF_REG_0].range = meta.mem_size" explicitly to make it more
> obvious :)
'regs[BPF_REG_0].range = meta.mem_size' would be great.  No strong
opinion here.

> >
> > >       } else if (base_type(ret_type) == RET_PTR_TO_MEM_OR_BTF_ID) {
> > >               const struct btf_type *t;
> > > @@ -14132,6 +14146,25 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> > >                       goto patch_call_imm;
> > >               }
> > >
> > > +             if (insn->imm == BPF_FUNC_dynptr_from_skb) {
> > > +                     if (!may_access_direct_pkt_data(env, NULL, BPF_WRITE))
> > > +                             insn_buf[0] = BPF_MOV32_IMM(BPF_REG_4, true);
> > > +                     else
> > > +                             insn_buf[0] = BPF_MOV32_IMM(BPF_REG_4, false);
> > > +                     insn_buf[1] = *insn;
> > > +                     cnt = 2;
> > > +
> > > +                     new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> > > +                     if (!new_prog)
> > > +                             return -ENOMEM;
> > > +
> > > +                     delta += cnt - 1;
> > > +                     env->prog = new_prog;
> > > +                     prog = new_prog;
> > > +                     insn = new_prog->insnsi + i + delta;
> > > +                     goto patch_call_imm;
> > > +             }
> > Have you considered to reject bpf_dynptr_write()
> > at prog load time?
> It's possible to reject bpf_dynptr_write() at prog load time but would
> require adding tracking in the verifier for whether a dynptr is
> read-only or not. Do you think it's better to reject it at load time
> instead of returning NULL at runtime?
The check_helper_call above seems to know 'meta.type == BPF_DYNPTR_TYPE_SKB'.
Together with may_access_direct_pkt_data(), would it be enough ?
Then no need to do patching for BPF_FUNC_dynptr_from_skb here.

Since we are on bpf_dynptr_write, what is the reason
on limiting it to the skb_headlen() ?  Not implying one
way is better than another.  would like to undertand the reason
behind it since it is not clear in the commit message.