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

Peter Zijlstra
2022-08-15T15:42:00Z

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