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

[Joanne Koong <joannelkoong@gmail.com>]

On Tue, Jan 31, 2023 at 11:59 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue, Jan 31, 2023 at 10:30 AM Joanne Koong <joannelkoong@gmail.com> wrote:
> >
> > 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?
>
> There is no unclone and there is no pull in xdp.
> The main idea of this semantics of bpf_dynptr_slice is to make it
> work the same way on skb and xdp for _read_ case.
> Writes are going to be different between skb and xdp anyway.
> In some rare cases the writes can be the same for skb and xdp
> with this bpf_dynptr_slice + bpf_dynptr_write logic,
> but that's a minor feature addition of the api.

bpf_dynptr_read works the same way on skb and xdp. bpf_dynptr_read
takes in a buffer as well, so what is the added benefit of
bpf_dynptr_slice?

>
> I'd say in skb cases the progs do reads and either drop
> or forward the skb.
> Writes to skb are done from time to time too, because
> they're a pain to do correctly.
> nat is the main use case for skb rewrites.
> In xdp cases the progs do parse, drop, rewrite, xmit more or less equally.