Re: [PATCHv3 bpf-next 2/2] bpf: Move bpf_dispatcher function out of ftrace locations

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Sep 03, 2022 at 03:11:54PM +0200, Jiri Olsa 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.
> 
> 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 pointer 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: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  arch/x86/Kconfig    | 1 +
>  include/linux/bpf.h | 7 +++++++
>  2 files changed, 8 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..e267625557cb 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -924,7 +924,14 @@ int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs);
>  	},							\
>  }
>  
> +#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,				\

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>