Re: [PATCH v9 bpf-next 10/14] bpf: add multi-buffer support to xdp copy helpers

[Eelco Chaudron <echaudro@redhat.com>]

On 23 Jun 2021, at 1:49, John Fastabend wrote:

> Lorenzo Bianconi wrote:
>> From: Eelco Chaudron <echaudro@redhat.com>
>>
>> This patch adds support for multi-buffer for the following helpers:
>>   - bpf_xdp_output()
>>   - bpf_perf_event_output()
>>
>> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
>> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
>> ---
>
> Ah ok so at least xdp_output will work with all bytes. But this is
> getting close to having access into the frags so I think doing
> the last bit shouldn't be too hard?


Guess you are talking about multi-buffer access in the XDP program?

I did suggest an API a while back, https://lore.kernel.org/bpf/FD3E6E08-DE78-4FBA-96F6-646C93E88631@redhat.com/ but I had/have not time to work on it. Guess the difficult part is to convince the verifier to allow the data to be accessed.

>
>>  kernel/trace/bpf_trace.c                      |   3 +
>>  net/core/filter.c                             |  72 +++++++++-
>>  .../selftests/bpf/prog_tests/xdp_bpf2bpf.c    | 127 ++++++++++++------
>>  .../selftests/bpf/progs/test_xdp_bpf2bpf.c    |   2 +-
>>  4 files changed, 160 insertions(+), 44 deletions(-)
>>
>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>> index d2d7cf6cfe83..ee926ec64f78 100644
>> --- a/kernel/trace/bpf_trace.c
>> +++ b/kernel/trace/bpf_trace.c
>> @@ -1365,6 +1365,7 @@ static const struct bpf_func_proto bpf_perf_event_output_proto_raw_tp = {
>>
>>  extern const struct bpf_func_proto bpf_skb_output_proto;
>>  extern const struct bpf_func_proto bpf_xdp_output_proto;
>> +extern const struct bpf_func_proto bpf_xdp_get_buff_len_trace_proto;
>>
>>  BPF_CALL_3(bpf_get_stackid_raw_tp, struct bpf_raw_tracepoint_args *, args,
>>  	   struct bpf_map *, map, u64, flags)
>> @@ -1460,6 +1461,8 @@ tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>>  		return &bpf_sock_from_file_proto;
>>  	case BPF_FUNC_get_socket_cookie:
>>  		return &bpf_get_socket_ptr_cookie_proto;
>> +	case BPF_FUNC_xdp_get_buff_len:
>> +		return &bpf_xdp_get_buff_len_trace_proto;
>>  #endif
>>  	case BPF_FUNC_seq_printf:
>>  		return prog->expected_attach_type == BPF_TRACE_ITER ?
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index b0855f2d4726..f7211b7908a9 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -3939,6 +3939,15 @@ const struct bpf_func_proto bpf_xdp_get_buff_len_proto = {
>>  	.arg1_type	= ARG_PTR_TO_CTX,
>>  };
>>
>> +BTF_ID_LIST_SINGLE(bpf_xdp_get_buff_len_bpf_ids, struct, xdp_buff)
>> +
>> +const struct bpf_func_proto bpf_xdp_get_buff_len_trace_proto = {
>> +	.func		= bpf_xdp_get_buff_len,
>> +	.gpl_only	= false,
>> +	.arg1_type	= ARG_PTR_TO_BTF_ID,
>> +	.arg1_btf_id	= &bpf_xdp_get_buff_len_bpf_ids[0],
>> +};
>> +
>>  BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
>>  {
>>  	void *data_hard_end = xdp_data_hard_end(xdp); /* use xdp->frame_sz */
>> @@ -4606,10 +4615,56 @@ static const struct bpf_func_proto bpf_sk_ancestor_cgroup_id_proto = {
>>  };
>>  #endif
>>
>> -static unsigned long bpf_xdp_copy(void *dst_buff, const void *src_buff,
>> +static unsigned long bpf_xdp_copy(void *dst_buff, const void *ctx,
>>  				  unsigned long off, unsigned long len)
>>  {
>> -	memcpy(dst_buff, src_buff + off, len);
>> +	struct xdp_buff *xdp = (struct xdp_buff *)ctx;
>> +	struct skb_shared_info *sinfo;
>> +	unsigned long base_len;
>> +
>> +	if (likely(!xdp_buff_is_mb(xdp))) {
>> +		memcpy(dst_buff, xdp->data + off, len);
>> +		return 0;
>> +	}
>> +
>> +	base_len = xdp->data_end - xdp->data;
>> +	sinfo = xdp_get_shared_info_from_buff(xdp);
>> +	do {
>> +		const void *src_buff = NULL;
>> +		unsigned long copy_len = 0;
>> +
>> +		if (off < base_len) {
>> +			src_buff = xdp->data + off;
>> +			copy_len = min(len, base_len - off);
>> +		} else {
>> +			unsigned long frag_off_total = base_len;
>> +			int i;
>> +
>> +			for (i = 0; i < sinfo->nr_frags; i++) {
>> +				skb_frag_t *frag = &sinfo->frags[i];
>> +				unsigned long frag_len, frag_off;
>> +
>> +				frag_len = skb_frag_size(frag);
>> +				frag_off = off - frag_off_total;
>> +				if (frag_off < frag_len) {
>> +					src_buff = skb_frag_address(frag) +
>> +						   frag_off;
>> +					copy_len = min(len,
>> +						       frag_len - frag_off);
>> +					break;
>> +				}
>> +				frag_off_total += frag_len;
>> +			}
>> +		}
>> +		if (!src_buff)
>> +			break;
>> +
>> +		memcpy(dst_buff, src_buff, copy_len);
>> +		off += copy_len;
>> +		len -= copy_len;
>> +		dst_buff += copy_len;
>
> This block reads odd to be because it requires looping over the frags
> multiple times? Why not something like this,
>
>   if (off < base_len) {
>    src_buff = xdp->data + off
>    copy_len = min...
>    memcpy(dst_buff, src_buff, copy_len)
>    off += copylen
>    len -= copylen
>    dst_buff += copylen;
>   }
>
>   for (i = 0; i , nr_frags; i++) {
>      frag = ...
>      ...
>      if frag_off < fraglen
>         ...
>         memcpy()
>         update(off, len, dst_buff)
>   }
>
>
> Maybe use a helper to set off,len and dst_buff if worried about the
> duplication. Seems cleaner than walking through 0..n-1 frags for
> each copy.

You are right it looks odd, will re-write this in the next iteration.

>> +	} while (len);
>> +
>>  	return 0;
>>  }