[bpf-next,01/10,bpf] : adding bpf_xdp_adjust_tail helper

Adding new bpf helper which would allow us to manipulate
xdp's data_end pointer, and allow us to reduce packet's size
indended use case: to generate ICMP messages from XDP context,
where such message would contain truncated original packet.

Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
---
 include/uapi/linux/bpf.h | 10 +++++++++-
 net/core/filter.c        | 29 ++++++++++++++++++++++++++++-
 2 files changed, 37 insertions(+), 2 deletions(-)

Hi Nikita,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on bpf-next/master]

url:    https://github.com/0day-ci/linux/commits/Nikita-V-Shirokov/introduction-of-bpf_xdp_adjust_tail/20180417-211905
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
config: i386-randconfig-s1-201815 (attached as .config)
compiler: gcc-6 (Debian 6.4.0-9) 6.4.0 20171026
reproduce:
        # save the attached .config to linux build tree
        make ARCH=i386 

All warnings (new ones prefixed by >>):

   net/core/filter.c: In function '____bpf_xdp_adjust_tail':
>> net/core/filter.c:2726:2: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
     void *data_end = xdp->data_end + offset;
     ^~~~

vim +2726 net/core/filter.c

  2719	
  2720	BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
  2721	{
  2722		/* only shrinking is allowed for now. */
  2723		if (unlikely(offset > 0))
  2724			return -EINVAL;
  2725	
> 2726		void *data_end = xdp->data_end + offset;
  2727	
  2728		if (unlikely(data_end < xdp->data + ETH_HLEN))
  2729			return -EINVAL;
  2730	
  2731		xdp->data_end = data_end;
  2732	
  2733		return 0;
  2734	}
  2735	

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

On Mon, Apr 16, 2018 at 11:51:22PM -0700, Nikita V. Shirokov wrote:
> Adding new bpf helper which would allow us to manipulate
> xdp's data_end pointer, and allow us to reduce packet's size
> indended use case: to generate ICMP messages from XDP context,
> where such message would contain truncated original packet.
> 
> Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
> ---
>  include/uapi/linux/bpf.h | 10 +++++++++-
>  net/core/filter.c        | 29 ++++++++++++++++++++++++++++-
>  2 files changed, 37 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index c5ec89732a8d..9a2d1a04eb24 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -755,6 +755,13 @@ union bpf_attr {
>   *     @addr: pointer to struct sockaddr to bind socket to
>   *     @addr_len: length of sockaddr structure
>   *     Return: 0 on success or negative error code
> + *
> + * int bpf_xdp_adjust_tail(xdp_md, delta)
> + *     Adjust the xdp_md.data_end by delta. Only shrinking of packet's
> + *     size is supported.
> + *     @xdp_md: pointer to xdp_md
> + *     @delta: A negative integer to be added to xdp_md.data_end
> + *     Return: 0 on success or negative on error
>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -821,7 +828,8 @@ union bpf_attr {
>  	FN(msg_apply_bytes),		\
>  	FN(msg_cork_bytes),		\
>  	FN(msg_pull_data),		\
> -	FN(bind),
> +	FN(bind),			\
> +	FN(xdp_adjust_tail),
>  
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>   * function eBPF program intends to call
> diff --git a/net/core/filter.c b/net/core/filter.c
> index d31aff93270d..6c8ac7b548d6 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2717,6 +2717,30 @@ static const struct bpf_func_proto bpf_xdp_adjust_head_proto = {
>  	.arg2_type	= ARG_ANYTHING,
>  };
>  
> +BPF_CALL_2(bpf_xdp_adjust_tail, struct xdp_buff *, xdp, int, offset)
> +{
> +	/* only shrinking is allowed for now. */
> +	if (unlikely(offset > 0))
> +		return -EINVAL;

why allow offset == 0 ?
It's a nop. xdp_adjust_head allows it, but it's not a reason
to repeat the same here.
Like we may decide to do something with offset==0 in the future.
Let's keep it reserved.

In the subject please replace
[bpf]: adding bpf_xdp_adjust_tail helper
with
bpf: adding bpf_xdp_adjust_tail helper

"[bpf] foo bar" subject used to be llvm patch convention,
but lately we switched it to kernel style as well with "bpf: foo bar"