linux-kernel - Re: [PATCH] recordmcount: avoid using ABS symbol as reference

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Date:   Mon, 7 Jun 2021 08:37:27 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Mark-PK Tsai <mark-pk.tsai@...iatek.com>
Cc:     Matthias Brugger <matthias.bgg@...il.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Matt Helsley <mhelsley@...are.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-mediatek@...ts.infradead.org, yj.chiang@...iatek.com
Subject: Re: [PATCH] recordmcount: avoid using ABS symbol as reference

On Mon, 7 Jun 2021 at 04:42, Mark-PK Tsai <mark-pk.tsai@...iatek.com> wrote:
>
> Avoid using ABS symbol, which won't be relocate, as reference.
>
> On arm64 platform, if there's shndx equals SHN_ABS(0xfff1).
>
> Section Headers:
> [Nr]    Name                         Type      Address          Off      Size   ES  Flg Lk     Inf    Al
> [65521] .text.n_tty_receive_buf      PROGBITS  0000000000000000 3cdab520 000054 00  AX  0      0      4
> [65522] .rela.text.n_tty_receive_buf RELA      0000000000000000 3cdab578 000030 18  I   152076 65521  8
>

A RELA section's r_info field points to the section to which it
applies. This is why in the example above section #65522 points to
section #65521. This has nothing to do with the numerical value of
SHN_ABS.

> find_secsym_ndx, which use r_info in rela section to find the reference
> symbol, may take ABS symbol as base.
>
> Symbol table '.symtab' contains 453285 entries:
>    Num:    Value          Size Type    Bind   Vis       Ndx Name
>      6: 0000000000000002     0 NOTYPE  LOCAL  DEFAULT   ABS section_count
>
> Which cause an invalid address in __mcount_loc.
>

Could you give a better account of the error you are trying to address?

Also, arm64 no longer defines a section_count symbol (since v5.11), so
please make sure that the diagnostics of the issue you are addressing
are accurate for mainline.

> Signed-off-by: Mark-PK Tsai <mark-pk.tsai@...iatek.com>
> ---
>  scripts/recordmcount.h | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/scripts/recordmcount.h b/scripts/recordmcount.h
> index f9b19524da11..9b69167fb7ff 100644
> --- a/scripts/recordmcount.h
> +++ b/scripts/recordmcount.h
> @@ -526,6 +526,10 @@ static int find_secsym_ndx(unsigned const txtndx,
>         for (symp = sym0, t = nsym; t; --t, ++symp) {
>                 unsigned int const st_bind = ELF_ST_BIND(symp->st_info);
>
> +               /* avoid absolute symbols */
> +               if (symp->st_shndx == SHN_ABS)
> +                       continue;
> +
>                 if (txtndx == get_symindex(symp, symtab, symtab_shndx)
>                         /* avoid STB_WEAK */
>                     && (STB_LOCAL == st_bind || STB_GLOBAL == st_bind)) {
> --
> 2.18.0
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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