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

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Date:   Fri, 19 Aug 2022 13:45:34 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Jiri Olsa <olsajiri@...il.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC] ftrace: Add support to keep some functions out of ftrace

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@...dmis.org> wrote:
> > >
> > > On Thu, 18 Aug 2022 22:27:07 +0200
> > > Jiri Olsa <olsajiri@...il.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,				\