linux-kernel - Re: [PATCH] recordmcount: Support empty section from recent binutils

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Date:   Wed, 30 Mar 2022 09:51:27 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Joel Stanley <joel@....id.au>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] recordmcount: Support empty section from recent binutils



Le 29/03/2022 à 00:31, Joel Stanley a écrit :
> On Mon, 29 Nov 2021 at 22:43, Christophe Leroy
> <christophe.leroy@...roup.eu> wrote:
>>
>>
>>
>> Le 29/11/2021 à 18:43, Steven Rostedt a écrit :
>>> On Fri, 26 Nov 2021 08:43:23 +0000
>>> LEROY Christophe <christophe.leroy@...roup.eu> wrote:
>>>
>>>> Le 24/11/2021 à 15:43, Christophe Leroy a écrit :
>>>>> Looks like recent binutils (2.36 and over ?) may empty some section,
>>>>> leading to failure like:
>>>>>
>>>>>      Cannot find symbol for section 11: .text.unlikely.
>>>>>      kernel/kexec_file.o: failed
>>>>>      make[1]: *** [scripts/Makefile.build:287: kernel/kexec_file.o] Error 1
>>>>>
>>>>> In order to avoid that, ensure that the section has a content before
>>>>> returning it's name in has_rel_mcount().
>>>>
>>>> This patch doesn't work, on PPC32 I get the following message with this
>>>> patch applied:
>>>>
>>>> [    0.000000] ftrace: No functions to be traced?
>>>>
>>>> Without the patch I get:
>>>>
>>>> [    0.000000] ftrace: allocating 22381 entries in 66 pages
>>>> [    0.000000] ftrace: allocated 66 pages with 2 groups
>>>
>>> Because of this report, I have not applied this patch (even though I was
>>> about to push it to Linus).
>>>
>>> I'm pulling it from my queue until this gets resolved.
>>>
>>
>> I have no idea on how to fix that for the moment.
>>
>> With GCC 10 (binutils 2.36) an objdump -x on kernel/kexec_file.o gives:
>>
>> 0000000000000000 l    d  .text.unlikely 0000000000000000 .text.unlikely
>> 0000000000000000  w    F .text.unlikely 0000000000000038
>> .arch_kexec_apply_relocations_add
>> 0000000000000038  w    F .text.unlikely 0000000000000038
>> .arch_kexec_apply_relocations
>>
>>
>> With GCC 11 (binutils 2.37) the same gives:
>>
>> 0000000000000000  w    F .text.unlikely 0000000000000038
>> .arch_kexec_apply_relocations_add
>> 0000000000000038  w    F .text.unlikely 0000000000000038
>> .arch_kexec_apply_relocations
>>
>>
>> The problem is that recordmcount drops weak symbols, and it doesn't find
>> any non-weak symbol in .text.unlikely
>>
>> Explication given at
>> https://elixir.bootlin.com/linux/v5.16-rc2/source/scripts/recordmcount.h#L506
>>
>> I have no idea on what to do.
> 
> Did you end up finding a solution for this issue?
> 

Not really, my solution was to switch to the kernel compiler at 
https://mirrors.edge.kernel.org/pub/tools/crosstool/ which embeds 
binutils 2.36

But it looks like using objtool instead of recordmcount doesn't exhibit 
the problem.

https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220318105140.43914-4-sv@linux.ibm.com/

Christophe