RE: [PATCH] Add skb_store_bytes() for BPF_PROG_TYPE_CGROUP_SKB

-----Original Message-----
From: Martin KaFai Lau <redacted>
Sent: Wednesday, December 22, 2021 3:51 PM
To: Tyler Wear <redacted>
Cc: Yonghong Song <redacted>; Tyler Wear (QUIC) <redacted>; netdev@vger.kernel.org; bpf@vger.kernel.org;
maze@google.com
Subject: Re: [PATCH] Add skb_store_bytes() for BPF_PROG_TYPE_CGROUP_SKB

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On Wed, Dec 22, 2021 at 10:49:45PM +0000, Tyler Wear wrote:

quoted

quoted

On 12/21/21 6:27 PM, Tyler Wear wrote:

quoted

Need to modify the ds field to support upcoming Wifi QoS Alliance
spec. Instead of adding generic function for just modifying the ds
field, add skb_store_bytes for BPF_PROG_TYPE_CGROUP_SKB. This
allows other fields in the network and transport header to be
modified in the future.

Could change tag from "[PATCH]" to "[PATCH bpf-next]"?
Please also indicate the version of the patch, so in this case, it should be "[PATCH bpf-next v2]".

I think you can add more contents in the commit message about why
existing bpf_setsockopt() won't work and why CGROUP_UDP[4|6]_SENDMSG is not preferred.
These have been discussed in v1 of this patch and they are valuable for people to understand full context and reasoning.

quoted

Signed-off-by: Tyler Wear <redacted>
---
  net/core/filter.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c index

6102f093d59a..0c25aa2212a2 100644

--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -7289,6 +7289,8 @@ static const struct bpf_func_proto *
  cg_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
  {
      switch (func_id) {
+     case BPF_FUNC_skb_store_bytes:
+             return &bpf_skb_store_bytes_proto;

Typically different 'case's are added in chronological order to
people can guess what is added earlier and what is added later. Maybe add the new helper after BPF_FUNC_perf_event_output?

quoted

      case BPF_FUNC_get_local_storage:
              return &bpf_get_local_storage_proto;
      case BPF_FUNC_sk_fullsock:

Please add a test case to exercise the new usage of
bpf_skb_store_bytes() helper. You may piggy back on some existing cg_skb progs if it is easier to do.

Would it be sufficient to change the dscp value in
tools/testing/selftests/bpf/progs/test_sock_fields.c via
bpf_skb_store_bytes()

test_sock_fields focus on sk instead of skb, so it will not be a good fit.

load_bytes_relative.c may be a better fit.
The minimal is to write the dscp value by bpf_skb_store_bytes() and be able to read it back at the receiver side (e.g.
by making a TCP connection like load_bytes_relative).

Unable to run any bpf tests do to errors below. These occur with and without the new patch. Is this a known issue?
Is the new test case required since bpf_skb_store_bytes() is already a tested function for other prog types?

libbpf: failed to find BTF for extern 'bpf_testmod_invalid_mod_kfunc' [18] section: -2
Error: failed to open BPF object file: No such file or directory
libbpf: failed to find BTF info for global/extern symbol 'my_tid'
Error: failed to link '/local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/linked_funcs1.o': Unknown error -2 (-2)
libbpf: failed to find BTF for extern 'bpf_kfunc_call_test1' [27] section: -2
Error: failed to open BPF object file: No such file or directory
make: *** [Makefile:484: /local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/test_ksyms_module.skel.h] Error 255
make: *** Deleting file '/local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/test_ksyms_module.skel.h'
make: *** Waiting for unfinished jobs....
make: *** [Makefile:484: /local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/kfunc_call_test_subprog.skel.h] Error 255
make: *** Deleting file '/local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/kfunc_call_test_subprog.skel.h'
make: *** [Makefile:482: /local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/linked_funcs.skel.h] Error 254
libbpf: failed to find BTF info for global/extern symbol 'input_rodata_weak'
Error: failed to link '/local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/linked_vars1.o': Unknown error -2 (-2)
make: *** [Makefile:482: /local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/linked_vars.skel.h] Error 254
libbpf: failed to find BTF for extern 'tcp_cong_avoid_ai' [27] section: -2
Error: failed to open BPF object file: No such file or directory
make: *** [Makefile:486: /local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/bpf_cubic.skel.h] Error 255
make: *** Deleting file '/local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/bpf_cubic.skel.h'
libbpf: failed to find BTF for extern 'bpf_kfunc_call_test1' [28] section: -2
Error: failed to open BPF object file: No such file or directory
make: *** [Makefile:486: /local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/kfunc_call_test.lskel.h] Error 255
make: *** Deleting file '/local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/kfunc_call_test.lskel.h'
libbpf: failed to find BTF for extern 'tcp_reno_cong_avoid' [38] section: -2
Error: failed to open BPF object file: No such file or directory
libbpf: failed to find BTF for extern 'bpf_testmod_invalid_mod_kfunc' [18] section: -2
Error: failed to open BPF object file: No such file or directory
make: *** [Makefile:486: /local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/bpf_dctcp.skel.h] Error 255
make: *** Deleting file '/local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/bpf_dctcp.skel.h'
make: *** [Makefile:486: /local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/test_ksyms_module.lskel.h] Error 255
make: *** Deleting file '/local/mnt/workspace/linux-stable/tools/testing/selftests/bpf/test_ksyms_module.lskel.h'