Date:   Tue, 31 Jan 2023 16:11:47 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Martin KaFai Lau <martin.lau@...ux.dev>,
        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
Subject: Re: [PATCH v9 bpf-next 3/5] bpf: Add skb dynptrs

On Mon, Jan 30, 2023 at 9:30 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Mon, Jan 30, 2023 at 08:43:47PM -0800, Andrii Nakryiko wrote:
> > 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:
>
> bpf_skb_pull_data() is even more unreliable, since it's a bigger allocation.
> I like preallocated approach more, so we're in agreement here.
>
> > > >
> > > >
> > > > 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.
>
> I don't follow. Why two equivalent 'len' args ?
> Just to allow 'len' to be a variable instead of constant ?
> It's unusual for the verifier to have 'len' before 'buffer',
> but this is fixable.

Right, I don't like two lens as well. And no, len can't be variable,
it has to be a constant known at verification time. We could define
bpf_dynptr_slice as

void *bpf_dynptr_slice(const struct bpf_dynptr *ptr, u32 offset, void
*buffer, u32 buffer__sz) and it would follow current conventions,
though feels a bit weird.

But either way we'd have to teach verifier to take buffer__sz and mark
it as the size of PTR_TO_MEM returned from bpf_dynptr_slice.

All this is doable.

>
> How about adding 'rd_only vs rdwr' flag ?
> Then MEM_RDONLY for ret value of bpf_dynptr_slice can be set by the verifier
> and in run-time bpf_dynptr_slice() wouldn't need to check for skb->cloned.
> if (rd_only) return skb_header_pointer()
> if (rdwr) bpf_try_make_writable(); return skb->data + off;
> and final bpf_dynptr_write() is not needed.
>
> But that doesn't work for xdp, since there is no pull.
>
> It's not clear how to deal with BPF_F_RECOMPUTE_CSUM though.
> Expose __skb_postpull_rcsum/__skb_postpush_rcsum as kfuncs?
> But that defeats Andrii's goal to use dynptr as a generic wrapper.
> skb is quite special.
>
> Maybe something like:
> void *bpf_dynptr_slice(const struct bpf_dynptr *ptr, u32 offset, u32 len,
>                        void *buffer, u32 buffer__sz)
> {
>   if (skb_cloned()) {
>     skb_copy_bits(skb, offset, buffer, len);
>     return buffer;
>   }
>   return skb_header_pointer(...);
> }
>
> When prog is just parsing the packet it doesn't need to finalize with bpf_dynptr_write.
> The prog can always write into the pointer followed by if (p == buf) bpf_dynptr_write.
> No need for rdonly flag, but extra copy is there in case of cloned which
> could have been avoided with extra rd_only flag.

Yep, given we are designing bpf_dynptr_slice for performance, extra
copy on reads is unfortunate. ro/rw flag or have separate
bpf_dynptr_slice_rw vs bpf_dynptr_slice_ro?

>
> In case of xdp it will be:
> void *bpf_dynptr_slice(const struct bpf_dynptr *ptr, u32 offset, u32 len,
>                        void *buffer, u32 buffer__sz)
> {
>    ptr = bpf_xdp_pointer(xdp, offset, len);
>    if (ptr)
>       return ptr;
>    bpf_xdp_copy_buf(xdp, offset, buffer, len, false); /* copy into buf */
>    return buffer;
> }
>
> bpf_dynptr_write will use bpf_xdp_copy_buf(,true); /* copy into xdp */
>
> > >
> > > 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?
>
> Yes and No. bpf_skb_store_bytes is doing pull followed by memcpy,
> while xdp_store_bytes does scatter gather copy into frags.
> We should probably add similar copy to skb case to avoid allocations and pull.
> Then in case of:
>  if (p == buf) {
>       bpf_dynptr_write(dp, buf, 16);
>  }
>
> the write will guarantee to succeed for both xdp and skb and the user
> doesn't need to add error checking for alloc failures in case of skb.
>

That seems like a nice guarantee, agreed.

> > >
> > > > 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.
>
> It looks to me that dynptr on top of skb, xdp, local can work as generic reader,
> but dynptr as a generic writer doesn't look possible.
> BPF_F_RECOMPUTE_CSUM and BPF_F_INVALIDATE_HASH are special to skb.
> There is also bpf_skb_change_proto and crazy complex bpf_skb_adjust_room.
> I don't think writing into skb vs xdp vs ringbuf are generalizable.
> The prog needs to do a ton more work to write into skb correctly.

If that's the case, then yeah, bpf_dynptr_write() can just return
error for skb/xdp dynptrs?

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