[PATCH bpf-next 2/2] bpf: Move bpf_dispatcher function out of ftrace locations

[Jiri Olsa <jolsa@kernel.org>]

The dispatcher function is attached/detached to trampoline by
dispatcher update function. At the same time it's available as
ftrace attachable function.

After discussion [1] the proposed solution is to use compiler
attributes to alter bpf_dispatcher_##name##_func function:

  - remove it from being instrumented with __no_instrument_function__
    attribute, so ftrace has no track of it

  - but still generate 5 nop instructions with patchable_function_entry(5)
    attribute, which are expected by bpf_arch_text_poke used by
    dispatcher update function

Enabling HAVE_DYNAMIC_FTRACE_NO_PATCHABLE option for x86, so
__patchable_function_entries functions are not part of ftrace/mcount
locations.

The dispatcher code is generated and attached only for x86 so it's safe
to keep bpf_dispatcher func in patchable_function_entry locations for
other archs.

[1] https://lore.kernel.org/bpf/20220722110811.124515-1-jolsa@kernel.org/
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 arch/x86/Kconfig    | 1 +
 include/linux/bpf.h | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f9920f1341c8..089c20cefd2b 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -284,6 +284,7 @@ config X86
 	select PROC_PID_ARCH_STATUS		if PROC_FS
 	select HAVE_ARCH_NODE_DEV_GROUP		if X86_SGX
 	imply IMA_SECURE_AND_OR_TRUSTED_BOOT    if EFI
+	select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
 
 config INSTRUCTION_DECODER
 	def_bool y
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 9c1674973e03..945d5414bb62 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -925,6 +925,8 @@ int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs);
 }
 
 #define DEFINE_BPF_DISPATCHER(name)					\
+	__attribute__((__no_instrument_function__))			\
+	__attribute__((patchable_function_entry(5)))			\
 	noinline __nocfi unsigned int bpf_dispatcher_##name##_func(	\
 		const void *ctx,					\
 		const struct bpf_insn *insnsi,				\
-- 
2.37.2