Add support to directly attach BPF program to ftrace

Hi,
These patches make it possible to attach BPF programs directly to tracepoints
using ftrace (/sys/kernel/debug/tracing) without needing the process doing the
attach to be alive. This has the following benefits:

1. Simplified Security: In Android, we have finer-grained security controls to
specific ftrace trace events using SELinux labels. We control precisely who is
allowed to enable an ftrace event already. By adding a node to ftrace for
attaching BPF programs, we can use the same mechanism to further control who is
allowed to attach to a trace event.

2. Process lifetime: In Android we are adding usecases where a tracing program
needs to be attached all the time to a tracepoint, for the full life time of
the system. Such as to gather statistics where there no need for a detach for
the full system lifetime. With perf or bpf(2)'s BPF_RAW_TRACEPOINT_OPEN, this
means keeping a process alive all the time.  However, in Android our BPF loader
currently (for hardeneded security) involves just starting a process at boot
time, doing the BPF program loading, and then pinning them to /sys/fs/bpf.  We
don't keep this process alive all the time. It is more suitable to do a
one-shot attach of the program using ftrace and not need to have a process
alive all the time anymore for this. Such process also needs elevated
privileges since tracepoint program loading currently requires CAP_SYS_ADMIN
anyway so by design Android's bpfloader runs once at init and exits.

This series add a new bpf file to /sys/kernel/debug/tracing/events/X/Y/bpf
The following commands can be written into it:
attach:<fd>     Attaches BPF prog fd to tracepoint
detach:<fd>     Detaches BPF prog fd to tracepoint

Reading the bpf file will show all the attached programs to the tracepoint.

Joel Fernandes (Google) (4):
Move bpf_raw_tracepoint functionality into bpf_trace.c
trace/bpf: Add support for attach/detach of ftrace events to BPF
lib/bpf: Add support for ftrace event attach and detach
selftests/bpf: Add test for ftrace-based BPF attach/detach

include/linux/bpf_trace.h                     |  16 ++
include/linux/trace_events.h                  |   1 +
kernel/bpf/syscall.c                          |  69 +-----
kernel/trace/bpf_trace.c                      | 225 ++++++++++++++++++
kernel/trace/trace.h                          |   1 +
kernel/trace/trace_events.c                   |   8 +
tools/lib/bpf/bpf.c                           |  53 +++++
tools/lib/bpf/bpf.h                           |   4 +
tools/lib/bpf/libbpf.map                      |   2 +
.../raw_tp_writable_test_ftrace_run.c         |  89 +++++++
10 files changed, 410 insertions(+), 58 deletions(-)
create mode 100644 tools/testing/selftests/bpf/prog_tests/raw_tp_writable_test_ftrace_run.c

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2.22.0.410.gd8fdbe21b5-goog