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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Sat, 24 Jun 2023 17:55:51 +0900
Message-ID: <CAK7LNAST13Ju4XNH6YDiy_QbfV_2wSGzv6-axLhTPwC54qUvfA@mail.gmail.com>
Subject: Re: [PATCH 1/3] modpost: factor out inst location calculation to section_rel()
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        Nathan Chancellor <nathan@kernel.org>,
        Nicolas Schier <nicolas@fjasle.eu>
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On Sat, Jun 24, 2023 at 2:01=E2=80=AFAM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Thu, Jun 22, 2023 at 10:38=E2=80=AFPM Masahiro Yamada <masahiroy@kerne=
l.org> wrote:
> >
> > On Fri, Jun 23, 2023 at 3:25=E2=80=AFAM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > On Tue, Jun 20, 2023 at 5:05=E2=80=AFAM Masahiro Yamada <masahiroy@ke=
rnel.org> wrote:
> > > >
> > > > diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> > > > index 6e0b8be32648..2551ac9d5bd3 100644
> > > > --- a/scripts/mod/modpost.c
> > > > +++ b/scripts/mod/modpost.c
> > > > @@ -1519,17 +1512,20 @@ static void section_rel(struct module *mod,=
 struct elf_info *elf,
> > > >                 r_sym =3D ELF_R_SYM(r.r_info);
> > > >  #endif
> > > >                 r.r_addend =3D 0;
> > > > +
> > > > +               loc =3D sym_get_data_by_offset(elf, fsecndx, r.r_of=
fset);
> > >
> > > Can we compute `loc` only for the three machine types?
> >
> >
> >
> > I believe you can compute the location in the same way for any architec=
ture
> > because it is mentioned in ELF spec.
>
> Sure, but perhaps it's wasted work for other machine types?


I guess you missed the following code:


switch (elf->hdr->e_machine) {
case EM_386:
        ...
        break;
case EM_ARM:
        ...
        break;
case EM_MIPS:
        ...
        break;
default:
        fatal("Please add code to calculate addend for this architecture\n"=
);
}


I believe other machines never call this function.
If it occurred, fatal() would immediately errors out,
but I have not heard such a breakage for far.

I believe only i386, mips and arm use REL.
The other architectures are RELA.


--=20
Best Regards
Masahiro Yamada