Re: [PATCH 1/2] mkimage: avoid copying relocations for sections that won't be copied.

[Peter Jones]

Anyone want to have a look at this patchset to make gcc > 7.2.1 (with
optional plugins enabled) work?

On Wed, Jan 31, 2018 at 11:26:59AM -0500, Peter Jones wrote:
> Some versions of gcc include a plugin called "annobin", and in some
> build systems this is enabled by default.  This plugin creates special
> ELF note sections to track which ABI-breaking features are used by a
> binary, as well as a series of relocations to annotate where.
> 
> If grub is compiled with this feature, then when grub-mkimage translates
> the binary to another file format which does not strongly associate
> relocation data with sections (i.e. when platform is *-efi), these
> relocations appear to be against the .text section rather than the
> original note section.  When the binary is loaded by the PE runtime
> loader, hilarity ensues.
> 
> This issue is not necessarily limited to the annobin, but could arise
> any time there are relocations in sections that are not represented in
> grub-mkimage's output.
> 
> This patch seeks to avoid this issue by only including relocations that
> refer to sections which will be included in the final binary.
> 
> As an aside, this should also obviate the need to avoid -funwind-tables,
> -fasynchronous-unwind-tables, and any sections similar to .eh_frame in
> the future.  I've tested it on x86-64-efi with the following gcc command
> line options (as recorded by -grecord-gcc-flags), but I still need to
> test the result on some other platforms that have been problematic in
> the past (especially ARM Aarch64) before I feel comfortable making
> changes to the configure.ac bits:
> 
> GNU C11 7.2.1 20180116 (Red Hat 7.2.1-7) -mno-mmx -mno-sse -mno-sse2 
> -mno-sse3 -mno-3dnow -msoft-float -mno-stack-arg-probe -mcmodel=large 
> -mno-red-zone -m64 -mtune=generic -march=x86-64 -g3 -Os -freg-struct-return 
> -fno-stack-protector -ffreestanding -funwind-tables 
> -fasynchronous-unwind-tables -fno-strict-aliasing -fstack-clash-protection 
> -fno-ident -fplugin=annobin
> 
> Signed-off-by: Peter Jones <address@hidden>
> ---
>  util/grub-mkimagexx.c | 81 
> ++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 73 insertions(+), 8 deletions(-)
> 
> diff --git a/util/grub-mkimagexx.c b/util/grub-mkimagexx.c
> index a2bb05439f0..b016b061c8c 100644
> --- a/util/grub-mkimagexx.c
> +++ b/util/grub-mkimagexx.c
> @@ -708,6 +708,13 @@ arm_get_trampoline_size (Elf_Ehdr *e,
>  }
>  #endif
>  
> +static int
> +SUFFIX (is_kept_section) (Elf_Shdr *s, const struct 
> grub_install_image_target_desc *image_target);
> +static int
> +SUFFIX (is_kept_reloc_section) (Elf_Shdr *s, const struct 
> grub_install_image_target_desc *image_target,
> +                             Elf_Shdr *sections, Elf_Half section_entsize, 
> Elf_Half num_sections,
> +                             const char *strtab);
> +
>  /* Deal with relocation information. This function relocates addresses
>     within the virtual address space starting from 0. So only relative
>     addresses can be fully resolved. Absolute addresses must be relocated
> @@ -747,6 +754,14 @@ SUFFIX (relocate_addresses) (Elf_Ehdr *e, Elf_Shdr 
> *sections,
>       Elf_Shdr *target_section;
>       Elf_Word j;
>  
> +     if (!SUFFIX (is_kept_section) (s, image_target) &&
> +         !SUFFIX (is_kept_reloc_section) (s, image_target, sections, 
> section_entsize, num_sections, strtab))
> +       {
> +         grub_util_info ("not translating relocations for omitted section 
> %s",
> +                     strtab + grub_le_to_cpu32 (s->sh_name));
> +         continue;
> +       }
> +
>       symtab_section = (Elf_Shdr *) ((char *) sections
>                                        + (grub_target_to_host32 (s->sh_link)
>                                           * section_entsize));
> @@ -1656,6 +1671,13 @@ make_reloc_section (Elf_Ehdr *e, struct 
> grub_mkimage_layout *layout,
>       Elf_Addr section_address;
>       Elf_Word j;
>  
> +     if (!SUFFIX (is_kept_reloc_section) (s, image_target, sections, 
> section_entsize, num_sections, strtab))
> +       {
> +         grub_util_info ("not translating the skipped relocation section %s",
> +                         strtab + grub_le_to_cpu32 (s->sh_name));
> +         continue;
> +       }
> +
>       grub_util_info ("translating the relocation section %s",
>                       strtab + grub_le_to_cpu32 (s->sh_name));
>  
> @@ -1731,6 +1753,56 @@ SUFFIX (is_bss_section) (Elf_Shdr *s, const struct 
> grub_install_image_target_des
>         == SHF_ALLOC) && (grub_target_to_host32 (s->sh_type) == SHT_NOBITS);
>  }
>  
> +/* Determine if a section is going to be in the final output */
> +static int
> +SUFFIX (is_kept_section) (Elf_Shdr *s, const struct 
> grub_install_image_target_desc *image_target)
> +{
> +  /* We keep .text and .data */
> +  if (SUFFIX (is_text_section) (s, image_target)
> +      || SUFFIX (is_data_section) (s, image_target))
> +    return 1;
> +
> +  /* And we keep .bss if we're producing PE binaries  or the target doesn't
> +   * have a relocating loader.  Platforms other than EFI and U-boot shouldn't
> +   * have .bss in their binaries as we build with -Wl,-Ttext.
> +   */
> +  if (SUFFIX (is_bss_section) (s, image_target)
> +      && (image_target->id == IMAGE_EFI || !is_relocatable (image_target)))
> +    return 1;
> +
> +  /* Otherwise this is not a section we're keeping in the final output. */
> +  return 0;
> +}
> +
> +static int
> +SUFFIX (is_kept_reloc_section) (Elf_Shdr *s, const struct 
> grub_install_image_target_desc *image_target,
> +                             Elf_Shdr *sections, Elf_Half section_entsize, 
> Elf_Half num_sections,
> +                             const char *strtab)
> +{
> +  int i;
> +  int r = 0;
> +  const char *name = strtab + grub_host_to_target32 (s->sh_name);
> +
> +  if (!strncmp (name, ".rela.", 6))
> +    name += 5;
> +  else if (!strncmp (name, ".rel.", 5))
> +    name += 4;
> +  else
> +    return 1;
> +
> +  for (i = 0, s = sections; i < num_sections;
> +       i++, s = (Elf_Shdr *) ((char *) s + section_entsize))
> +    {
> +      const char *sname = strtab + grub_host_to_target32 (s->sh_name);
> +      if (strcmp (sname, name))
> +     continue;
> +
> +      return SUFFIX (is_kept_section) (s, image_target);
> +    }
> +
> +  return r;
> +}
> +
>  /* Return if the ELF header is valid.  */
>  static int
>  SUFFIX (check_elf_header) (Elf_Ehdr *e, size_t size, const struct 
> grub_install_image_target_desc *image_target)
> @@ -2087,14 +2159,7 @@ SUFFIX (grub_mkimage_load_image) (const char 
> *kernel_path,
>    for (i = 0, s = sections;
>         i < num_sections;
>         i++, s = (Elf_Shdr *) ((char *) s + section_entsize))
> -    if (SUFFIX (is_data_section) (s, image_target)
> -     /* Explicitly initialize BSS
> -        when producing PE32 to avoid a bug in EFI implementations.
> -        Platforms other than EFI and U-boot shouldn't have .bss in
> -        their binaries as we build with -Wl,-Ttext.
> -     */
> -     || (SUFFIX (is_bss_section) (s, image_target) && (image_target->id == 
> IMAGE_EFI || !is_relocatable (image_target)))
> -     || SUFFIX (is_text_section) (s, image_target))
> +    if (SUFFIX (is_kept_section) (s, image_target))
>        {
>       if (grub_target_to_host32 (s->sh_type) == SHT_NOBITS)
>         memset (out_img + section_addresses[i], 0,
> -- 
> 2.15.0
> 

-- 
  Peter