Date:   Mon, 30 Jan 2023 20:43:47 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Martin KaFai Lau <martin.lau@...ux.dev>
Cc:     Joanne Koong <joannelkoong@...il.com>, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...nel.org, ast@...nel.org,
        netdev@...r.kernel.org, memxor@...il.com, kernel-team@...com,
        bpf@...r.kernel.org,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH v9 bpf-next 3/5] bpf: Add skb dynptrs

On Mon, Jan 30, 2023 at 5:49 PM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
>
> On 1/30/23 5:04 PM, Andrii Nakryiko wrote:
> > On Mon, Jan 30, 2023 at 2:31 PM Alexei Starovoitov
> > <alexei.starovoitov@...il.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.
>
> A fifth __sz arg may do:
> bpf_dynptr_slice(const struct bpf_dynptr *ptr, u32 offset, u32 len, void
> *buffer, u32 buffer__sz);

We'll need to make sure that buffer__sz is >= len (or preferably not
require extra size at all). We can check that at runtime, of course,
but rejecting too small buffer at verification time would be a better
experience.

>
> The bpf prog usually has buffer in the stack for the common small header parsing.

sure, that would work for small chunks

>
> One side note is the bpf_dynptr_slice() still needs to check if the skb is
> cloned or not even the off/len is within the head range.

yep, and the above snippet will still do the right thing with
bpf_dynptr_write(), right? bpf_dynptr_write() will have to pull
anyways, if I understand correctly?

>
> > 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.
>
> xdp does not have the bpf_skb_pull_data() equivalent, so xdp prog will still
> need the write back handling.
>

Sure, unfortunately, can't have everything. I'm just thinking how to
make bpf_dynptr_data() generically usable. Think about some common BPF
routine that calculates hash for all bytes pointed to by dynptr,
regardless of underlying dynptr type; it can iterate in small chunks,
get memory slice, if possible, but fallback to generic
bpf_dynptr_read() if doesn't. This will work for skb, xdp, LOCAL,
RINGBUF, any other dynptr type.

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