Merge branch 'tcp-bpf-cc'

Martin Lau says:

====================
This series introduces BPF STRUCT_OPS.  It is an infra to allow
implementing some specific kernel's function pointers in BPF.
The first use case included in this series is to implement
TCP congestion control algorithm in BPF  (i.e. implement
struct tcp_congestion_ops in BPF).

There has been attempt to move the TCP CC to the user space
(e.g. CCP in TCP).   The common arguments are faster turn around,
get away from long-tail kernel versions in production...etc,
which are legit points.

BPF has been the continuous effort to join both kernel and
userspace upsides together (e.g. XDP to gain the performance
advantage without bypassing the kernel).  The recent BPF
advancements (in particular BTF-aware verifier, BPF trampoline,
BPF CO-RE...) made implementing kernel struct ops (e.g. tcp cc)
possible in BPF.

The idea is to allow implementing tcp_congestion_ops in bpf.
It allows a faster turnaround for testing algorithm in the
production while leveraging the existing (and continue growing) BPF
feature/framework instead of building one specifically for
userspace TCP CC.

Please see individual patch for details.

The bpftool support will be posted in follow-up patches.

v4:
- Expose tcp_ca_find() to tcp.h in patch 7.
  It is used to check the same bpf-tcp-cc
  does not exist to guarantee the register()
  will succeed.
- set_memory_ro() and then set_memory_x() only after all
  trampolines are written to the image in patch 6. (Daniel)
  spinlock is replaced by mutex because set_memory_*
  requires sleepable context.

v3:
- Fix kbuild error by considering CONFIG_BPF_SYSCALL (kbuild)
- Support anonymous bitfield in patch 4 (Andrii, Yonghong)
- Push boundary safety check to a specific arch's trampoline function
  (in patch 6) (Yonghong).
  Reuse the WANR_ON_ONCE check in arch_prepare_bpf_trampoline() in x86.
- Check module field is 0 in udata in patch 6 (Yonghong)
- Check zero holes in patch 6 (Andrii)
- s/_btf_vmlinux/btf/ in patch 5 and 7 (Andrii)
- s/check_xxx/is_xxx/ in patch 7 (Andrii)
- Use "struct_ops/" convention in patch 11 (Andrii)
- Use the skel instead of bpf_object in patch 11 (Andrii)
- libbpf: Decide BPF_PROG_TYPE_STRUCT_OPS at open phase by using
          find_sec_def()
- libbpf: Avoid a debug message at open phase (Andrii)
- libbpf: Add bpf_program__(is|set)_struct_ops() for consistency (Andrii)
- libbpf: Add "struct_ops" to section_defs (Andrii)
- libbpf: Some code shuffling in init_kern_struct_ops() (Andrii)
- libbpf: A few safety checks (Andrii)

v2:
- Dropped cubic for now.  They will be reposted
  once there are more clarity in "jiffies" on both
  bpf side (about the helper) and
  tcp_cubic side (some of jiffies usages are being replaced
  by tp->tcp_mstamp)
- Remove unnecssary check on bitfield support from btf_struct_access()
  (Yonghong)
- BTF_TYPE_EMIT macro (Yonghong, Andrii)
- value_name's length check to avoid an unlikely
  type match during truncation case (Yonghong)
- BUILD_BUG_ON to ensure no trampoline-image overrun
  in the future (Yonghong)
- Simplify get_next_key() (Yonghong)
- Added comment to explain how to check mandatory
  func ptr in net/ipv4/bpf_tcp_ca.c (Yonghong)
- Rename "__bpf_" to "bpf_struct_ops_" for value prefix (Andrii)
- Add comment to highlight the bpf_dctcp.c is not necessarily
  the same as tcp_dctcp.c. (Alexei, Eric)
- libbpf: Renmae "struct_ops" to ".struct_ops" for elf sec (Andrii)
- libbpf: Expose struct_ops as a bpf_map (Andrii)
- libbpf: Support multiple struct_ops in SEC(".struct_ops") (Andrii)
- libbpf: Add bpf_map__attach_struct_ops()  (Andrii)
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>