Re: [PATCH 5/7] modpost: detect section mismatch for R_ARM_THM_{MOVW_ABS_NC,MOVT_ABS}

[Masahiro Yamada]

On Thu, Jun 1, 2023 at 9:23 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Thu, 1 Jun 2023 at 14:10, Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
> >
> > When CONFIG_THUMB2_KERNEL is enabled, modpost fails to detect some
> > types of section mismatches.
> >
> >   [test code]
> >
> >     #include <linux/init.h>
> >
> >     int __initdata foo;
> >     int get_foo(void) { return foo; }
> >
> > It is apparently a bad reference, but modpost does not report anything.
> >
> > The test code above produces the following relocations.
> >
> >   Relocation section '.rel.text' at offset 0x1e8 contains 2 entries:
> >    Offset     Info    Type            Sym.Value  Sym. Name
> >   00000000  0000052f R_ARM_THM_MOVW_AB 00000000   .LANCHOR0
> >   00000004  00000530 R_ARM_THM_MOVT_AB 00000000   .LANCHOR0
> >
> > Currently, R_ARM_THM_MOVW_ABS_NC and R_ARM_THM_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.
> >
> > One more thing to note for Thumb instructions - the st_value is an odd
> > value, so you need to mask the bit 0 to get the offset. Otherwise, you
> > will get an off-by-one error in the nearest symbol look-up.
> >
> > It is documented in "ELF for the ARM Architecture" [1]:
> >
> >   * If the symbol addresses a Thumb instruction, its value is the address
> >     of the instruction with bit zero set (in a relocatable object, the
> >     section offset with bit zero set).
> >
> >   * For the purposes of relocation the value used shall be the address
> >     of the instruction (st_value & ~1).
> >
> > [1]: https://github.com/ARM-software/abi-aa/blob/main/aaelf32/aaelf32.rst
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> > ---
> >
> >  scripts/mod/modpost.c | 31 ++++++++++++++++++++++++++-----
> >  1 file changed, 26 insertions(+), 5 deletions(-)
> >
> > diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> > index 32d56efe3f3b..528aa9175e84 100644
> > --- a/scripts/mod/modpost.c
> > +++ b/scripts/mod/modpost.c
> > @@ -1082,7 +1082,8 @@ static Elf_Sym *find_nearest_sym(struct elf_info *elf, Elf_Addr addr,
> >  {
> >         Elf_Sym *sym;
> >         Elf_Sym *near = NULL;
> > -       Elf_Addr distance;
> > +       Elf_Addr sym_addr, distance;
> > +       bool is_arm = (elf->hdr->e_machine == EM_ARM);
> >
> >         for (sym = elf->symtab_start; sym < elf->symtab_stop; sym++) {
> >                 if (get_secindex(elf, sym) != secndx)
> > @@ -1090,10 +1091,19 @@ static Elf_Sym *find_nearest_sym(struct elf_info *elf, Elf_Addr addr,
> >                 if (!is_valid_name(elf, sym))
> >                         continue;
> >
> > -               if (addr >= sym->st_value)
> > -                       distance = addr - sym->st_value;
> > +               sym_addr = sym->st_value;
> > +
> > +               /*
> > +                * For ARM Thumb instruction, the bit 0 of st_value is set.
> > +                * Mask it to get the address.
> > +                */
> > +               if (is_arm)
> > +                        sym_addr &= ~1;
> > +
>
> This is only appropriate for STT_FUNC symbols. If this is a data
> reference, bit 0 could be a valid address bit.


Thanks for catching it.

I will fix it as follows:

    /*
     * For ARM Thumb instruction, the bit 0 of st_value is set if
     * the symbol is STT_FUNC type. Mask it to get the address.
     */
    if (is_arm && ELF_ST_TYPE(sym->st_info) == STT_FUNC)
            sym_addr &= ~1;




-- 
Best Regards
Masahiro Yamada