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

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Aug 16, 2022 at 08:56:33AM +0200, Jiri Olsa wrote:
> On Mon, Aug 15, 2022 at 05:48:44PM +0200, Peter Zijlstra wrote:
> > On Mon, Aug 15, 2022 at 08:35:53AM -0700, Alexei Starovoitov wrote:
> > > On Mon, Aug 15, 2022 at 8:28 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > > >
> > > > On Mon, Aug 15, 2022 at 08:17:42AM -0700, Alexei Starovoitov wrote:
> > > > > It's hiding a fake function from ftrace, since it's not a function
> > > > > and ftrace infra shouldn't show it tracing logs.
> > > > > In other words it's a _notrace_ function with nop5.
> > > >
> > > > Then make it a notrace function with a nop5 in it. That isn't hard.
> > > 
> > > That's exactly what we're trying to do.
> > 
> > All the while claiming ftrace is broken while it is not.
> > 
> > > Jiri's patch is one way to achieve that.
> > 
> > Fairly horrible way.
> > 
> > > What is your suggestion?
> > 
> > Mailed it already.
> > 
> > > Move it from C to asm ?
> > 
> > Would be much better than proposed IMO.
> 
> nice, that would be independent of the compiler atributes
> and config checking..  will check on this one ;-)

how about something like below?

dispatcher code is generated only for x86_64, so that will be covered
by the assembly version (free of ftrace table) other archs stay same

jirka


----
diff --git a/arch/x86/net/Makefile b/arch/x86/net/Makefile
index 383c87300b0d..94964002eaae 100644
--- a/arch/x86/net/Makefile
+++ b/arch/x86/net/Makefile
@@ -7,4 +7,5 @@ ifeq ($(CONFIG_X86_32),y)
         obj-$(CONFIG_BPF_JIT) += bpf_jit_comp32.o
 else
         obj-$(CONFIG_BPF_JIT) += bpf_jit_comp.o
+        obj-$(CONFIG_BPF_JIT) += bpf_dispatcher.o
 endif
diff --git a/arch/x86/net/bpf_dispatcher.S b/arch/x86/net/bpf_dispatcher.S
new file mode 100644
index 000000000000..65790a1286e8
--- /dev/null
+++ b/arch/x86/net/bpf_dispatcher.S
@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#include <linux/linkage.h>
+#include <asm/nops.h>
+#include <asm/nospec-branch.h>
+
+	.text
+SYM_FUNC_START(bpf_dispatcher_xdp_func)
+	ASM_NOP5
+	JMP_NOSPEC rdx
+SYM_FUNC_END(bpf_dispatcher_xdp_func)
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index a627a02cf8ab..03b54c820b95 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -924,7 +924,7 @@ int arch_prepare_bpf_dispatcher(void *image, s64 *funcs, int num_funcs);
 }
 
 #define DEFINE_BPF_DISPATCHER(name)					\
-	noinline __nocfi unsigned int bpf_dispatcher_##name##_func(	\
+	noinline __nocfi unsigned int __weak bpf_dispatcher_##name##_func(\
 		const void *ctx,					\
 		const struct bpf_insn *insnsi,				\
 		bpf_func_t bpf_func)					\