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

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Date:   Mon, 29 Nov 2021 18:56:43 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     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 29/11/2021 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.

Thanks
Christophe