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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 2 Aug 2022 10:58:24 -0700
Message-ID: <CAKwvOdk3-UdsxDwjgF=_=irsGLeDj6LF45oeV+6a5d9CeY-Fkg@mail.gmail.com>
Subject: Re: [PATCH 2/3] modpost: use more reliable way to get fromsec in section_rel(a)()
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     linux-kbuild@vger.kernel.org,
        Michal Marek <michal.lkml@markovi.net>,
        linux-kernel@vger.kernel.org, Fangrui Song <maskray@google.com>
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On Sat, Jul 30, 2022 at 10:37 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> The section name of Rel and Rela starts with ".rel" and ".rela"
> respectively (but, I do not know whether this is specification or
> convention).
>
> For example, ".rela.text" holds relocation entries applied to the
> ".text" section.
>
> So, the code chops the ".rel" or ".rela" prefix to get the name of
> the section to which the relocation applies.
>
> However, I do not like to skip 4 or 5 bytes blindly because it is
> potential memory overrun.
>
> The ELF specification provides a more reliable way to do this.
>
>  - The sh_info field holds extra information, whose interpretation
>    depends on the section type
>
>  - If the section type is SHT_REL or SHT_RELA, the sh_info field holds
>    the section header index of the section to which the relocation
>    applies.
>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Yes, this seems much safer; thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>
>  scripts/mod/modpost.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index 148b38699889..c6a055c0291e 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -1723,8 +1723,7 @@ static void section_rela(const char *modname, struct elf_info *elf,
>         Elf_Rela *start = (void *)elf->hdr + sechdr->sh_offset;
>         Elf_Rela *stop  = (void *)start + sechdr->sh_size;
>
> -       fromsec = sech_name(elf, sechdr);
> -       fromsec += strlen(".rela");
> +       fromsec = sec_name(elf, sechdr->sh_info);
>         /* if from section (name) is know good then skip it */
>         if (match(fromsec, section_white_list))
>                 return;
> @@ -1776,8 +1775,7 @@ static void section_rel(const char *modname, struct elf_info *elf,
>         Elf_Rel *start = (void *)elf->hdr + sechdr->sh_offset;
>         Elf_Rel *stop  = (void *)start + sechdr->sh_size;
>
> -       fromsec = sech_name(elf, sechdr);
> -       fromsec += strlen(".rel");
> +       fromsec = sec_name(elf, sechdr->sh_info);
>         /* if from section (name) is know good then skip it */
>         if (match(fromsec, section_white_list))
>                 return;
> --
> 2.34.1
>


-- 
Thanks,
~Nick Desaulniers