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

[Jiri Olsa]

On Wed, Aug 17, 2022 at 09:57:45AM -0700, Alexei Starovoitov wrote:
> On Wed, Aug 17, 2022 at 2:29 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> >
> > 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@xxxxxxxxxxxxx> 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)
> 
> Wait. Why asm ? Did you try Peter's suggestion:
> __attribute__((__no_instrument_function__))
> __attribute__((patchable_function_entry(5)))

ah so this suggestion came in the other thread after the asm one.. ok, will check

jirka