On Fri, Aug 26, 2022 at 08:46:08PM +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 This is typically spelled like: 'notrace' in the kernel. > - 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@xxxxxxxxxx/ > Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx> > --- > 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, \ What makes that whole dispatcher thing x86 only? AFAICT it is only under BPF_JIT here and could be used by anyone. ARM64 for instance has BPG_JIT and builds net/core/filter.c. And ARM64 very much does use patchable_function_entry() for its ftrace implementation.
