Introduce BPF trampoline

Introduce BPF trampoline that works as a bridge between kernel functions and
BPF programs. The first use case is fentry/fexit BPF programs that are roughly
equivalent to kprobe/kretprobe. Unlike k[ret]probe there is practically zero
overhead to call a set of BPF programs before or after kernel function. In the
future patches networking use cases will be explored. For example: BPF
trampoline can be used to call XDP programs from drivers with direct calls or
wrapping BPF program into another BPF program.

The patch set depends on register_ftrace_direct() API. It's not upstream yet
and available in [1]. The first patch is a hack to workaround this dependency.
The idea is to land this set via bpf-next tree and land register_ftrace_direct
via Steven's ftrace tree. Then during the merge window revert the first patch.
Steven,
do you think it's workable?
As an alternative we can route register_ftrace_direct patches via bpf-next ?

Peter's static_call patches [2] are solving the issue of indirect call overhead
for large class of kernel use cases, but unfortunately don't help calling into
a set of BPF programs.  BPF trampoline's first goal is to translate kernel
calling convention into BPF calling convention. The second goal is to call a
set of programs efficiently. In the future we can replace BPF_PROG_RUN_ARRAY
with BPF trampoline variant. Another future work is to add support for static_key,
static_jmp and static_call inside generated BPF trampoline.

Please see patch 3 for details.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/rostedt/l...
[2]
https://lore.kernel.org/lkml/20191007082708.01393931.1@in...

Alexei Starovoitov (7):
  bpf, ftrace: temporary workaround
  bpf: refactor x86 JIT into helpers
  bpf: Introduce BPF trampoline
  libbpf: Add support to attach to fentry/fexit tracing progs
  selftest/bpf: Simple test for fentry/fexit
  bpf: Add kernel test functions for fentry testing
  selftests/bpf: Add test for BPF trampoline

 arch/x86/kernel/ftrace.c                      |  36 ++
 arch/x86/net/bpf_jit_comp.c                   | 352 +++++++++++++++---
 include/linux/bpf.h                           |  90 +++++
 include/uapi/linux/bpf.h                      |   2 +
 kernel/bpf/Makefile                           |   1 +
 kernel/bpf/btf.c                              |  73 +++-
 kernel/bpf/core.c                             |  31 ++
 kernel/bpf/syscall.c                          |  53 ++-
 kernel/bpf/trampoline.c                       | 252 +++++++++++++
 kernel/bpf/verifier.c                         |  39 ++
 net/bpf/test_run.c                            |  41 ++
 tools/include/uapi/linux/bpf.h                |   2 +
 tools/lib/bpf/bpf_helpers.h                   |  13 +
 tools/lib/bpf/libbpf.c                        |  55 ++-
 tools/lib/bpf/libbpf.h                        |   2 +
 tools/lib/bpf/libbpf.map                      |   1 +
 .../selftests/bpf/prog_tests/fentry_test.c    |  65 ++++
 .../selftests/bpf/prog_tests/kfree_skb.c      |  37 +-
 .../testing/selftests/bpf/progs/fentry_test.c |  90 +++++
 tools/testing/selftests/bpf/progs/kfree_skb.c |  52 +++
 20 files changed, 1215 insertions(+), 72 deletions(-)
 create mode 100644 kernel/bpf/trampoline.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/fentry_test.c
 create mode 100644 tools/testing/selftests/bpf/progs/fentry_test.c

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2.23.0