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

[Martin KaFai Lau <martin.lau@linux.dev>]

On 1/30/23 5:04 PM, Andrii Nakryiko 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.

A fifth __sz arg may do:
bpf_dynptr_slice(const struct bpf_dynptr *ptr, u32 offset, u32 len, void 
*buffer, u32 buffer__sz);

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

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.

> 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.