Re: [PATCH] kprobes: Enable tracing for mololithic kernel images

Christophe Leroy
2022-06-09T08:48:39Z

Le 08/06/2022 à 18:12, Song Liu a écrit :
> On Wed, Jun 8, 2022 at 7:21 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>>
>> Hi Jarkko,
>>
>> On Wed, 8 Jun 2022 08:25:38 +0300
>> Jarkko Sakkinen <jarkko@kernel.org> wrote:
>>
>>> On Wed, Jun 08, 2022 at 10:35:42AM +0800, Guo Ren wrote:
>>>> .
>>>>
>>>> On Wed, Jun 8, 2022 at 8:02 AM Jarkko Sakkinen <jarkko@profian.com> wrote:
>>>>>
>>>>> Tracing with kprobes while running a monolithic kernel is currently
>>>>> impossible because CONFIG_KPROBES is dependent of CONFIG_MODULES. This
>>>>> dependency is a result of kprobes code using the module allocator for the
>>>>> trampoline code.
>>>>>
>>>>> Detaching kprobes from modules helps to squeeze down the user space,
>>>>> e.g. when developing new core kernel features, while still having all
>>>>> the nice tracing capabilities.
>>>>>
>>>>> For kernel/ and arch/*, move module_alloc() and module_memfree() to
>>>>> module_alloc.c, and compile as part of vmlinux when either CONFIG_MODULES
>>>>> or CONFIG_KPROBES is enabled. In addition, flag kernel module specific
>>>>> code with CONFIG_MODULES.
>>>>>
>>>>> As the result, kprobes can be used with a monolithic kernel.
>>>> It's strange when MODULES is n, but vmlinux still obtains module_alloc.
>>>>
>>>> Maybe we need a kprobe_alloc, right?
>>>
>>> Perhaps not the best name but at least it documents the fact that
>>> they use the same allocator.
>>>
>>> Few years ago I carved up something "half-way there" for kprobes,
>>> and I used the name text_alloc() [*].
>>>
>>> [*] https://lore.kernel.org/all/20200724050553.1724168-1-jarkko.sakkinen@linux.intel.com/
>>
>> Yeah, I remember that. Thank you for updating your patch!
>> I think the idea (split module_alloc() from CONFIG_MODULE) is good to me.
>> If module support maintainers think this name is not good, you may be
>> able to rename it as text_alloc() and make the module_alloc() as a
>> wrapper of it.
>
> IIUC, most users of module_alloc() use it to allocate memory for text, except
> that module code uses it for both text and data. Therefore, I guess calling it
> text_alloc() is not 100% accurate until we change the module code (to use
> a different API to allocate memory for data).

When CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC, module code uses
module_alloc() for text and vmalloc() for data, see function
move_module() in kernel/module/main.c

>
> Thanks,
> Song
>
>>
>> Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
>> for kprobe side.
>>
>> Thank you,
>>
>> --
>> Masami Hiramatsu (Google) <mhiramat@kernel.org>