Received-SPF: pass (google.com: domain of linux-kernel+bounces-209583-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Message-ID: <d0743ed8-d26c-3471-ed5d-66ec9e46db5e@huawei.com>
Date: Tue, 11 Jun 2024 17:36:47 +0800
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101
 Thunderbird/102.6.0
Subject: Re: [RFC PATCH] ftrace: Skip __fentry__ location of overridden weak
 functions
Content-Language: en-US
To: Peter Zijlstra <peterz@infradead.org>
CC: <rostedt@goodmis.org>, <mcgrof@kernel.org>, <mhiramat@kernel.org>,
	<mark.rutland@arm.com>, <mathieu.desnoyers@efficios.com>,
	<jpoimboe@kernel.org>, <linux-modules@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <linux-trace-kernel@vger.kernel.org>,
	<bpf@vger.kernel.org>
References: <20240607115211.734845-1-zhengyejian1@huawei.com>
 <20240607150228.GR8774@noisy.programming.kicks-ass.net>
 <57e499a4-e26d-148f-317d-233e873d11b4@huawei.com>
 <20240611092157.GU40213@noisy.programming.kicks-ass.net>
From: Zheng Yejian <zhengyejian1@huawei.com>
In-Reply-To: <20240611092157.GU40213@noisy.programming.kicks-ass.net>
Content-Type: text/plain; charset="UTF-8"; format=flowed
Content-Transfer-Encoding: 7bit

On 2024/6/11 17:21, Peter Zijlstra wrote:
> On Tue, Jun 11, 2024 at 09:56:51AM +0800, Zheng Yejian wrote:
>> On 2024/6/7 23:02, Peter Zijlstra wrote:
> 
>>> Oh gawd, sodding weak functions again.
>>>
>>> I would suggest changing scipts/kallsyms.c to emit readily identifiable
>>> symbol names for all the weak junk, eg:
>>>
>>>     __weak_junk_NNNNN
>>>
>>
>> Sorry for the late reply, I just had a long noon holiday :>
>>
>> scripts/kallsyms.c is compiled and used to handle symbols in vmlinux.o
>> or vmlinux.a, see kallsyms_step() in scripts/link-vmlinux.sh, those
>> overridden weak symbols has been removed from symbol table of vmlinux.o
>> or vmlinux.a. But we can found those symbols from original xx/xx.o file,
>> for example, the weak free_initmem() in in init/main.c is overridden,
>> its symbol is not in vmlinx but is still in init/main.o .
>>
>> How about traversing all origin xx/xx.o and finding all weak junk symbols ?
> 
> You don't need to. ELF symbl tables have an entry size for FUNC type
> objects, this means that you can readily find holes in the text and fill
> them with a symbol.
> 
> Specifically, you can check the mcount locations against the symbol
> table and for every one that falls in a hole, generate a new junk
> symbol.
> 
> Also see 4adb23686795 where objtool adds these holes to the
> ignore/unreachable code check.
> 
> 
> The lack of size for kallsyms is in a large part what is causing the
> problems.

Thanks for your suggestions, I'll try it soon.

--

Thanks,
ZYJ