Re: [PATCH v2] BaseTools X64: fold PLT relocations into simple relative references

["Gao, Liming" <liming.gao@intel.com>]

Reviewed-by: Liming Gao <liming.gao@intel.com>
> -----Original Message-----
> From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of
> Ard Biesheuvel
> Sent: Friday, August 05, 2016 10:36 PM
> To: Shi, Steven <steven.shi@intel.com>; Zhu, Yonghong
> <yonghong.zhu@intel.com>; Gao, Liming <liming.gao@intel.com>; Justen,
> Jordan L <jordan.l.justen@intel.com>; edk2-devel@lists.01.org
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Subject: [edk2] [PATCH v2] BaseTools X64: fold PLT relocations into simple
> relative references
> 
> For X64/GCC, we use position independent code with hidden visibility
> to inform the compiler that symbols references are never resolved at
> runtime, which removes the need for PLTs and GOTs. However, in some
> cases GCC has been reported to still emit PLT based relocations, which
> we need to handle in the ELF to PE/COFF perform by GenFw.
> 
> Unlike GOT based relocations, which are non-trivial to handle since the
> indirections in the code can not be fixed up easily (although relocation
> types exist for X64 that annotate relocation targets as suitable for
> relaxation), PLT relocations simply point to jump targets, and we can
> relax such relocations by resolving them using the symbol directly rather
> than via a PLT entry that does nothing more than tail call the function
> we already know it is going to call (since all symbol references are
> resolved in the same module).
> 
> So handle R_X86_64_PLT32 as a R_X86_64_PC32 relocation.
> 
> Suggested-by: Steven Shi <steven.shi@intel.com>
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  BaseTools/Source/C/GenFw/Elf64Convert.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/BaseTools/Source/C/GenFw/Elf64Convert.c
> b/BaseTools/Source/C/GenFw/Elf64Convert.c
> index 944c94b8f8b4..708c1a1d91a7 100644
> --- a/BaseTools/Source/C/GenFw/Elf64Convert.c
> +++ b/BaseTools/Source/C/GenFw/Elf64Convert.c
> @@ -785,6 +785,17 @@ WriteSections64 (
>              *(INT32 *)Targ = (INT32)((INT64)(*(INT32 *)Targ) - SymShdr->sh_addr
> + mCoffSectionsOffset[Sym->st_shndx]);
>              VerboseMsg ("Relocation:  0x%08X", *(UINT32*)Targ);
>              break;
> +
> +          case R_X86_64_PLT32:
> +            //
> +            // Treat R_X86_64_PLT32 relocations as R_X86_64_PC32: this is
> +            // possible since we know all code symbol references resolve to
> +            // definitions in the same module (UEFI has no shared libraries),
> +            // and so there is never a reason to jump via a PLT entry,
> +            // allowing us to resolve the reference using the symbol directly.
> +            //
> +            VerboseMsg ("Treating R_X86_64_PLT32 as R_X86_64_PC32 ...");
> +            /* fall through */
>            case R_X86_64_PC32:
>              //
>              // Relative relocation: Symbol - Ip + Addend
> @@ -935,6 +946,7 @@ WriteRelocations64 (
>              switch (ELF_R_TYPE(Rel->r_info)) {
>              case R_X86_64_NONE:
>              case R_X86_64_PC32:
> +            case R_X86_64_PLT32:
>                break;
>              case R_X86_64_64:
>                VerboseMsg ("EFI_IMAGE_REL_BASED_DIR64 Offset: 0x%08X",
> --
> 2.7.4
> 
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