bpf: Fix the build on BPF_SYSCALL=y && !CONFIG_TRACING kernels, make it more configurable

So bpf_tracing.o depends on CONFIG_BPF_SYSCALL - but that's not its only
dependency, it also depends on the tracing infrastructure and on kprobes,
without which it will fail to build with:

  In file included from kernel/trace/bpf_trace.c:14:0:
  kernel/trace/trace.h: In function ‘trace_test_and_set_recursion’:
  kernel/trace/trace.h:491:28: error: ‘struct task_struct’ has no member named ‘trace_recursion’
    unsigned int val = current->trace_recursion;
  [...]

It took quite some time to trigger this build failure, because right now
BPF_SYSCALL is very obscure, depends on CONFIG_EXPERT. So also make BPF_SYSCALL
more configurable, not just under CONFIG_EXPERT.

If BPF_SYSCALL, tracing and kprobes are enabled then enable the bpf_tracing
gateway as well.

We might want to make this an interactive option later on, although
I'd not complicate it unnecessarily: enabling BPF_SYSCALL is enough of
an indicator that the user wants BPF support.

Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

init/Kconfig

@@ -1513,7 +1513,7 @@ config EVENTFD
 
 # syscall, maps, verifier
 config BPF_SYSCALL
-	bool "Enable bpf() system call" if EXPERT
+	bool "Enable bpf() system call"
 	select ANON_INODES
 	select BPF
 	default n

kernel/trace/Kconfig

@@ -432,6 +432,14 @@ config UPROBE_EVENT
 	  This option is required if you plan to use perf-probe subcommand
 	  of perf tools on user space applications.
 
+config BPF_EVENTS
+	depends on BPF_SYSCALL
+	depends on KPROBE_EVENT
+	bool
+	default y
+	help
+	  This allows the user to attach BPF programs to kprobe events.
+
 config PROBE_EVENTS
 	def_bool n
 

kernel/trace/Makefile

@@ -53,7 +53,7 @@ obj-$(CONFIG_EVENT_TRACING) += trace_event_perf.o
 endif
 obj-$(CONFIG_EVENT_TRACING) += trace_events_filter.o
 obj-$(CONFIG_EVENT_TRACING) += trace_events_trigger.o
-obj-$(CONFIG_BPF_SYSCALL) += bpf_trace.o
+obj-$(CONFIG_BPF_EVENTS) += bpf_trace.o
 obj-$(CONFIG_KPROBE_EVENT) += trace_kprobe.o
 obj-$(CONFIG_TRACEPOINTS) += power-traces.o
 ifeq ($(CONFIG_PM),y)