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

KP Singh
2022-08-28T23:01:40Z

On Fri, Aug 26, 2022 at 8:46 PM Jiri Olsa <jolsa@kernel.org> wrote:
>
> 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>

I read the whole discussion in
https://lore.kernel.org/bpf/CAADnVQLMY9Gbipx6Tc4dsRN3YSXo2CF8T=KwAvjU7nRDjYxRgQ@mail.gmail.com/

and this LGTM

Acked-by: KP Singh <kpsingh@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
>