Re: [PATCH v9 bpf-next 3/5] bpf: Add skb dynptrs

[Joanne Koong <joannelkoong@gmail.com>]

On Mon, Jan 30, 2023 at 5:04 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Mon, Jan 30, 2023 at 2:31 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Mon, Jan 30, 2023 at 02:04:08PM -0800, Martin KaFai Lau wrote:
> > > On 1/27/23 11:17 AM, Joanne Koong wrote:
> > > > @@ -8243,6 +8316,28 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> > > >             mark_reg_known_zero(env, regs, BPF_REG_0);
> > > >             regs[BPF_REG_0].type = PTR_TO_MEM | ret_flag;
> > > >             regs[BPF_REG_0].mem_size = meta.mem_size;
> > > > +           if (func_id == BPF_FUNC_dynptr_data &&
> > > > +               dynptr_type == BPF_DYNPTR_TYPE_SKB) {
> > > > +                   bool seen_direct_write = env->seen_direct_write;
> > > > +
> > > > +                   regs[BPF_REG_0].type |= DYNPTR_TYPE_SKB;
> > > > +                   if (!may_access_direct_pkt_data(env, NULL, BPF_WRITE))
> > > > +                           regs[BPF_REG_0].type |= MEM_RDONLY;
> > > > +                   else
> > > > +                           /*
> > > > +                            * Calling may_access_direct_pkt_data() will set
> > > > +                            * env->seen_direct_write to true if the skb is
> > > > +                            * writable. As an optimization, we can ignore
> > > > +                            * setting env->seen_direct_write.
> > > > +                            *
> > > > +                            * env->seen_direct_write is used by skb
> > > > +                            * programs to determine whether the skb's page
> > > > +                            * buffers should be cloned. Since data slice
> > > > +                            * writes would only be to the head, we can skip
> > > > +                            * this.
> > > > +                            */
> > > > +                           env->seen_direct_write = seen_direct_write;
> > > > +           }
> > >
> > > [ ... ]
> > >
> > > > @@ -9263,17 +9361,26 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
> > > >                             return ret;
> > > >                     break;
> > > >             case KF_ARG_PTR_TO_DYNPTR:
> > > > +           {
> > > > +                   enum bpf_arg_type dynptr_arg_type = ARG_PTR_TO_DYNPTR;
> > > > +
> > > >                     if (reg->type != PTR_TO_STACK &&
> > > >                         reg->type != CONST_PTR_TO_DYNPTR) {
> > > >                             verbose(env, "arg#%d expected pointer to stack or dynptr_ptr\n", i);
> > > >                             return -EINVAL;
> > > >                     }
> > > > -                   ret = process_dynptr_func(env, regno, insn_idx,
> > > > -                                             ARG_PTR_TO_DYNPTR | MEM_RDONLY);
> > > > +                   if (meta->func_id == special_kfunc_list[KF_bpf_dynptr_from_skb])
> > > > +                           dynptr_arg_type |= MEM_UNINIT | DYNPTR_TYPE_SKB;
> > > > +                   else
> > > > +                           dynptr_arg_type |= MEM_RDONLY;
> > > > +
> > > > +                   ret = process_dynptr_func(env, regno, insn_idx, dynptr_arg_type,
> > > > +                                             meta->func_id);
> > > >                     if (ret < 0)
> > > >                             return ret;
> > > >                     break;
> > > > +           }
> > > >             case KF_ARG_PTR_TO_LIST_HEAD:
> > > >                     if (reg->type != PTR_TO_MAP_VALUE &&
> > > >                         reg->type != (PTR_TO_BTF_ID | MEM_ALLOC)) {
> > > > @@ -15857,6 +15964,14 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > > >                desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
> > > >             insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
> > > >             *cnt = 1;
> > > > +   } else if (desc->func_id == special_kfunc_list[KF_bpf_dynptr_from_skb]) {
> > > > +           bool is_rdonly = !may_access_direct_pkt_data(env, NULL, BPF_WRITE);
> > >
> > > Does it need to restore the env->seen_direct_write here also?
> > >
> > > It seems this 'seen_direct_write' saving/restoring is needed now because
> > > 'may_access_direct_pkt_data(BPF_WRITE)' is not only called when it is
> > > actually writing the packet. Some refactoring can help to avoid issue like
> > > this.
> > >
> > > While at 'seen_direct_write', Alexei has also pointed out that the verifier
> > > needs to track whether the (packet) 'slice' returned by bpf_dynptr_data()
> > > has been written. It should be tracked in 'seen_direct_write'. Take a look
> > > at how reg_is_pkt_pointer() and may_access_direct_pkt_data() are done in
> > > check_mem_access(). iirc, this reg_is_pkt_pointer() part got loss somewhere
> > > in v5 (or v4?) when bpf_dynptr_data() was changed to return register typed
> > > PTR_TO_MEM instead of PTR_TO_PACKET.
> >
> > btw tc progs are using gen_prologue() approach because data/data_end are not kfuncs
> > (nothing is being called by the bpf prog).
> > In this case we don't need to repeat this approach. If so we don't need to
> > set seen_direct_write.
> > Instead bpf_dynptr_data() can call bpf_skb_pull_data() directly.
> > And technically we don't need to limit it to skb head. It can handle any off/len.
> > It will work for skb, but there is no equivalent for xdp_pull_data().
> > I don't think we can implement xdp_pull_data in all drivers.
> > That's massive amount of work, but we need to be consistent if we want
> > dynptr to wrap both skb and xdp.
> > We can say dynptr_data is for head only, but we've seen bugs where people
> > had to switch from data/data_end to load_bytes.
> >
> > Also bpf_skb_pull_data is quite heavy. For progs that only want to parse
> > the packet calling that in bpf_dynptr_data is a heavy hammer.
> >
> > It feels that we need to go back to skb_header_pointer-like discussion.
> > Something like:
> > bpf_dynptr_slice(const struct bpf_dynptr *ptr, u32 offset, u32 len, void *buffer)
> > Whether buffer is a part of dynptr or program provided is tbd.
>
> making it hidden within dynptr would make this approach unreliable
> (memory allocations, which can fail, etc). But if we ask users to pass
> it directly, then it should be relatively easy to use in practice with
> some pre-allocated per-CPU buffer:
>
>
> struct {
>   __int(type, BPF_MAP_TYPE_PERCPU_ARRAY);
>   __int(max_entries, 1);
>   __type(key, int);
>   __type(value, char[4096]);
> } scratch SEC(".maps");
>
>
> ...
>
>
> struct dyn_ptr *dp = bpf_dynptr_from_skb(...).
> void *p, *buf;
> int zero = 0;
>
> buf = bpf_map_lookup_elem(&scratch, &zero);
> if (!buf) return 0; /* can't happen */
>
> p = bpf_dynptr_slice(dp, off, 16, buf);
> if (p == NULL) {
>    /* out of range */
> } else {
>    /* work with p directly */
> }
>
> /* if we wrote something to p and it was copied to buffer, write it back */
> if (p == buf) {
>     bpf_dynptr_write(dp, buf, 16);
> }
>
>
> We'll just need to teach verifier to make sure that buf is at least 16
> byte long.

I'm confused what the benefit of passing in the buffer is. If it's to
avoid the uncloning, this will still need to happen if the user writes
back the data to the skb (which will be the majority of cases). If
it's to avoid uncloning if the user only reads the data of a writable
prog, then we could add logic in the verifier so that we don't pull
the data in this case; the uncloning might still happen regardless if
another part of the program does a direct write. If the benefit is to
avoid needing to pull the data, then can't the user just use
bpf_dynptr_read, which takes in a buffer?

>
>
> But I wonder if for simple cases when users are mostly sure that they
> are going to access only header data directly we can have an option
> for bpf_dynptr_from_skb() to specify what should be the behavior for
> bpf_dynptr_slice():
>
>  - either return NULL for anything that crosses into frags (no
> surprising perf penalty, but surprising NULLs);
>  - do bpf_skb_pull_data() if bpf_dynptr_data() needs to point to data
> beyond header (potential perf penalty, but on NULLs, if off+len is
> within packet).
>
> And then bpf_dynptr_from_skb() can accept a flag specifying this
> behavior and store it somewhere in struct bpf_dynptr.
>
> Thoughts?