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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Fri, 23 Jun 2023 14:37:20 +0900
Message-ID: <CAK7LNATX+_kr+zKskV7NahpasZBV0P+Ke3g_nW=+vRwcsFL1+g@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 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@kernel=
.org> wrote:
> >
> > All the addend_*_rel() functions calculate the instruction location in
> > the same way.
> >
> > Factor out the similar code to the caller. Squash reloc_location() too.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> > ---
> >
> >  scripts/mod/modpost.c | 28 ++++++++++++----------------
> >  1 file changed, 12 insertions(+), 16 deletions(-)
> >
> > 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
> > @@ -1256,16 +1256,9 @@ static void check_section_mismatch(struct module=
 *mod, struct elf_info *elf,
> >                                  tosec, taddr);
> >  }
> >
> > -static unsigned int *reloc_location(struct elf_info *elf,
> > -                                   Elf_Shdr *sechdr, Elf_Rela *r)
> > -{
> > -       return sym_get_data_by_offset(elf, sechdr->sh_info, r->r_offset=
);
> > -}
> > -
> > -static int addend_386_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_=
Rela *r)
> > +static int addend_386_rel(uint32_t *location, Elf_Rela *r)
> >  {
> >         unsigned int r_typ =3D ELF_R_TYPE(r->r_info);
> > -       unsigned int *location =3D reloc_location(elf, sechdr, r);
> >
> >         switch (r_typ) {
> >         case R_386_32:
> > @@ -1302,11 +1295,10 @@ static int32_t sign_extend32(int32_t value, int=
 index)
> >         return (int32_t)(value << shift) >> shift;
> >  }
> >
> > -static int addend_arm_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_=
Rela *r)
> > +static int addend_arm_rel(void *loc, struct elf_info *elf, Elf_Rela *r=
)
> >  {
> >         unsigned int r_typ =3D ELF_R_TYPE(r->r_info);
> >         Elf_Sym *sym =3D elf->symtab_start + ELF_R_SYM(r->r_info);
> > -       void *loc =3D reloc_location(elf, sechdr, r);
> >         uint32_t inst, upper, lower, sign, j1, j2;
> >         int32_t offset;
> >
> > @@ -1396,11 +1388,10 @@ static int addend_arm_rel(struct elf_info *elf,=
 Elf_Shdr *sechdr, Elf_Rela *r)
> >         return 0;
> >  }
> >
> > -static int addend_mips_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf=
_Rela *r)
> > +static int addend_mips_rel(uint32_t *location, Elf_Rela *r)
> >  {
> >         unsigned int r_typ =3D ELF_R_TYPE(r->r_info);
> > -       unsigned int *location =3D reloc_location(elf, sechdr, r);
> > -       unsigned int inst;
> > +       uint32_t inst;
> >
> >         if (r_typ =3D=3D R_MIPS_HI16)
> >                 return 1;       /* skip this */
> > @@ -1502,6 +1493,8 @@ static void section_rel(struct module *mod, struc=
t elf_info *elf,
> >                 return;
> >
> >         for (rel =3D start; rel < stop; rel++) {
> > +               void *loc;
> > +
> >                 r.r_offset =3D TO_NATIVE(rel->r_offset);
> >  #if KERNEL_ELFCLASS =3D=3D ELFCLASS64
> >                 if (elf->hdr->e_machine =3D=3D EM_MIPS) {
> > @@ -1519,17 +1512,20 @@ static void section_rel(struct module *mod, str=
uct 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_offset=
);
>
> Can we compute `loc` only for the three machine types?


I believe you can compute the location in the same way for any architecture
because it is mentioned in ELF spec.

https://refspecs.linuxfoundation.org/elf/elf.pdf

Page 36.


r_offset
  This member gives the location at which to apply the relocation action. F=
or
  a relocatable file, the value is the byte offset from the beginning of th=
e
  section to the storage unit affected by the relocation.


--=20
Best Regards
Masahiro Yamada