Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH -tip v8 11/13] x86/unwind: Recover kretprobe trampoline
 entry
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
        Daniel Xu <dxu@dxuuu.xyz>, linux-kernel@vger.kernel.org,
        bpf@vger.kernel.org, kuba@kernel.org, mingo@redhat.com,
        ast@kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, kernel-team@fb.com, yhs@fb.com,
        linux-ia64@vger.kernel.org,
        Abhishek Sagar <sagar.abhishek@gmail.com>,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>
References: <162399992186.506599.8457763707951687195.stgit@devnote2>
 <162400002631.506599.2413605639666466945.stgit@devnote2>
 <YOLurg5mGHdBc+fz@hirez.programming.kicks-ass.net>
 <20210706004257.9e282b98f447251a380f658f@kernel.org>
 <YOQMV8uE/2bVkPOY@hirez.programming.kicks-ass.net>
 <20210706111136.7c5e9843@oasis.local.home>
 <YOVj2VoyrcOvJfEB@hirez.programming.kicks-ass.net>
 <20210707191510.cb48ca4a20f0502ce6c46508@kernel.org>
 <YOWACec65qVdTD1y@hirez.programming.kicks-ass.net>
 <20210707194530.766a9c8364f3b2d7714ca590@kernel.org>
 <20210707222925.87ecc1391d0ab61db3d8398e@kernel.org>
From:   Matt Wu <wuqiang.matt@bytedance.com>
Message-ID: <3fc578e0-5b26-6067-d026-5b5d230d6720@bytedance.com>
Date:   Wed, 7 Jul 2021 22:42:47 +0800
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0)
 Gecko/20100101 Thunderbird/78.11.0
MIME-Version: 1.0
In-Reply-To: <20210707222925.87ecc1391d0ab61db3d8398e@kernel.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 2021/7/7 PM9:29, Masami Hiramatsu wrote:
> On Wed, 7 Jul 2021 19:45:30 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
>> On Wed, 7 Jul 2021 12:20:57 +0200
>> Peter Zijlstra <peterz@infradead.org> wrote:
>>
>>> On Wed, Jul 07, 2021 at 07:15:10PM +0900, Masami Hiramatsu wrote:
>>>
>>>> I actually don't want to keep this feature because no one use it.
>>>> (only systemtap needs it?)
>>>
>>> Yeah, you mentioned systemtap, but since that's out-of-tree I don't
>>> care. Their problem.
> 
> Yeah, maybe it is not hard to update.
> 
>>>
>>>> Anyway, if we keep the idea-level compatibility (not code level),
>>>> what we need is 'void *data' in the struct kretprobe_instance.
>>>> User who needs it can allocate their own instance data for their
>>>> kretprobes when initialising it and sets in their entry handler.
>>>>
>>>> Then we can have a simple kretprobe_instance.
>>>
>>> When would you do the alloc? When installing the retprobe, but that
>>> might be inside the allocator, which means you can't call the allocator
>>> etc.. :-)
>>
>> Yes, so the user may need to allocate a pool right before register_kretprobe().
>> (whether per-kretprobe or per-task or global pool, that is user's choice.)
>>
>>>
>>> If we look at struct ftrace_ret_stack, it has a few fixed function
>>> fields. The calltime one is all that is needed for the kretprobe
>>> example code.
>>
>> kretprobe consumes 3 fields, a pointer to 'struct kretprobe' (which
>> stores callee function address in 'kretprobe::kp.addr'), a return
>> address and a frame pointer (*).
>  > Oops, I forgot to add "void *data" for storing user data.
> 

Should use "struct kretprobe_holder *rph", since "struct kretprobe" belongs
to 3rd-party module (which might be unloaded any time).

User's own pool might not work if the module can be unloaded. Better manage
the pool in kretprobe_holder, which needs no changes from user side.