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/20220722110811.124515-1-jolsa@kernel.org/
  [2] https://lore.kernel.org/bpf/969a14281a7791c334d476825863ee449964dd0c.camel@linux.ibm.com/Suggested-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NJiri Olsa <jolsa@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/bpf/20220903131154.420467-3-jolsa@kernel.org