linux-kernel - Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

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Date:   Mon, 7 Jun 2021 13:54:37 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>
Cc:     Fāng-ruì Sòng <maskray@...gle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, lma@...ihalf.com,
        Guenter Roeck <groeck@...gle.com>,
        Juergen Gross <jgross@...e.com>, lb@...ihalf.com,
        LKML <linux-kernel@...r.kernel.org>, mbenes@...e.com,
        Radosław Biernacki <rad@...ihalf.com>,
        upstream@...ihalf.com,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

On Mon, Jun 7, 2021 at 2:46 AM Peter Zijlstra <peterz@...radead.org> wrote:
>

Thanks, the below diff resolves the linker error reported in
https://github.com/ClangBuiltLinux/linux/issues/1388

Both readelf implementations seem happy with the results, too.

Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>

Nathan,
Can you please test the below diff and see if that resolves your boot
issue reported in:
https://github.com/ClangBuiltLinux/linux/issues/1384

> ---
>  tools/objtool/elf.c | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
>
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index 743c2e9d0f56..41bca1d13d8e 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -717,7 +717,7 @@ static int elf_add_string(struct elf *elf, struct section *strtab, char *str)
>
>  struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
>  {
> -       struct section *symtab;
> +       struct section *symtab, *symtab_shndx;
>         struct symbol *sym;
>         Elf_Data *data;
>         Elf_Scn *s;
> @@ -769,6 +769,29 @@ struct symbol *elf_create_undef_symbol(struct elf *elf, const char *name)
>         symtab->len += data->d_size;
>         symtab->changed = true;
>
> +       symtab_shndx = find_section_by_name(elf, ".symtab_shndx");
> +       if (symtab_shndx) {
> +               s = elf_getscn(elf->elf, symtab_shndx->idx);
> +               if (!s) {
> +                       WARN_ELF("elf_getscn");
> +                       return NULL;
> +               }
> +
> +               data = elf_newdata(s);
> +               if (!data) {
> +                       WARN_ELF("elf_newdata");
> +                       return NULL;
> +               }
> +
> +               data->d_buf = &sym->sym.st_size; /* conveniently 0 */
> +               data->d_size = sizeof(Elf32_Word);
> +               data->d_align = 4;
> +               data->d_type = ELF_T_WORD;
> +
> +               symtab_shndx->len += 4;
> +               symtab_shndx->changed = true;
> +       }
> +
>         sym->sec = find_section_by_index(elf, 0);
>
>         elf_add_symbol(elf, sym);


The only thing that's still different is that the `file` command still
prints "no section header."

$ find . -name \*.lto.o | xargs file | rev | cut -d , -f 1 | rev |
sort | uniq -c
      1  no section header
   8377  not stripped
      1  too many section headers (33683)
      1  too many section headers (50758)
$ file --version
file-5.39

That's drivers/gpu/drm/amd/amdgpu/amdgpu.lto.o, fs/xfs/xfs.lto.o,
drivers/gpu/drm/i915/i915.lto.o, respectively.  I'm not sure that's a
problem, yet, and whether 9bc0bb50727c8ac69fbb33fb937431cf3518ff37 is
even related yet; those might just be huge drivers and figured it was
reporting somewhere in case it ever comes up again.  CONFIG_LTO
implies -ffunction-sections -fdata-sections, and
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION explicitly sets those, too.
-- 
Thanks,
~Nick Desaulniers