linux-kernel - Re: [PATCH v6 03/20] modpost: detect section mismatch for R_ARM_MOVW_ABS_NC and R_ARM_MOVT

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Message-ID: <CAK7LNAR8UE8298b=WxyUkG99pe=yb_m=qdsiW+jMcACg=jvCwA@mail.gmail.com>
Date:   Wed, 24 May 2023 09:04:49 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        Nicolas Schier <nicolas@...sle.eu>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Fangrui Song <maskray@...gle.com>
Subject: Re: [PATCH v6 03/20] modpost: detect section mismatch for
 R_ARM_MOVW_ABS_NC and R_ARM_MOVT_ABS

On Tue, May 23, 2023 at 3:03 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> + linux-arm-kernel
>
> On Sun, May 21, 2023 at 9:05 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > ARM defconfig misses to detect some section mismatches.
> >
> >   [test code]
> >
> >     #include <linux/init.h>
> >
> >     int __initdata foo;
> >     int get_foo(int x) { return foo; }
> >
> > It is apparently a bad reference, but modpost does not report anything
> > for ARM defconfig (i.e. multi_v7_defconfig).
> >
> > The test code above produces the following relocations.
> >
> >   Relocation section '.rel.text' at offset 0x200 contains 2 entries:
> >    Offset     Info    Type            Sym.Value  Sym. Name
> >   00000000  0000062b R_ARM_MOVW_ABS_NC 00000000   .LANCHOR0
> >   00000004  0000062c R_ARM_MOVT_ABS    00000000   .LANCHOR0
> >
> >   Relocation section '.rel.ARM.exidx' at offset 0x210 contains 2 entries:
> >    Offset     Info    Type            Sym.Value  Sym. Name
> >   00000000  0000022a R_ARM_PREL31      00000000   .text
> >   00000000  00001000 R_ARM_NONE        00000000   __aeabi_unwind_cpp_pr0
> >
> > Currently, R_ARM_MOVW_ABS_NC and R_ARM_MOVT_ABS are just skipped.
> >
> > Add code to handle them. I checked arch/arm/kernel/module.c to learn
> > how the offset is encoded in the instruction.
> >
> > The referenced symbol in relocation might be a local anchor.
> > If is_valid_name() returns false, let's search for a better symbol name.
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> > ---
> >
> >  scripts/mod/modpost.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> > index 34fbbd85bfde..ed2301e951a9 100644
> > --- a/scripts/mod/modpost.c
> > +++ b/scripts/mod/modpost.c
> > @@ -1108,7 +1108,7 @@ static inline int is_valid_name(struct elf_info *elf, Elf_Sym *sym)
> >  /**
> >   * Find symbol based on relocation record info.
> >   * In some cases the symbol supplied is a valid symbol so
> > - * return refsym. If st_name != 0 we assume this is a valid symbol.
> > + * return refsym. If is_valid_name() == true, we assume this is a valid symbol.
> >   * In other cases the symbol needs to be looked up in the symbol table
> >   * based on section and address.
> >   *  **/
> > @@ -1121,7 +1121,7 @@ static Elf_Sym *find_tosym(struct elf_info *elf, Elf64_Sword addr,
> >         Elf64_Sword d;
> >         unsigned int relsym_secindex;
> >
> > -       if (relsym->st_name != 0)
> > +       if (is_valid_name(elf, relsym))
> >                 return relsym;
> >
> >         /*
> > @@ -1312,11 +1312,19 @@ static int addend_arm_rel(struct elf_info *elf, Elf_Shdr *sechdr, Elf_Rela *r)
> >         unsigned int r_typ = ELF_R_TYPE(r->r_info);
> >         Elf_Sym *sym = elf->symtab_start + ELF_R_SYM(r->r_info);
> >         unsigned int inst = TO_NATIVE(*reloc_location(elf, sechdr, r));
> > +       int offset;
> >
> >         switch (r_typ) {
> >         case R_ARM_ABS32:
> >                 r->r_addend = inst + sym->st_value;
> >                 break;
> > +       case R_ARM_MOVW_ABS_NC:
> > +       case R_ARM_MOVT_ABS:
> > +               offset = ((inst & 0xf0000) >> 4) | (inst & 0xfff);
> > +               offset = (offset ^ 0x8000) - 0x8000;
>
> The code in arch/arm/kernel/module.c then right shifts the offset by
> 16 for R_ARM_MOVT_ABS. Is that necessary?


I replied to Ard's email, but just in case.


modpost does not need to write back the fixed instruction.
modpost is only interested in the offset address.

So, the right-shift by 16 is unneeded.






-- 
Best Regards
Masahiro Yamada