bpf: Move bpf_dispatcher function out of ftrace locations

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.

Adding attributes to bpf_dispatcher_XXX function on x86_64 so it's
kept out of ftrace locations and has 5 byte nop generated at entry.

These attributes need to be arch specific as pointed out by Ilya
Leoshkevic in here [2].

The dispatcher image is generated only for x86_64 arch, so the
code can stay as is for other archs.

[1] https://lore.kernel.org/bpf/[e-mail]/
[2] https://lore.kernel.org/bpf/[e-mail]/

Suggested-by: Peter Zijlstra (Intel) <[e-mail]>
Signed-off-by: Jiri Olsa <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

arch/x86/Kconfig

···
 	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

include/linux/bpf.h

···
 	},							\
 }
 
+#ifdef CONFIG_X86_64
+#define BPF_DISPATCHER_ATTRIBUTES __attribute__((patchable_function_entry(5)))
+#else
+#define BPF_DISPATCHER_ATTRIBUTES
+#endif
+
 #define DEFINE_BPF_DISPATCHER(name)					\
+	notrace BPF_DISPATCHER_ATTRIBUTES				\
 	noinline __nocfi unsigned int bpf_dispatcher_##name##_func(	\
 		const void *ctx,					\
 		const struct bpf_insn *insnsi,				\