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

From: Peter Zijlstra <peterz@infradead.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Jiri Olsa <olsajiri@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, bpf <bpf@vger.kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [RFC] ftrace: Add support to keep some functions out of ftrace
Date: Mon, 15 Aug 2022 17:41:27 +0200	[thread overview]
Message-ID: <YvppJ7TjMXD3cSdZ@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <YvpmAnFldR0iwAFC@worktop.programming.kicks-ass.net>

On Mon, Aug 15, 2022 at 05:28:02PM +0200, Peter Zijlstra 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.
> 
> The whole problem is that it isn't a notrace function and you're abusing
> a __fentry__ site.

https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=x86/fineibt&id=8d075bdf11193f1d276bf19fa56b4b8dfe24df9e

foo.c:

__attribute__((__no_instrument_function__))
__attribute__((patchable_function_entry(5)))
void my_func(void)
{
}

void my_foo(void)
{
}

gcc -c foo.c -pg -mfentry -mcmodel=kernel -fno-PIE -O2

foo.o:     file format elf64-x86-64

Disassembly of section .text:

0000000000000000 <my_func>:
   0:   f3 0f 1e fa             endbr64 
   4:   90                      nop
   5:   90                      nop
   6:   90                      nop
   7:   90                      nop
   8:   90                      nop
   9:   c3                      ret    
   a:   66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)

0000000000000010 <my_foo>:
  10:   f3 0f 1e fa             endbr64 
  14:   e8 00 00 00 00          call   19 <my_foo+0x9>  15: R_X86_64_PLT32      __fentry__-0x4
  19:   c3                      ret    