Re: [PATCH] selftests/ftrace: Account for fprobe attachment at creation

[Google]

On Fri, 10 Apr 2026 12:32:43 +0800
Cao Ruichuang <create0818@xxxxxxx> wrote:

> Hi Masami,
> 
> I reran this in clean QEMU on two kernels and got different results.
> 
> 1. Ubuntu distro kernel:
>    Linux 6.8.0-100-generic #100-Ubuntu SMP PREEMPT_DYNAMIC
>    Tue Jan 13 16:40:06 UTC 2026
> 
>    baseline count=2
>    after_create123 count=4
>    after_enable1/2/3 count=4
> 
>    baseline enabled_functions:
>      __hid_bpf_tail_call ...
> 
>    after_create123 enabled_functions:
>      kernel_clone (2) R ->arch_ftrace_ops_list_func+0x0/0x280
>      kmem_cache_free (1) R tramp: ... ->fprobe_handler+0x0/0x40
>      __hid_bpf_tail_call ...
> 
> 2. Current source-tree kernel built from the clean snapshot of my patch
>    branch:
>    Linux 7.0.0-rc6 #2 SMP PREEMPT_DYNAMIC Fri Apr 10 12:19:39 CST 2026
> 
>    baseline count=0
>    after_create123 count=0
>    after_enable1 count=1
>    after_enable2 count=1
>    after_enable3 count=2
> 
>    after_create123 enabled_functions:
>      <empty>
> 
>    after_enable3 enabled_functions:
>      kernel_clone (2) ->arch_ftrace_ops_list_func+0x0/0x200
>      kmem_cache_free (1) tramp: ... ->fprobe_ftrace_entry+0x0/0x220
> 
> So the behavior I reported earlier reproduces on that Ubuntu 6.8 kernel,
> but not on the current source-tree kernel. I think my earlier conclusion
> was too broad.

Thanks for reporting the difference of behaviors.

> 
> I will stop pushing this testcase change for now unless I can narrow down
> which kernel change caused the difference.

OK.

Thanks!

> 
> Thanks,
> Cao Ruichuang
> 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>