linux-kernel - Re: [PATCH] ftrace: Do not reference symbols in sections without size

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Message-ID: <20210215200639.67141685@oasis.local.home>
Date:   Mon, 15 Feb 2021 20:06:39 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] ftrace: Do not reference symbols in sections without
 size

On Mon, 15 Feb 2021 18:05:04 -0600
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Mon, Feb 15, 2021 at 04:44:46PM -0500, Steven Rostedt wrote:
> > From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> > 
> > Starting with binutils 2.36, sections were being removed if they had weak
> > functions that were optimized out. Unfortunately, these weak functions would
> > leave references to mcount/fentry calls, that would make recordmcount fail
> > to find the symbol that matched the call to fentry.  
> 
> Binutils 2.36 isn't removing sections, just section *symbols*.
> 
> > Before returning the symbol of the section to create the mcount location,
> > check if that section size is greater than zero. If it has no size, skip
> > referencing that mcount call location.  
> 
> How does this even work?  The .text.unlikely section isn't empty:
> 
>   # readelf -SW kernel/kexec_file.o

Bah, because I messed up, and realized I was using the first section (shdr0)
and not the one it was being used for.

Strange, that when I applied this to the latest kernel on a my build
system (binutils 2.35), it still created all the necessary mcount
locations??

I'll look more into this.

Thanks for pointing this out.

-- Steve



>   
>   Section Headers:
>     [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
>     [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
>     [ 1] .text             PROGBITS        0000000000000000 000040 001acf 00  AX  0   0  1
>     [ 2] .rela.text        RELA            0000000000000000 0049e0 001b30 18   I 27   1  8
>     [ 3] .data             PROGBITS        0000000000000000 001b20 000e80 00  WA  0   0 32
>     [ 4] .rela.data        RELA            0000000000000000 006510 0010e0 18   I 27   3  8
>     [ 5] .bss              NOBITS          0000000000000000 0029a0 000060 00  WA  0   0 32
>     [ 6] .rodata           PROGBITS        0000000000000000 0029a0 000b80 00   A  0   0 32
>     [ 7] .rodata.str1.1    PROGBITS        0000000000000000 003520 0001c6 01 AMS  0   0  1
>     [ 8] .text.unlikely    PROGBITS        0000000000000000 0036e6 000038 00  AX  0   0  1
>