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

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Date:   Mon, 15 Feb 2021 18:05:04 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, 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
  
  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

-- 
Josh