Re: [RFC] ftrace: Add support to keep some functions out of ftrace

Jiri Olsa
2022-08-19T11:45:45Z

On Thu, Aug 18, 2022 at 11:32:55PM +0200, Jiri Olsa wrote:
> On Thu, Aug 18, 2022 at 02:00:21PM -0700, Alexei Starovoitov wrote:
> > On Thu, Aug 18, 2022 at 1:50 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > > On Thu, 18 Aug 2022 22:27:07 +0200
> > > Jiri Olsa <olsajiri@gmail.com> wrote:
> > >
> > > > ok, so the problem with __attribute__((patchable_function_entry(5))) is that
> > > > it puts function address into __patchable_function_entries section, which is
> > > > one of ftrace locations source:
> > > >
> > > > #define MCOUNT_REC() . = ALIGN(8); \
> > > > __start_mcount_loc = .; \
> > > > KEEP(*(__mcount_loc)) \
> > > > KEEP(*(__patchable_function_entries)) \
> > > > __stop_mcount_loc = .; \
> > > > ...
> > > >
> > > >
> > > > it looks like __patchable_function_entries is used for other than x86 archs,
> > > > so we perhaps we could have x86 specific MCOUNT_REC macro just with
> > > > __mcount_loc section?
> > >
> > > So something like this:
> > >
> > > #ifdef CONFIG_X86
> > > # define NON_MCOUNT_PATCHABLE KEEP(*(__patchable_function_entries))
> > > # define MCOUNT_PATCHABLE
> > > #else
> > > # define NON_MCOUNT_PATCHABLE
> > > # define MCOUNT_PATCHABLE KEEP(*(__patchable_function_entries))
> > > #endif
> > >
> > > #define MCOUNT_REC() . = ALIGN(8); \
> > > __start_mcount_loc = .; \
> > > KEEP(*(__mcount_loc)) \
> > > MCOUNT_PATCHABLE \
> > > __stop_mcount_loc = .; \
> > > NON_MCOUNT_PATCHABLE \
> > > ...
> > >
> > > ??
> >
> > That's what more or less Peter's patch is doing:
> > Here it is again for reference:
> > https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?id=8d075bdf11193f1d276bf19fa56b4b8dfe24df9e
>
> ah nice, and discards the __patchable_function_entries section, great
>

tested change below with Peter's change above and it seems to work,
once it get merged I'll send full patch

thanks,
jirka

---
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 39bd36359c1e..39b6807058e9 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -924,6 +924,8 @@ 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, \