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

["Shi, Steven" <steven.shi@intel.com>]

Hi Ard,
I don't see you add below code for case R_X86_64_PLT32. Is it right?

            *(UINT32 *)Targ = (UINT32) (*(UINT32 *)Targ
              + (mCoffSectionsOffset[Sym->st_shndx] - SymShdr->sh_addr)
              - (SecOffset - SecShdr->sh_addr));


Steven Shi
Intel\SSG\STO\UEFI Firmware

Tel: +86 021-61166522
iNet: 821-6522

> -----Original Message-----
> From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org]
> Sent: Thursday, August 04, 2016 4:46 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: mischief@offblast.org; Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Subject: [PATCH] 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 | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/BaseTools/Source/C/GenFw/Elf64Convert.c
> b/BaseTools/Source/C/GenFw/Elf64Convert.c
> index 944c94b8f8b4..7cbff0df0996 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
> --
> 2.7.4